home Safety Eight simple transistor circuits for beginner radio amateurs. Cognitive experiments with transistors For beginner radio amateurs about the boundary voltage of a transistor

Eight simple transistor circuits for beginner radio amateurs. Cognitive experiments with transistors For beginner radio amateurs about the boundary voltage of a transistor

Several schemes are given simple devices and nodes that can be made by novice radio amateurs.

Single stage AF amplifier

This is the simplest design that allows you to demonstrate the amplifying capabilities of a transistor. True, the voltage gain is small - it does not exceed 6, so the scope of such a device is limited.

Nevertheless, it can be connected to, say, a detector radio receiver (it must be loaded with a 10 kΩ resistor) and, using the BF1 headphone, listen to the transmission of a local radio station.

The amplified signal is fed to the input sockets X1, X2, and the supply voltage (as in all other designs of this author, it is 6 V - four galvanic cells with a voltage of 1.5 V connected in series) is fed to the X3, X4 sockets.

Divider R1R2 sets the bias voltage at the base of the transistor, and resistor R3 provides current feedback, which contributes to the temperature stabilization of the amplifier.

Rice. 1. Scheme of a single-stage AF amplifier on a transistor.

How does stabilization take place? Suppose that under the influence of temperature, the collector current of the transistor has increased. Accordingly, the voltage drop across the resistor R3 will increase. As a result, the emitter current will decrease, and hence the collector current - it will reach its original value.

The load of the amplifying stage is a headphone with a resistance of 60 .. 100 Ohms. It is not difficult to check the operation of the amplifier, you need to touch the X1 input jack, for example, a weak buzz should be heard with tweezers in the phone, as a result of alternating current pickup. The collector current of the transistor is about 3 mA.

Two-stage ultrasonic frequency converter on transistors of different structures

It is designed with direct coupling between stages and deep negative DC feedback, which makes its mode independent of temperature. environment. The basis of temperature stabilization is the resistor R4, which works similarly to the resistor R3 in the previous design.

The amplifier is more "sensitive" compared to a single-stage one - the voltage gain reaches 20. The input jacks can be fed AC voltage with an amplitude of not more than 30 mV, otherwise there will be distortion heard in the head phone.

They check the amplifier by touching the X1 input jack with tweezers (or just a finger) - the phone will hear loud noise. The amplifier consumes a current of about 8 mA.

Rice. 2. Scheme of a two-stage AF amplifier on transistors different structure.

This design can be used to amplify weak signals such as from a microphone. And of course, it will significantly amplify the signal 34 taken from the load of the detector receiver.

Two-stage ultrasonic frequency converter on transistors of the same structure

Here, a direct connection between the cascades is also used, but the stabilization of the operating mode is somewhat different from previous designs.

Assume that the collector current of the transistor VT1 has decreased. The voltage drop across this transistor will increase, which will lead to an increase in the voltage across the resistor R3 included in the emitter circuit of the transistor VT2.

Due to the connection of the transistors through the resistor R2, the base current of the input transistor will increase, which will lead to an increase in its collector current. As a result, the initial change in the collector current of this transistor will be compensated.

Rice. 3. Scheme of a two-stage AF amplifier on transistors of the same structure.

The sensitivity of the amplifier is very high - the gain reaches 100. The gain is highly dependent on the capacitance of the capacitor C2 - if you turn it off, the gain will decrease. The input voltage should be no more than 2 mV.

The amplifier works well with a detector receiver, with an electret microphone and other sources. weak signal. The current consumed by the amplifier is about 2 mA.

It is made on transistors of different structures and has a voltage gain of about 10. The highest input voltage can be 0.1 V.

The first two-stage amplifier is assembled on a VT1 transistor, the second - on VT2 and VTZ of different structures. The first stage amplifies signal 34 in terms of voltage, and both half-waves are the same. The second one amplifies the current signal, but the cascade on the VT2 transistor “works” with positive half-waves, and on the VТЗ transistor - with negative ones.

Rice. 4. Push-pull AF power amplifier on transistors.

The DC mode is chosen so that the voltage at the junction point of the emitters of the transistors of the second stage is approximately half the voltage of the power source.

This is achieved by turning on the resistor R2 feedback The collector current of the input transistor, flowing through the diode VD1, leads to a voltage drop across it. which is the bias voltage at the bases of the output transistors (relative to their emitters), - it allows you to reduce the distortion of the amplified signal.

The load (several parallel-connected headphones or a dynamic head) is connected to the amplifier through an oxide capacitor C2.

If the amplifier will work on a dynamic head (with a resistance of 8 -.10 ohms), the capacitance of this capacitor should be at least twice as large , but with a lower load output.

This is the so-called voltage boost circuit, in which a small positive feedback voltage is supplied to the base circuit of the output transistors, which equalizes the operating conditions of the transistors.

Two-level voltage indicator

Such a device can be used. for example, to indicate the “depletion” of the battery or to indicate the level of the reproduced signal in a household tape recorder. The layout of the indicator will allow you to demonstrate the principle of its operation.

Rice. 5. Scheme of a two-level voltage indicator.

In the lower position of the variable resistor R1 engine according to the diagram, both transistors are closed, the LEDs HL1, HL2 are off. When moving the slider of the resistor up, the voltage across it increases. When it reaches the opening voltage of the transistor VT1, the HL1 LED will flash

If you continue to move the engine. there will come a moment when, after the diode VD1, the transistor VT2 opens. The HL2 LED will also flash. In other words, a low voltage at the input of the indicator causes only the HL1 LED to glow, and more than both LEDs.

Gradually reducing the input voltage variable resistor, note that the HL2 LED goes out first, and then HL1. The brightness of the LEDs depends on the limiting resistors R3 and R6 as their resistances increase, the brightness decreases.

To connect the indicator to a real device, you need to disconnect the top terminal of the variable resistor from the positive wire of the power source and apply a controlled voltage to the extreme terminals of this resistor. By moving its engine, the threshold of the indicator is selected.

When monitoring only the voltage of the power source, it is permissible to install the AL307G green LED in place of HL2.

It gives out light signals according to the principle less than the norm - the norm - more than the norm. To do this, the indicator uses two red LEDs and one green LED.

Rice. 6. Three-level voltage indicator.

At a certain voltage on the engine of the variable resistor R1 (the voltage is normal), both transistors are closed and (works) only green LED HL3. Moving the resistor slider up the circuit leads to an increase in voltage (more than normal), the transistor VT1 opens on it.

LED HL3 goes out, and HL1 lights up. If the engine is moved down and thus the voltage on it is reduced ('less than normal'), the transistor VT1 will close, and VT2 will open. The following picture will be observed: first, the HL1 LED will go out, then it will light up and soon HL3 will go out, and finally HL2 will flash.

Due to the low sensitivity of the indicator, a smooth transition is obtained from the extinction of one LED to the ignition of another, for example, HL1 has not yet completely gone out, but HL3 is already on.

Schmitt trigger

As you know, this device is usually used to convert a slowly changing voltage into a rectangular signal. When the engine of the variable resistor R1 is in the lower position according to the circuit, the transistor VT1 is closed.

The voltage on its collector is high, as a result, the transistor VT2 is open, which means that the HL1 LED is lit. A voltage drop forms on the resistor R3.

Rice. 7. Simple Schmitt trigger on two transistors.

By slowly moving the variable resistor slider up the circuit, it will be possible to reach the moment when the transistor VT1 suddenly opens and VT2 closes. This will happen when the voltage on the base of VT1 exceeds the voltage drop across the resistor R3.

The LED will turn off. If after that you move the slider down, the trigger will return to its original position - the LED will flash. This will happen when the voltage on the slider is less than the LED off voltage.

Waiting multivibrator

Such a device has one stable state and switches to another only when an input signal is applied. In this case, the multivibrator generates an impulse of its duration, regardless of the duration of the input. We will verify this by conducting an experiment with the layout of the proposed device.

Rice. eight. circuit diagram waiting multivibrator.

In the initial state, the transistor VT2 is open, the LED HL1 is lit. Now it is enough to briefly close the sockets X1 and X2 so that the current pulse through the capacitor C1 opens the transistor VT1. The voltage on its collector will decrease and the capacitor C2 will be connected to the base of the transistor VT2 in such polarity that it will close. The LED will turn off.

The capacitor starts to discharge, the discharge current will flow through the resistor R5, keeping the transistor VT2 in the closed state. As soon as the capacitor is discharged, the transistor VT2 will open again and the multivibrator will go back to standby mode.

The duration of the pulse generated by the multivibrator (the duration of being in an unstable state) does not depend on the duration of the trigger, but is determined by the resistance of the resistor R5 and the capacitance of the capacitor C2.

If you connect a capacitor of the same capacity in parallel with C2, the LED will remain off twice as long.

I. Bokomchev. R-06-2000.

Field-effect transistors in the practice of beginner radio amateurs

This article is intended for the "Beginner Radio Amateur" section. Long before the appearance in the journal "Radio" No. 9 - 2007 of the article by V. Andryushkevich "Measuring the parameters of field-effect transistors", guided by the same principles and tasks, I made a device similar to that described in the article, but, in my opinion, much simpler circuitry and technologically. I think beginner radio amateurs will appreciate it. On the other hand, the device of V. Andryushkevich is more accurate and versatile, created on a more modern element base, with good ergonomic properties, in short - of a higher level.

At one time, the author faced the problem of selecting common field-effect transistors (FETs) for installation in specific circuits of amplifiers, source followers, mixers, etc. a combined device for measuring the most commonly used parameters in the practice of radio amateurs: drain current, cutoff voltage, slope of the characteristic.

First, a little theory. It is presented only for further practical application and understanding of the operation of the device, and no more. Therefore, the physics of the work of the PT and some theoretical provisions are omitted. It is on the practical aspect of the applicable provisions that the emphasis is placed. I hope that for beginner radio amateurs a short description of the operation of the device will be useful and applicable in creating a real design.

Transfer (control) characteristic of field-effect transistors with managing p-n- transition.

The figure below shows a circuit for measuring the drain current of a field-effect transistor. In the notation: gate - s, drain - s, source - i. In addition to the drain current, the most important characteristic of the FET is the cutoff voltage Uots. This is the voltage between the gate and the source (Uz), at which the drain current is almost 0, although it is usually taken as 10 μA.

If Uzi is equal to 0, then the drain current of the FET will be maximum and is called the saturation current, or full current open channel, or the initial drain current. Denoted by Ic.beginning. (sometimes Ic.o).

If a bias voltage is applied to the FET gate (it is Uzi, in Fig. 1 this is a 1.5v battery), and Uot is reflected on the abscissa, and Ic.beginning on the ordinate. and other values of the drain current at different Uzi (bias), then you can build a curve called volt-ampere characteristic Fri. Thus, as can be seen from the graph, Ic depends on the Uot value.

Determination of the characteristic slope (S) according to the assembled circuit (Fig. 1) is carried out according to the formula:

S = Is.beginning – Ic/Us., where Ic is the selected optimal drain current at which the FET will operate.

On its straight section, which always is located on the graph from 0 to Uots./2 and is called quadratic, choose the drain current Ic, at which the FET will work most efficiently and not introduce nonlinear distortions into the work standard scheme linear amplifier (Fig. 3). Usually this is half of the quadratic section: Ures./2, then Uzi will be approximately equal to Ures./4.

In practice, Uzi is equal to the voltage drop across Rн (Uн). That is, you can choose the optimal current Ic from the S curve and then determine Uzi (there are corresponding graphs in the reference books - the dependence of S on Ic and Uzi, and vice versa). Further, according to Ohm's law, determine Rn, which must be placed in the source circuit of the FET of a linear amplifier. Suppose that Ic = 6mA is selected, while from the data on the S-characteristic Uzi = Un = 0.7 v. Then Rn \u003d Un / Ic \u003d 0.7 v / 0.006 A \u003d 116 Ohm.

Another option is also possible: knowing from the characteristics or measurements of Uots. it is possible to determine Uzi (= 1/4 Uots.) and then, according to the S schedule, determine Ic, and then the value of Rn.

In a working FET amplifier, you can measure Un (voltage drop across Rn) without soldering and, knowing the value of Rn from the circuit, calculate Ic. For example, Ic \u003d Un / Rn \u003d 0.7 v / 116 Ohm \u003d 0.006 A (6mA). Comparing the obtained data with the table-passport one can choose Rn for the optimal Ic.

Definition of Uots. possible according to the scheme in Fig.4.

Since Ic depends on Uzi, the S-characteristic can change (shift). It also changes when the PT is exposed to ambient temperature. To get to a thermostable point, choose Uzi = Uots. - 0.63v. In practice, for real FETs at a fixed Uzi, Ic varies from 0.1 to 0.5 mA (in the reference literature there are corresponding graphs of this transfer characteristic).

On the current-voltage characteristics of the FET Usi is in the range up to Usi.nas. - saturation voltage drain - source, and usually does not exceed 2v (for KP303, and sometimes more for other PTs). This feature is called day off.

Scheme and work with the device.

The real scheme of the device for measuring the parameters of the FET does not differ from the above schemes for measuring Ic and Uots. It's just that the device has become more versatile, a kind of stand for measuring PT parameters.

When Ic is known (desired, optimal, from directories), Ic.nach is first determined. To do this, set the type of the PT channel with the switches SA2 and SA3 (“n - p channel”), and the switch SA4 (“Parameter”) is set to the position “Is.begin”. The microammeter (multimeter) is connected to the XT2 terminals. Having connected the PT to the XT4 terminal block, turn on the device, press the SB1 “Measurement” button and read Ic.

Next, Ic is determined by moving the switch SA4 to the "Ic" position. With this resistor R2 (“Set Uzi”) change (on the scale of this resistor) Uots. from the value at which the drain current will be minimal (about 10 μA) to a value close to ¼ Uots. The microammeter will show Ic: together with the Uzi value on the graph, they form a point on the quadratic section of the curve. Then the steepness of the characteristic (S) of the PT is calculated:

S = Ic.beginning - Ic/Uzi, where Uzi =1/4Uot.(empirically selected ratio).

You can first determine Uots. (switch SA4 in the appropriate position), divide this value by 4, getting Uzi, and after that Ic according to the schedule.

When measuring Uot. (when the multimeter is connected to the terminals of the voltmeter) it is important, if you use the same multimeter, do not forget to close the terminals of the XT2 milli(micro)ammeter with jumper S1.

Usi is usually equal to 10 v. In the device, you can change it, because. reference books sometimes show VAC graphs at a different voltage. The same can be said about Uzi - its value can be changed. For these purposes, adjustable positive and negative voltage stabilizers are used, which are used to power the drain circuit of the FET from 2 to 15 v, and the gate circuit - from 0 to -5 v. Sometimes, when measuring the parameters of 2 gate FETs, it is required to apply a positive voltage to the second gate. For this purpose, a SA2.2 switch is installed in the device, which changes the polarity of the voltage received from the bias stabilizer to the opposite. Actually, this is the only reason this switch is not combined with the channel type switch. The “K” terminal on the XT4 bar can be used (or an additional one can be installed) to connect the second gate by switching it with the output of the bias voltage regulator (not shown in the diagram).

Voltage regulators should be calibrated - then you do not need to use additional terminals and devices for measuring Usi and Uzi. In order not to swap the multimeter probes during measurements, the XT2 and XT3 terminals are connected in the circuit through the corresponding diode bridges, and the polarity of the supply voltages is reversed by the SA2 switch. The values of the voltages themselves should be set as given in the reference books.

You can often hear about the danger of damage to the PT by static electricity induced from the mains through the PSU (also from a soldering iron, from hands, clothes, etc.). Of course, it is optimal to power the device from Krona and an AA type element, while the risk of damage to the PT by network static is minimal. And if the voltages of the indicated batteries are sufficient for measuring low-power FETs, then this should be done - insert these two batteries into the device. On the other hand, my practical experience with the manufactured device has never led to damage to the FET. It is obvious that this contributed certain properties design and compliance with the usual rules when working with field-effect transistors. Teflon interwinding insulation is used in the T1 transformer, power is supplied to the FET connected to the device in the circuit through the SB1 “Measurement” button. By the way, the transformer that is most accessible and suitable for this device in terms of the voltage on the secondary windings is TVK-70L2.

The simplest rule is that the FET leads before and when connected to the instrument terminals should always be shorted (a few turns of soft tinned thin wire around the leads at the base of the transistor). During measurements, the wire, of course, is removed.

The device is mounted in the body of an old AVO-63, where it was possible to place the power supply and use the standard pointer measuring head. Appearance device is shown in Fig.6. The outputs of the FET under test are connected to the connector at the end of the short cable from the power supply unit of the personal computer.

In conclusion, it should be noted that the above scheme is not a dogma, and when translated into a real device for a radio amateur, there is a whole field of possibilities and options for changing circuitry and design.

Vasily Kononenko (RA0CCN).

A transistor is a semiconductor device that can amplify, convert, and generate electrical signals. The first workable bipolar transistor was invented in 1947. Germanium served as a material for its manufacture. And already in 1956, the silicon transistor was born.

In a bipolar transistor, two types of charge carriers are used - electrons and holes, which is why such transistors are called bipolar. In addition to bipolar, there are unipolar (field) transistors, which use only one type of carrier - electrons or holes. This article will cover.

Most silicon transistors are n-p-n, which is also due to the manufacturing technology, although silicon transistors also exist. pnp type, but there are somewhat fewer of them than n-p-n structures. Such transistors are used as part of complementary pairs (transistors of different conductivity with the same electrical parameters). For example, KT315 and KT361, KT815 and KT814, and in the output stages of the transistor UMZCH KT819 and KT818. In imported amplifiers, a powerful complementary pair 2SA1943 and 2SC5200 is very often used.

Often, p-n-p structure transistors are called direct conduction transistors, and structures n-p-n reverse. For some reason, this name is almost never found in the literature, but in the circle of radio engineers and radio amateurs it is used everywhere, everyone immediately understands what it is about. Figure 1 shows a schematic device of transistors and their conventional graphic symbols.

Picture 1.

In addition to differences in type of conductivity and material, bipolar transistors are classified by power and operating frequency. If the power dissipation on the transistor does not exceed 0.3 W, such a transistor is considered low-power. At a power of 0.3 ... 3 W, the transistor is called a medium power transistor, and at a power of more than 3 W, the power is considered high. Modern transistors are able to dissipate power of several tens and even hundreds of watts.

Transistors amplify electrical signals not equally well: with increasing frequency, the amplification of the transistor stage drops, and at a certain frequency it stops altogether. Therefore, to operate in a wide frequency range, transistors are produced with different frequency properties.

According to the operating frequency, transistors are divided into low-frequency ones - the operating frequency is not more than 3 MHz, mid-frequency - 3 ... 30 MHz, high-frequency - over 30 MHz. If the operating frequency exceeds 300 MHz, then these are already microwave transistors.

In general, over 100 different transistor parameters are given in serious thick reference books, which also indicates a huge number of models. And the number of modern transistors is such that it is no longer possible to put them in full in any reference book. And the lineup is constantly increasing, allowing to solve almost all the tasks set by the developers.

There are many transistor circuits (just remember the number of at least household equipment) for amplifying and converting electrical signals, but, for all their diversity, these circuits consist of separate cascades, which are based on transistors. To achieve the required signal amplification, it is necessary to use several amplification stages connected in series. To understand how amplifying stages work, you need to become more familiar with transistor switching circuits.

By itself, the transistor will not be able to amplify anything. Its amplifying properties lie in the fact that small changes in the input signal (current or voltage) lead to significant changes in the voltage or current at the output of the stage due to the consumption of energy from an external source. It is this property that is widely used in analog circuits - amplifiers, television, radio, communications, etc.

To simplify the presentation, circuits based on transistors of the n-p-n structure will be considered here. Everything that will be said about these transistors applies equally to p-n-p transistors. It is enough just to reverse the polarity of the power supplies, and, if any, to get a working circuit.

In total, there are three such circuits: a circuit with a common emitter (CE), a circuit with a common collector (OC) and a circuit with a common base (OB). All these schemes are shown in Figure 2.

Figure 2.

But before proceeding to the consideration of these circuits, you should get acquainted with how the transistor works in the key mode. This introduction should make it easier to understand in boost mode. In a certain sense, the key circuit can be considered as a kind of circuit with OE.

Transistor operation in key mode

Before studying the operation of a transistor in signal amplification mode, it is worth remembering that transistors are often used in a key mode.

This mode of operation of the transistor has been considered for a long time. In the August issue of the magazine "Radio" in 1959, an article by G. Lavrov "Semiconductor triode in key mode" was published. The author of the article suggested changing the duration of the pulses in the control winding (OC). Now this method of regulation is called PWM and is used quite often. The diagram from the magazine of that time is shown in Figure 3.

Figure 3

But the key mode is used not only in PWM systems. Often a transistor simply turns something on and off.

In this case, a relay can be used as a load: an input signal is applied - the relay is turned on, no - the relay signal is turned off. Light bulbs are often used instead of relays in key mode. Usually this is done to indicate: the light bulb is either on or off. A diagram of such a key stage is shown in Figure 4. Key stages are also used to work with LEDs or with optocouplers.

Figure 4

In the figure, the cascade is controlled by a conventional contact, although it may be a digital microcircuit or instead. A car bulb, this is used to illuminate the dashboard in the Zhiguli. Attention should be paid to the fact that 5V is used for control, and the switched collector voltage is 12V.

There is nothing strange in this, since voltages in this circuit do not play any role, only currents matter. Therefore, the light bulb can be at least 220V, if the transistor is designed to operate at such voltages. The collector source voltage must also match the operating voltage of the load. With the help of such cascades, the load is connected to digital microcircuits or microcontrollers.

In this scheme, the base current controls the collector current, which, due to the energy of the power source, is several tens or even hundreds of times more (depending on the collector load) than the base current. It is easy to see that there is an increase in current. When the transistor is operating in the key mode, it is usually used to calculate the cascade by the value called in the reference books "current gain in the large signal mode" - in the reference books it is denoted by the letter β. This is the ratio of the collector current, determined by the load, to the minimum possible base current. In the form of a mathematical formula, it looks like this: β = Ik / Ib.

For most modern transistors, the coefficient β is large enough, as a rule, from 50 and above, therefore, when calculating the key stage, it can be taken equal to only 10. Even if the base current turns out to be greater than the calculated one, the transistor will not open more from this, then and key mode.

To light the bulb shown in Figure 3, Ib \u003d Ik / β \u003d 100mA / 10 \u003d 10mA, this is at least. With a control voltage of 5V on the base resistor Rb, minus the voltage drop in the B-E section, 5V - 0.6V = 4.4V will remain. The resistance of the base resistor will be: 4.4V / 10mA = 440 ohms. A resistor with a resistance of 430 ohms is selected from the standard range. The voltage of 0.6V is the voltage at the B-E junction, and you should not forget about it when calculating!

So that the base of the transistor does not remain “hanging in the air” when the control contact is opened, the B-E junction is usually shunted with a resistor Rbe, which reliably closes the transistor. This resistor should not be forgotten, although for some reason it is not in some circuits, which can lead to false operation of the noise stage. Actually, everyone knew about this resistor, but for some reason they forgot, and once again stepped on the "rake".

The value of this resistor must be such that when the contact opens, the voltage at the base would not be less than 0.6V, otherwise the cascade will be uncontrollable, as if section B-E just short-circuited. In practice, the resistor Rbe is set with a nominal value of about ten times more than Rb. But even if the value of Rb is 10Kom, the circuit will work quite reliably: the potentials of the base and emitter will be equal, which will lead to the closing of the transistor.

Such a key cascade, if it is in good condition, can turn on the light bulb at full incandescence, or turn it off completely. In this case, the transistor can be fully on (saturation state) or fully closed (cutoff state). Immediately, by itself, the conclusion suggests itself that between these "boundary" states there is such a thing when the light bulb shines half-heartedly. Is the transistor half open or half closed in this case? It's like filling a glass: an optimist sees the glass as half full, while a pessimist sees it as half empty. This mode of operation of the transistor is called amplifying or linear.

Transistor operation in signal amplification mode

Almost all modern electronic equipment consists of microcircuits in which transistors are “hidden”. It is enough just to choose the operating mode of the operational amplifier in order to obtain the required gain or bandwidth. But, despite this, cascades on discrete ("loose") transistors are often used, and therefore an understanding of the operation of the amplifying cascade is simply necessary.

The most common transistor connection compared to OK and OB is the common emitter (CE) circuit. The reason for this prevalence, first of all, is the high voltage and current gain. The highest gain of the OE stage is provided when half of the voltage of the power supply Epit/2 drops across the collector load. Accordingly, the second half falls on the plot K-E transistor. This is achieved by setting the cascade, which will be discussed below. This mode of amplification is called class A.

When the transistor with OE is turned on, the output signal at the collector is in antiphase with the input signal. As disadvantages, it can be noted that the input resistance of the OE is small (no more than a few hundred ohms), and the output resistance is within tens of kΩ.

If in the switching mode the transistor is characterized by a current gain in the large signal mode β, then in the amplification mode the "current gain in the small signal mode" is used, denoted in the reference books h21e. This designation came from the representation of the transistor in the form of a quadripole. The letter "e" indicates that the measurements were made when the transistor with a common emitter was turned on.

The coefficient h21e, as a rule, is somewhat larger than β, although it can also be used in calculations in the first approximation. All the same, the spread of parameters β and h21e is so large even for one type of transistor that the calculations are only approximate. After such calculations, as a rule, it is required to adjust the scheme.

The gain of the transistor depends on the thickness of the base, so it cannot be changed. Hence the large variation in the gain of transistors taken even from one box (read one batch). For low-power transistors, this coefficient ranges from 100 ... 1000, and for powerful ones it is 5 ... 200. The thinner the base, the higher the coefficient.

The simplest circuit for switching on an OE transistor is shown in Figure 5. This is just a small piece from Figure 2, shown in the second part of the article. Such a circuit is called a fixed base current circuit.

Figure 5

The scheme is extremely simple. The input signal is applied to the base of the transistor through the decoupling capacitor C1, and, being amplified, is taken from the collector of the transistor through the capacitor C2. The purpose of capacitors is to protect the input circuits from the constant component of the input signal (just remember carbon or electret microphone) and provide the necessary bandwidth of the cascade.

Resistor R2 is the collector load of the stage, and R1 supplies a DC bias to the base. With the help of this resistor, they try to make the voltage across the collector be Epit / 2. This state is called the operating point of the transistor, in this case the gain of the cascade is maximum.

Approximately the resistance of the resistor R1 can be determined by a simple formula R1 ≈ R2 * h21e / 1.5 ... 1.8. The coefficient 1.5…1.8 is substituted depending on the supply voltage: at low voltage (no more than 9V), the value of the coefficient is not more than 1.5, and starting from 50V, it approaches 1.8…2.0. But, indeed, the formula is so approximate that the resistor R1 most often has to be selected, otherwise the required value of Epit / 2 on the collector will not be obtained.

The collector resistor R2 is set as a condition of the problem, since the collector current and the gain of the cascade as a whole depend on its value: the greater the resistance of the resistor R2, the higher the gain. But with this resistor you need to be careful, the collector current must be less than the maximum allowable for of this type transistor.

The scheme is very simple, but this simplicity gives it negative properties, and this simplicity comes at a cost. Firstly, the amplification of the cascade depends on the specific instance of the transistor: I replaced the transistor during repair, - select the offset again, bring it to the operating point.

Secondly, from the ambient temperature, with increasing temperature, the reverse collector current Ico increases, which leads to an increase in the collector current. And where, then, is half the supply voltage on the Epit / 2 collector, that same operating point? As a result, the transistor heats up even more, after which it fails. To get rid of this dependence, or at least reduce it to a minimum, additional negative feedback elements are introduced into the transistor cascade - OOS.

Figure 6 shows a circuit with a fixed bias voltage.

Figure 6

It would seem that the voltage divider Rb-k, Rb-e will provide the required initial bias of the cascade, but in fact, such a cascade has all the disadvantages of a fixed current circuit. Thus, the circuit shown is just a variation of the fixed current circuit shown in Figure 5.

Circuits with thermal stabilization

The situation is somewhat better in the case of applying the schemes shown in Figure 7.

Figure 7

In a collector-stabilized circuit, the bias resistor R1 is not connected to the power supply, but to the collector of the transistor. In this case, if the reverse current increases with increasing temperature, the transistor opens more strongly, the collector voltage decreases. This decrease leads to a decrease in the bias voltage applied to the base through R1. The transistor starts to close, the collector current decreases to an acceptable value, the position of the operating point is restored.

It is quite obvious that such a measure of stabilization leads to some reduction in the gain of the cascade, but this does not matter. The missing amplification, as a rule, is added by increasing the number of amplifying stages. But such an environmental protection allows you to significantly expand the operating temperature range of the cascade.

The circuitry of the cascade with emitter stabilization is somewhat more complicated. The amplifying properties of such cascades remain unchanged over an even wider temperature range than that of a collector-stabilized circuit. One more thing undeniable advantage, - when replacing the transistor, it is not necessary to re-select the operating modes of the cascade.

The emitter resistor R4, providing temperature stabilization, also reduces the gain of the cascade. It's for direct current. In order to eliminate the influence of the resistor R4 on the amplification of the alternating current, the resistor R4 is shunted by the capacitor Ce, which presents little resistance to the alternating current. Its value is determined by the frequency range of the amplifier. If these frequencies lie in the audio range, then the capacitance of the capacitor can be from units to tens and even hundreds of microfarads. For radio frequencies, this is already hundredths or thousandths, but in some cases the circuit works fine even without this capacitor.

In order to better understand how emitter stabilization works, it is necessary to consider the circuit for switching on a transistor with a common collector OK.

The Common Collector Circuit (CC) is shown in Figure 8. This circuit is a piece of Figure 2, from the second part of the article, which shows all three transistor switching circuits.

Figure 8

The load of the stage is the emitter resistor R2, the input signal is fed through the capacitor C1, and the output signal is taken through the capacitor C2. Here you can ask why this scheme is called OK? After all, if we recall the OE circuit, then it is clearly seen that the emitter is connected to the common wire of the circuit, relative to which the input signal is applied and the output signal is removed.

In the OK circuit, the collector is simply connected to the power source, and at first glance it seems that it has nothing to do with the input and output signal. But in fact, the EMF source (power battery) has a very small internal resistance; for a signal, this is practically one point, the same contact.

In more detail, the operation of the OK circuit can be seen in Figure 9.

Figure 9

It is known that for silicon transistors the voltage b-e transition is in the range of 0.5 ... 0.7V, so you can take it on average 0.6V, if you do not set out to carry out calculations with an accuracy of tenths of a percent. Therefore, as seen in Figure 9, output voltage will always be less than the input by the value of Ub-e, namely by those same 0.6V. Unlike the OE circuit, this circuit does not invert the input signal, it simply repeats it, and even reduces it by 0.6V. This circuit is also called an emitter follower. Why is such a scheme needed, what is its use?

The OK circuit amplifies the current signal by h21e times, which means that the input impedance of the circuit is h21e times greater than the resistance in the emitter circuit. In other words, without fear of burning the transistor, apply voltage directly to the base (without a limiting resistor). Simply take the base pin and connect it to the +U power rail.

The high input impedance allows you to connect a high impedance (complex impedance) input source, such as a piezoelectric pickup. If such a pickup is connected to the cascade according to the OE scheme, then the low input impedance of this cascade will simply “land” the pickup signal - “the radio will not play”.

A distinctive feature of the OK circuit is that its collector current Ik depends only on the load resistance and the voltage of the input signal source. In this case, the parameters of the transistor do not play any role here at all. Such circuits are said to be covered by 100% voltage feedback.

As shown in Figure 9, the current in the emitter load (aka the emitter current) In = Ik + Ib. Taking into account that the base current Ib is negligible compared to the collector current Ik, it can be assumed that the load current is equal to the collector current In = Ik. The current in the load will be (Uin - Ube) / Rn. In this case, we will assume that Ube is known and is always equal to 0.6V.

It follows that the collector current Ik = (Uin - Ube) / Rn depends only on the input voltage and load resistance. The load resistance can be changed over a wide range, however, it is not necessary to be particularly zealous. After all, if instead of Rn you put a nail - a hundredth, then no transistor will survive!

The OK circuit makes it quite easy to measure the static current transfer coefficient h21e. How to do this is shown in Figure 10.

Figure 10.

First, measure the load current as shown in Figure 10a. In this case, the base of the transistor does not need to be connected anywhere, as shown in the figure. After that, the base current is measured in accordance with Figure 10b. Measurements should in both cases be made in the same quantities: either in amperes or in milliamps. The power supply voltage and load must remain the same for both measurements. To find out the static current transfer coefficient, it is enough to divide the load current by the base current: h21e ≈ In / Ib.

It should be noted that with an increase in the load current, h21e somewhat decreases, and with an increase in the supply voltage, it increases. Emitter followers are often built in a push-pull configuration using complementary pairs of transistors to increase the output power of the device. Such an emitter follower is shown in Figure 11.

Figure 11.

Figure 12.

The inclusion of transistors according to the scheme with a common base ABOUT

Such a circuit provides only voltage gain, but has better frequency properties compared to the OE circuit: the same transistors can operate at higher frequencies. The main application of the OB circuit is antenna amplifiers of the UHF ranges. Scheme antenna amplifier shown in figure 12.

Electronics surrounds us everywhere. But almost no one thinks about how this whole thing works. In fact, everything is quite simple. That is what we will try to show today. And let's start with such an important element as a transistor. We will tell you what it is, what it does, and how a transistor works.

What is a transistor?

Transistor- a semiconductor device designed to control electric current.

Where are transistors used? Yes, everywhere! Virtually no modern technology can do without transistors. circuit diagram. They are widely used in the production of computer technology, audio and video equipment.

Times when Soviet microcircuits were the largest in the world, have passed, and the size of modern transistors is very small. So, the smallest of the devices have a size of the order of a nanometer!

Console nano denotes a magnitude of the order of ten to the minus ninth power.

However, there are giant specimens that are used mainly in the fields of energy and industry.

Exist different types transistors: bipolar and polar, direct and reverse conduction. However, the operation of these devices is based on the same principle. A transistor is a semiconductor device. As is known, charge carriers in a semiconductor are electrons or holes.

The region with an excess of electrons is denoted by the letter n(negative), and the region with hole conductivity p(positive).

How does a transistor work?

To make everything very clear, consider the work bipolar transistor (the most popular type).

(hereinafter referred to as simply a transistor) is a semiconductor crystal (most often used silicon or germanium), divided into three zones with different electrical conductivity. Zones are named accordingly collector, base and emitter. The transistor device and its schematic representation are shown in the figure below.

Separate transistors of direct and reverse conductivity. P-n-p transistors are called forward-conducting transistors, and NPN transistors- from the reverse.

Now about what are the two modes of operation of transistors. The very operation of the transistor is similar to the operation of a water tap or valve. Only instead of water - electricity. Two states of the transistor are possible - working (transistor open) and resting state (transistor closed).

What does it mean? When the transistor is closed, no current flows through it. In the open state, when a small control current is applied to the base, the transistor opens, and a large current begins to flow through the emitter-collector.

Physical processes in a transistor

And now more about why everything happens this way, that is, why the transistor opens and closes. Let's take a bipolar transistor. Let it be n-p-n transistor.

If you connect a power supply between the collector and emitter, the collector electrons will begin to be attracted to positive, but there will be no current between the collector and emitter. This is prevented by the base layer and the emitter layer itself.

If, however, an additional source is connected between the base and the emitter, electrons from the n region of the emitter will begin to penetrate into the region of the bases. As a result, the base region will be enriched with free electrons, some of which will recombine with holes, some will flow to the plus of the base, and some (most) will go to the collector.

Thus, the transistor turns open, and the emitter-collector current flows in it. If the base voltage is increased, the collector-emitter current will also increase. Moreover, with a small change in the control voltage, a significant increase in the current through the collector-emitter is observed. It is on this effect that the operation of transistors in amplifiers is based.

That's the whole point of how transistors work in a nutshell. You need to calculate the power amplifier on bipolar transistors overnight, or perform laboratory work to study the operation of the transistor? This is not a problem even for a beginner, if you use the help of our student service specialists.

Feel free to seek professional help with important matters like studying! And now that you already have an idea about transistors, we invite you to relax and watch the video of the Korn band “Twisted transistor”! For example, you decide to buy a practice report, contact the Correspondence Book.

In all experiments, transistors KT315B, diodes D9B, miniature incandescent lamps 2.5V x 0.068A are used. Headphones - high-resistance type TON-2. Variable capacitor - any, with a capacity of 15 ... 180 pF. The power supply battery consists of two 4.5V 3R12 batteries connected in series. The lamps can be replaced with series-connected LED type AL307A and a resistor with a nominal value of 1 kOhm.

EXPERIMENT 1
ELECTRICAL DIAGRAM (conductors, semiconductors and insulators)

Electric current is the directed movement of electrons from one pole to another under the influence of voltage (9 V battery).

All electrons have the same negative charge. Atoms of various substances have different number electrons. Most electrons are firmly bound to atoms, but there are also so-called "free", or valence, electrons. If a voltage is applied to the ends of the conductor, then free electrons will begin to move towards the positive pole of the battery.

In some materials, the movement of electrons is relatively free, they are called conductors; in others, movement is difficult, they are called semiconductors; thirdly, it is generally impossible; such materials are called insulators, or dielectrics.

Metals are good conductors current. Substances such as mica, porcelain, glass, silk, paper, cotton are insulators.

Semiconductors include germanium, silicon, etc. These substances become conductors when certain conditions. This property is used in the production of semiconductor devices - diodes, transistors.

Rice. 1. Determination of water conductivity

This experiment demonstrates the operation of a simple electrical circuit and the difference in conductance between conductors, semiconductors, and dielectrics.

Assemble the circuit as shown in Fig. 1, and bring the bare ends of the wires to the front of the board. Connect the bare ends together, the bulb will light up. This indicates that an electric current is passing through the circuit.

With two wires, you can test the conductivity of various materials. To accurately determine the conductivity of certain materials, special instruments are needed. (By the brightness of the lamp, one can only determine whether the material under study is a good or bad conductor.)

Attach the bare ends of the two conductors to a piece of dry wood at a short distance from each other. The light bulb will not light. This means that dry wood is a dielectric. If the bare ends of two conductors are attached to aluminum, copper or steel, the light bulb will burn. This suggests that metals are good conductors of electric current.

Dip the bare ends of the conductors into a glass of tap water (Fig. 1, a). The lamp does not light. This means that water is a poor conductor of current. If you add a little salt to the water and repeat the experiment (Fig. 1, b), the bulb will light up, which indicates the flow of current in the circuit.

The 56 ohm resistor in this circuit and in all subsequent experiments serves to limit the current in the circuit.

EXPERIMENT 2
DIODE ACTION

The purpose of this experiment is to demonstrate that a diode conducts well in one direction and does not conduct in the opposite direction.

Assemble the circuit as shown in Fig. 2, a. The lamp will be on. Rotate the diode by 180° (Fig. 2, b). The light bulb will not light.

And now let's try to understand the physical essence of the experiment.

Rice. 2. The action of a semiconductor diode in an electronic circuit.

The semiconductor substances germanium and silicon each have four free, or valence, electrons. Semiconductor atoms bind into dense crystals (crystal lattice) (Fig. 3, a).

Rice. 3. Crystal lattice of semiconductors.

If an impurity is introduced into a semiconductor having four valence electrons, for example, arsenic, which has five valence electrons (Fig. 3, b), then the fifth electron in the crystal will be free. Such impurities provide electronic conductivity, or n-type conductivity.

Impurities having a lower valency than semiconductor atoms have the ability to attach electrons to themselves; such impurities provide hole or p-type conductivity (Fig. 3c).

Rice. 4. pn junctions in a semiconductor diode.

A semiconductor diode consists of a junction of p- and n-type materials (p-n-junction) (Fig. 4, a). Depending on the polarity of the applied voltage, the p-n junction can either promote (Fig. 4, d) or prevent (Fig. 4, c) the passage of electric current. At the boundary of two semiconductors, even before the application of an external voltage, a binary electric layer is created with a local electric field of strength E 0 (Fig. 4, b).

If you pass through the diode alternating current, then the diode will pass only the positive half-wave (Fig. 4 d), and the negative one will not pass (see Fig. 4, c). The diode thus converts or "rectifies" the AC to DC.

EXPERIMENT 3
HOW A TRANSISTOR WORKS

This experiment clearly demonstrates the main function of the transistor, which is a current amplifier. A small drive current in the base circuit can cause a large current in the emitter-collector circuit. By changing the resistance of the base resistor, you can change the collector current.

Assemble the circuit (Fig. 5). Put resistors into the circuit in turn: 1 MΩ, 470 kΩ, 100 kΩ, 22 kΩ, 10 kΩ. You may notice that with 1 MΩ and 470 kΩ resistors, the light does not light; 100 kOhm - the light bulb barely burns; 22 kOhm - the light bulb burns brighter; full brightness is observed when a 10 kΩ base resistor is connected.

Rice. 6. Transistor with n-p-n structure.

Rice. 7. Transistor with p-n-p structure.

A transistor is essentially two semiconductor diodes that have one common area - the base. If, in this case, the region with p-conductivity turns out to be common, then a transistor with an n-p-n structure will be obtained (Fig. 6); if the common area is with n-conductivity, then the transistor will be with the p-n-p structure (Fig. 7).

The area of the transistor that emits (emigrates) current carriers is called the emitter; the area that collects current carriers is called the collector. The zone enclosed between these areas is called the base. The transition between the emitter and the base is called the emitter, and between the base and the collector - the collector.

On fig. 5 shows the inclusion of an n-p-n type transistor in an electrical circuit.

When included in the circuit of a transistor of the type p-n-p polarity switching on battery B is reversed.

For currents flowing through a transistor, there is a dependence

I e \u003d I b + I to

Transistors are characterized by a current gain, denoted by the letter β, which is the ratio of the increase in collector current to the change in base current.

The value of β ranges from several tens to several hundreds of units, depending on the type of transistor.

EXPERIMENT 4
PROPERTIES OF THE CAPACITOR

By studying the principle of operation of a transistor, you can demonstrate the properties of a capacitor. Assemble the circuit (Figure 8), but do not attach the 100uF electrolytic capacitor. Then connect it for a while to position A (Fig. 8, a). The lamp will turn on and off. This indicates that a capacitor charge current was flowing in the circuit. Now place the capacitor in position B (Fig. 8, b), while not touching the terminals with your hands, otherwise the capacitor may be discharged. The lamp will light up and go out, the capacitor has discharged. Now place the capacitor again in position A. It has been charged. Lay the capacitor aside for a while (10 seconds) on the insulating material, then place it in position B. The light will turn on and off. From this experiment it can be seen that the capacitor is able to accumulate and store electric charge for a long time. The accumulated charge depends on the capacitance of the capacitor.

Rice. 8. Scheme explaining the principle of the capacitor.

Rice. 9. Change in voltage and current on the capacitor over time.

Charge the capacitor by setting it to position A, then discharge it by connecting conductors with bare ends to the capacitor terminals (hold the conductor by the insulated part!), And place it in position B. The light will not light up. As can be seen from this experiment, a charged capacitor acts as a power source (battery) in the base circuit, but after use electric charge the light bulb goes out. On fig. 9 shows the dependences on time: capacitor charge voltage; charge current flowing in the circuit.

EXPERIMENT 5
TRANSISTOR AS A SWITCH

Assemble the circuit according to fig. 10, but do not install resistor R1 and transistor T1 into the circuit yet. Key B must be connected to the circuit at points A and E so that the connection point of resistors R3, R1 can be closed to a common wire (negative bus of the printed circuit board).

Rice. 10. The transistor in the circuit works like a switch.

Connect the battery, the lamp in the T2 collector circuit will be on. Now close the circuit with switch B. The light will go out, as the switch connects point A to the negative bus, thereby reducing the potential of point A, and therefore the potential of the base T2. If the switch is returned to its original position, the light will turn on. Now disconnect the battery and connect T1, do not connect the resistor R1. Connect the battery, the light will turn on again. As in the first case, the transistor T1 is open and an electric current passes through it. Now put a resistor R1 (470 kOhm) at points C and D. The light will go out. Remove the resistor and the bulb will light up again.

When the voltage at the collector T1 drops to zero (when a 470 kΩ resistor is installed), the transistor opens. The base of the transistor T2 is connected through T1 to the negative bus, and T2 is closed. The lamp goes out. Thus, the transistor T1 acts as a switch.

In previous experiments, the transistor was used as an amplifier, now it is used as a switch.

The possibilities of using a transistor as a key (switch) are given in experiments 6, 7.

EXPERIMENT 6
ALARM

A feature of this circuit is that the transistor T1, used as a key, is controlled by a photoresistor R2.

The photoresistor included in this kit changes its resistance from 2 kOhm in strong light to several hundred kOhm in the dark.

Assemble the circuit according to fig. 11. Depending on the lighting of the room where you are conducting the experiment, select the resistor R1 so that the bulb burns normally without dimming the photoresistor.

Rice. 11. Scheme alarm based on a photoresistor.

The state of the transistor T1 is determined by a voltage divider consisting of a resistor R1 and a photoresistor R2.

If the photoresistor is illuminated, its resistance is low, transistor T1 is closed, there is no current in its collector circuit. The state of the transistor T2 is determined by applying a positive potential by resistors R3 and R4 to the base of T2. Consequently, the transistor T2 opens, the collector current flows, the light is on.

When the photoresistor is darkened, its resistance increases greatly and reaches a value when the divider supplies voltage to the T1 base, sufficient to open it. The voltage at the collector T1 drops to almost zero, through the resistor R4 it closes the transistor T2, the light goes out.

In practice, in such circuits, other actuators (bell, relay, etc.) can be installed in the collector circuit of the transistor T2.

In this and subsequent circuits, a photoresistor of the SF2-9 type or similar can be used.

EXPERIMENT 7
AUTOMATIC LIGHT SWITCH

In contrast to experiment 6, in this experiment when the photoresistor R1 is dimmed, the light is on (Fig. 12).

Rice. 12. Scheme that turns on the light automatically.

When light hits the photoresistor, its resistance decreases greatly, which leads to the opening of the transistor T1, and consequently, to the closing of T2. The lamp does not light.

In the dark, the light turns on automatically.

This property can be used to turn lamps on and off depending on the amount of light.

EXPERIMENT 8
SIGNAL DEVICE

A distinctive feature of this scheme is its high sensitivity. In this and a number of subsequent experiments, a combined connection of transistors (composite transistor) is used (Fig. 13).

Rice. 13. Optoelectronic signaling device.

The principle of operation of this scheme does not differ from the scheme. At a certain resistance value of the resistors R1 + R2 and the resistance of the photoresistor R3, a current flows in the base circuit of the transistor T1. A current also flows in the collector circuit T1, but (3 times the current of the base T1. Let's assume that (β \u003d 100. All the current going through the emitter T1 must pass through the emitter-base T2 junction. Then the collector current T2 is β times more than the collector current T1, the collector current T1 is β times the base current T1, the collector current T2 is approximately 10,000 times the base current T1. Thus, the composite transistor can be considered as a single transistor with a very high gain and high sensitivity. of the composite transistor is that the transistor T2 must be powerful enough, while the transistor T1 controlling it can be low-power, since the current passing through it is 100 times less than the current passing through T2.

The performance of the circuit shown in Fig. 13 is determined by the illumination of the room where the experiment is being carried out, therefore it is important to select the resistance R1 of the divider of the upper arm so that the lamp does not burn in the illuminated room, but burns when the photoresistor is darkened by hand, the room is darkened with curtains, or when the light is turned off if the experiment is carried out in the evening.

EXPERIMENT 9
HUMIDITY SENSOR

In this circuit (Fig. 14), a compound transistor with high sensitivity is also used to determine the moisture content of the material. Base bias T1 is provided by resistor R1 and two bare-ended conductors.

Check the electrical circuit by lightly squeezing the bare ends of the two conductors with the fingers of both hands, without connecting them to each other. The resistance of the fingers is enough to trigger the circuit, and the bulb lights up.

Rice. 14. Scheme of the humidity sensor. The bare ends of the conductors penetrate the blotting paper.

Now pass the bare ends through blotting paper at a distance of about 1.5-2 cm, attach the other ends to the diagram according to fig. 14. Then moisten the blotting paper between the wires with water. The light bulb lights up (In this case, the decrease in resistance occurred due to the dissolution of salts in the paper with water.).

If the blotting paper is impregnated with saline, then dried and the experiment repeated, the efficiency of the experiment increases, the ends of the conductors can be separated to a greater distance.

EXPERIMENT 10
SIGNAL DEVICE

This scheme is similar to the previous one, the only difference is that the lamp lights up when the photoresistor is illuminated and goes out when darkened (Fig. 15).

Rice. 15. Signaling device on a photoresistor.

The circuit works as follows: with normal illumination of the photoresistor R1, the bulb will light up, since the resistance R1 is low, the transistor T1 is open. When the light is turned off, the lamp will turn off. The light of a flashlight or lit matches will cause the light bulb to burn again. The sensitivity of the circuit is adjusted by increasing or decreasing the resistance of the resistor R2.

EXPERIMENT 11
PRODUCT COUNTER

This experiment should be carried out in a semi-dark room. All the time when light falls on the photoresistor, indicator light L2 is on. If you place a piece of cardboard between the light source (light bulb L1 and photoresistor, light bulb L2 goes out. If you remove the cardboard, light bulb L2 lights up again (Fig. 16).

Rice. 16. Product counter.

In order for the experiment to be successful, it is necessary to adjust the circuit, i.e., select the resistance of the resistor R3 (the most suitable in this case is 470 ohms).

This scheme can practically be used to count a batch of products on a conveyor. If the light source and photoresistor are placed in such a way that a batch of products passes between them, the circuit turns on and off, as the light flow is interrupted by passing products. Instead of the L2 indicator light, a special counter is used.

EXPERIMENT 12
SIGNAL TRANSMISSION USING LIGHT

Rice. 23. Frequency divider on transistors.

Transistors T1 and T2 open in turn. The control signal is sent to the flip-flop. When transistor T2 is open, light L1 is off. Light bulb L2 lights up when transistor T3 is open. But transistors T3 and T4 open and close in turn, therefore, the light bulb L2 lights up with every second control signal sent by the multivibrator. Thus, the burning frequency of the light bulb L2 is 2 times less than the burning frequency of the light bulb L1.

This property can be used in an electric organ: the frequencies of all notes of the upper octave of the organ are divided in half and a tone is created an octave lower. The process may be repeated.

EXPERIMENT 18
SCHEME "AND" BY UNITS

In this experiment, the transistor is used as the key and the light bulb is the output indicator (Figure 24).

This circuit is logical. The bulb will light up if there is a high potential at the base of the transistor (point C).

Suppose points A and B are not connected to the negative bus, they have a high potential, therefore, there is also a high potential at point C, the transistor is open, the light is on.

Rice. 24. Logic element 2And on the transistor.

We accept conditionally: high potential - logical "1" - the light is on; low potential - logical "0" - the light is off.

Thus, if there are logical "1" at points A and B, there will also be "1" at point C.

Now let's connect point A to the negative bus. Its potential will become low (drop to "0" V). Point B has a high potential. Through the circuit R3 - D1 - the battery will flow current. Therefore, at point C there will be a low potential or "0". The transistor is closed, the light is off.

Let's connect point B to the ground. The current now flows through the circuit R3 - D2 - battery. The potential at point C is low, the transistor is closed, the light is off.

If both points are connected to ground, there will also be a low potential at point C.

Similar circuits can be used in an electronic examiner and other logic circuits, where the output signal will be only if there are simultaneous signals in two or more input channels.

Possible circuit states are shown in the table.

Truth table of the AND circuit

EXPERIMENT 19
SCHEME "OR" BY UNITS

This scheme is the opposite of the previous one. In order for there to be “0” at point C, it is necessary that there is also “0” at points A and B, that is, points A and B must be connected to the negative bus. In this case, the transistor will close, the light will go out (Fig. 25).

If now only one of the points, A or B, is connected to the negative bus, then at point C there will still be a high level, i.e. "1", the transistor is open, the light is on.

Rice. 25. Logic element 2OR on the transistor.

When connecting point B to the negative bus, current will flow through R2, D1 and R3. No current will flow through diode D2, since it is turned on in the opposite direction for conductivity. At point C, there will be about 9 V. The transistor is open, the light is on.

Now let's connect point A to the negative bus. The current will go through R1, D2, R3. The voltage at point C will be about 9 V, the transistor is open, the light is on.

OR circuit truth table

EXPERIMENT 20
"NOT" CIRCUIT (INVERTER)

This experiment demonstrates the operation of a transistor as an inverter - a device that can change the polarity of the output signal relative to the input to the opposite. In the experiments, the transistor was not part of the existing logic circuits, it only served to turn on the light bulb. If point A is connected to a negative bus, then its potential will drop to “0”, the transistor will close, the light will go out, at point B there is a high potential. This means a logical "1" (Fig. 26).

Rice. 26. The transistor works like an inverter.

If point A is not connected to the negative bus, that is, at point A - "1", then the transistor is open, the light is on, the voltage at point B is close to "0" or this is logical "0".

In this experiment, the transistor is integral part logic circuit and can be used to convert an OR circuit to a NOR circuit and an AND circuit to a NAND circuit.

NOT circuit truth table

EXPERIMENT 21
SCHEME "AND-NOT"

This experiment combines two experiments: 18 - scheme AND and 20 - scheme NOT (Fig. 27).

This circuit functions similarly to the circuit, forming "1" or "0" based on the transistor.

Rice. 27. Logic element 2I-NOT on a transistor.

The transistor is used as an inverter. If "1" appears on the base of the transistor, then the output point is "0" and vice versa.

If the potentials at point D are compared with the potentials at point C, it can be seen that they are inverted.

Truth table of the NAND circuit

EXPERIMENT 22
SCHEME "OR-NOT"

This experiment combines two experiments: - the OR circuit and - the NOT circuit (Fig. 28).

Rice. 28. Logic element 2OR-NOT on the transistor.

The circuit functions in exactly the same way as in experiment 20 (a “0” or “1” is generated based on the transistor). The only difference is that the transistor is used as an inverter: if “1” is at the input of the transistor, then “0” is at its output and vice versa.

Truth table of NOR circuit

EXPERIMENT 23
SCHEME "AND-NOT", ASSEMBLED ON TRANSISTORS

This circuit consists of two NOT logic circuits, the transistor collectors of which are connected at point C (Fig. 29).

If both points, A and B, are connected to the negative bus, then their potentials will become equal to "0". The transistors will close, there will be a high potential at point C, the bulb will not light.

Rice. 29. Logic element 2I-NOT.

If only point A is connected to the negative bus, at point B logical "1", T1 is closed, and T2 is open, collector current flows, the light is on, at point C logical "0".

If point B is connected to the negative bus, then the output will also be “0”, the light will be on, in this case T1 is open, T2 is closed.

And finally, if points A and B are logic "1" (not connected to the negative bus), both transistors are open. On their collectors "0", the current flows through both transistors, the light is on.

Truth table of the NAND circuit

EXPERIMENT 24
PHONE SENSOR AND AMPLIFIER

In the experimental scheme, both transistors are used as an amplifier sound signals(Fig. 30).

Rice. 30. Inductive phone sensor.

The signals are picked up and fed to the base of the transistor T1 with the help of an inductive coil L, then they are amplified and fed into the phone. When you have finished assembling the circuit on the board, position the ferrite rod near the phone, perpendicular to the incoming wires. Speech will be heard.

In this scheme and in the future, a ferrite rod with a diameter of 8 mm and a length of 100-160 mm, brand 600NN, is used as an inductive coil L. The winding contains approximately 110 turns of copper insulated wire with a diameter of 0.15..0.3 mm, type PEL or PEV.

EXPERIMENT 25
MICROPHONE AMPLIFIER

If an extra telephone is available (Figure 31), it can be used in place of the inductor in the previous experiment. As a result, we will have a sensitive microphone amplifier.

Rice. 31. Microphone amplifier.

Within assembled circuit you can get a semblance of a two-way communication device. Phone 1 can be used as the receiving device (connection at point A) and phone 2 as the output device (connection at point B). In this case, the second ends of both telephones must be connected to the negative bus.

EXPERIMENT 26
AMPLIFIER FOR PLAYER

With the help of a gramophone amplifier (Fig. 32), you can listen to recordings without disturbing the peace of those around you.

The circuit consists of two audio amplification stages. The input signal is the signal coming from the pickup.

Rice. 32. Amplifier for the player.

In the diagram, the letter A indicates the sensor. This sensor and capacitor C2 are a capacitive voltage divider to reduce the initial volume. Trimmer capacitor C3 and capacitor C4 are the secondary voltage divider. C3 controls the volume.

EXPERIMENT 27
"ELECTRONIC VIOLIN"

Here the multivibrator circuit is for making electronic music. The scheme is similar. The main difference is that the base bias resistor of transistor T1 is variable. A 22 kΩ resistor (R2) connected in series with a variable resistor provides the minimum base bias resistance T1 (Fig. 33).

Rice. 33. Multivibrator for creating music.

EXPERIMENT 28
FLASHING MORSE BUZZER

In this circuit, the multivibrator is designed to generate pulses with tone frequency. The lamp lights up when the circuit is powered on (Fig. 34).

The phone in this circuit is connected to the circuit between the collector of the transistor T2 through the capacitor C4 and the negative bus of the board.

Rice. 34. Generator for learning Morse code.

With this scheme, you can practice learning Morse code.

If you are not satisfied with the tone of the sound, swap capacitors C2 and C1.

EXPERIMENT 29
METRONOME

A metronome is a device for setting the rhythm (tempo), for example, in music. For these purposes, a pendulum metronome was previously used, which gave both a visual and audible designation of the tempo.

In this scheme, these functions are performed by a multivibrator. The tempo frequency is approximately 0.5 s (Fig. 35).

Rice. 35. Metronome.

Thanks to the telephone and the indicator light, it is possible to hear and visually feel the set rhythm.

EXPERIMENT 30
AUTOMATIC ALARM DEVICE WITH AUTOMATIC RESET

This circuit (Fig. 36) demonstrates the use of a single vibrator, the operation of which is described in experiment 14. In the initial state, the transistor T1 is open, and T2 is closed. The phone is used as a microphone here. Whistling into the microphone (you can just blow on it) or light tapping excites an alternating current in the microphone circuit. Negative signals, arriving at the base of transistor T1, close it, and therefore open transistor T2, a current appears in the collector circuit T2, and the bulb lights up. At this time, the capacitor C1 is charged through the resistor R1. The voltage of the charged capacitor C2 is sufficient to open the transistor T1, i.e., the circuit returns to its original state spontaneously, while the light goes out. The burning time of the bulb is about 4 s. If the capacitors C2 and C1 are interchanged, then the burning time of the bulb will increase to 30 s. If the resistor R4 (1 kOhm) is replaced by 470 kOhm, then the time will increase from 4 to 12 s.

Rice. 36. Acoustic signaling device.

This experiment can be presented as a trick that can be shown in a circle of friends. To do this, you need to remove one of the microphones of the phone and put it under the board near the light bulb so that the hole in the board coincides with the center of the microphone. Now, if you blow on the hole in the board, it will seem that you are blowing on a light bulb and therefore it lights up.

EXPERIMENT 31
BUZZER WITH MANUAL RESET

This circuit (Fig. 37) is similar in principle to the previous one, with the only difference that when switching, the circuit does not automatically return to the initial state, and this is done using switch B.

Rice. 37. Acoustic signaling device with manual reset.

The state of readiness of the circuit or the initial state will be when the transistor T1 is open, T2 is closed, the lamp is off.

A light whistle into the microphone gives a signal that turns off transistor T1, while opening transistor T2. The signal lamp lights up. It will burn until transistor T2 closes. To do this, it is necessary to short-circuit the base of the transistor T2 to the negative bus (“ground”) using key B. Other actuators, such as relays, can be connected to similar circuits.

EXPERIMENT 32
SIMPLE DETECTOR RECEIVER

For a beginner radio amateur, the design of radio receivers should begin with the simplest structures, for example, with a detector receiver, the diagram of which is shown in Fig. 38.

The detector receiver works as follows: electromagnetic waves sent on the air by radio stations, crossing the receiver antenna, induce voltage in it with a frequency corresponding to the frequency of the radio station signal. The induced voltage enters the input circuit L, C1. In other words, this circuit is called resonant, as it is pre-tuned to the frequency of the desired radio station. In the resonant circuit, the input signal is amplified tenfold and then fed to the detector.

Rice. 38. Detector receiver.

The detector is assembled on a semiconductor diode, which serves to rectify the modulated signal. The low frequency (audio) component will pass through the headphones and you will hear speech or music, depending on the transmission of that radio station. The high-frequency component of the detected signal, bypassing the headphones, will pass through the capacitor C2 to the ground. The capacitance of capacitor C2 determines the degree of filtering of the high-frequency component of the detected signal. Usually, the capacitance of the capacitor C2 is chosen in such a way that it represents a large resistance for audio frequencies, and its resistance is low for the high-frequency component.

As a capacitor C1, you can use any small-sized capacitor variable capacity with measurement limits 10...200 pF. AT this constructor to tune the circuit, a ceramic tuning capacitor of the KPK-2 type with a capacity of 25 to 150 pF is used.

The inductor L has the following parameters: number of turns - 110 ± 10, wire diameter - 0.15 mm, type - PEV-2, frame diameter of insulating material - 8.5 mm.

ANTENNA

A properly assembled receiver starts working immediately when an external antenna is connected to it, which is a piece of copper wire 0.35 mm in diameter, 15-20 m long, suspended on insulators at a certain height above the ground. The higher the antenna is above the ground, the better the reception of radio signals will be.

GROUNDING

The reception volume increases if ground is connected to the receiver. The ground wire should be short and have little resistance. Its end is connected to a copper pipe going deep into the ground.

EXPERIMENT 33
DETECTOR RECEIVER WITH LOW FREQUENCY AMPLIFIER

This circuit (Fig. 39) is similar to the previous detector receiver circuit, with the only difference being that the simplest amplifier low frequency, assembled on a transistor T. The low frequency amplifier serves to increase the power of the signals detected by the diode. Tuning diagram oscillatory circuit is connected to the diode through capacitor C2 (0.1 uF), and resistor R1 (100 kOhm) provides the diode with a constant bias.

Rice. 39. Detector receiver with a single-stage ULF.

For normal operation of the transistor, a 9 V power supply is used. Resistor R2 is necessary in order to provide voltage to the base of the transistor to create the necessary mode of its operation.

For this circuit, as in the previous experiment, an external antenna and ground are required.

EXPERIMENT 34

SIMPLE TRANSISTOR RECEIVER

The receiver (Fig. 40) differs from the previous one in that instead of diode D, a transistor is installed, which simultaneously works both as a detector of high-frequency oscillations and as a low-frequency amplifier.

Rice. 40. Single transistor receiver.

The detection of a high-frequency signal in this receiver is carried out at the base-emitter section, therefore, such a receiver does not require a special detector (diode). The transistor with an oscillating circuit is connected, as in the previous circuit, through a 0.1 μF capacitor and is decoupling. Capacitor C3 serves to filter the high-frequency component of the signal, which is also amplified by the transistor.

EXPERIMENT 35
REGENERATIVE RECEIVER

In this receiver (Fig. 41), regeneration is used to improve the sensitivity and selectivity of the circuit. This role is performed by the coil L2. The transistor in this circuit is turned on a little differently than in the previous one. The signal voltage from the input circuit is fed to the base of the transistor. The transistor detects and amplifies the signal. The high-frequency component of the signal does not immediately enter the filter capacitor C3, but first passes through the feedback winding L2, which is located on the same core with the loop coil L1. Due to the fact that the coils are placed on the same core, there is an inductive connection between them, and part of the amplified voltage of the high-frequency signal from the collector circuit of the transistor again enters the input circuit of the receiver. With the correct connection of the ends of the coupling coil L2, the feedback voltage supplied to the L1 circuit due to inductive coupling coincides in phase with the signal coming from the antenna, and the signal increases, as it were. This increases the sensitivity of the receiver. However, with a large inductive coupling, such a receiver can turn into a undamped oscillation generator, and a sharp whistle is heard in telephones. To eliminate excessive excitation, it is necessary to reduce the degree of coupling between the coils L1 and L2. This is achieved either by removing the coils from each other, or by reducing the number of turns of the L2 coil.

Rice. 41. Regenerative receiver.

It may happen that the feedback does not give the desired effect and the reception of stations that were well audible earlier, when the feedback is introduced, stops altogether. This suggests that instead of a positive feedback, a negative one has formed and the ends of the L2 coil need to be swapped.

At short distances from the radio station, the described receiver works well without external antenna, per magnetic antenna.

If the audibility of the radio station is low, you still need to connect an external antenna to the receiver.

The receiver with one ferrite antenna must be installed so that the electromagnetic waves coming from the radio station create the largest signal in the coil of the oscillatory circuit. Thus, when you have tuned in to the signal of the radio station with the help of a variable capacitor, if the audibility is poor, turn the circuit to receive signals in the phones at the volume you need.

EXPERIMENT 36
TWO-TRANSISTOR REGENERATIVE RECEIVER

This circuit (Fig. 42) differs from the previous one in that it uses a low-frequency amplifier assembled on T2 transistors.

With the help of a two-transistor regenerative receiver, you can receive a large number of radio stations.

Rice. 42. Regenerative receiver with a low frequency amplifier.

Although this kit (set no. 2) only has a long wave coil, the circuit can operate on both medium and short waves when using the appropriate tuning coils. You can make them yourself.

EXPERIMENT 37
"DIRECTION FINDER"

The scheme of this experiment is similar to the scheme of experiment 36 without antenna and ground.

Tune in to a powerful radio station. Take the board in your hands (it should be horizontal) and rotate until the sound (signal) disappears or at least decreases to a minimum. In this position, the axis of the ferrite points exactly to the transmitter. If you now turn the board 90°, the signals will be clearly audible. But more precisely, the location of the radio station can be determined by the graph-mathematical method, using a compass to determine the angle in azimuth.

To do this, you need to know the direction of the transmitter from different positions - A and B (Fig. 43, a).

Suppose we are at point A, we determined the direction of the transmitter, it is 60 °. Now let's move to point B, while measuring the distance AB. Let's determine the second direction of the transmitter location, it is 30°. The intersection of the two directions is the location of the transmitting station.

Rice. 43. Scheme of the direction finding of the radio station.

If you have a map with a location on it broadcasting stations, that is, the ability to accurately determine your location.

Tune into station A, let it be at a 45° angle, and then tune into station B; its azimuth is, say, 90°. Given these angles, draw lines on the map through points A and B, their intersection will give your location (Fig. 43, b).

In the same way, ships and planes orient themselves in the process of movement.

CHAIN CONTROL

In order for the circuits to work reliably during experiments, you need to make sure that the battery is charged, all connections are clean, and all nuts are securely screwed. The battery leads must be properly connected; when connecting, it is necessary to strictly observe the polarity of electrolytic capacitors and diodes.

COMPONENT CHECK

Diodes can be tested in; transistors - in; electrolytic capacitors (10 and 100 microfarads) - c. The head phone can also be checked by connecting it to the battery - a “crackle” will be heard in the earpiece.

Just about the complex. Programs. Iron. Internet. Windows

Eight simple transistor circuits for beginner radio amateurs. Cognitive experiments with transistors For beginner radio amateurs about the boundary voltage of a transistor

Single stage AF amplifier

Two-stage ultrasonic frequency converter on transistors of different structures

Two-stage ultrasonic frequency converter on transistors of the same structure

Two-level voltage indicator

Schmitt trigger

Waiting multivibrator

What is a transistor?

How does a transistor work?

Physical processes in a transistor

EXPERIMENT 1 ELECTRICAL DIAGRAM (conductors, semiconductors and insulators)

EXPERIMENT 2 DIODE ACTION

EXPERIMENT 3 HOW A TRANSISTOR WORKS

EXPERIMENT 4 PROPERTIES OF THE CAPACITOR

EXPERIMENT 5 TRANSISTOR AS A SWITCH

EXPERIMENT 6 ALARM

EXPERIMENT 7 AUTOMATIC LIGHT SWITCH

EXPERIMENT 8 SIGNAL DEVICE

EXPERIMENT 9 HUMIDITY SENSOR

EXPERIMENT 10 SIGNAL DEVICE

EXPERIMENT 11 PRODUCT COUNTER

EXPERIMENT 12 SIGNAL TRANSMISSION USING LIGHT

EXPERIMENT 18 SCHEME "AND" BY UNITS

EXPERIMENT 19 SCHEME "OR" BY UNITS

EXPERIMENT 20 "NOT" CIRCUIT (INVERTER)

EXPERIMENT 21 SCHEME "AND-NOT"

EXPERIMENT 22 SCHEME "OR-NOT"

EXPERIMENT 23 SCHEME "AND-NOT", ASSEMBLED ON TRANSISTORS

EXPERIMENT 24 PHONE SENSOR AND AMPLIFIER

EXPERIMENT 25 MICROPHONE AMPLIFIER

EXPERIMENT 26 AMPLIFIER FOR PLAYER

EXPERIMENT 27 "ELECTRONIC VIOLIN"

EXPERIMENT 28 FLASHING MORSE BUZZER

EXPERIMENT 29 METRONOME

EXPERIMENT 30 AUTOMATIC ALARM DEVICE WITH AUTOMATIC RESET

EXPERIMENT 31 BUZZER WITH MANUAL RESET

EXPERIMENT 32 SIMPLE DETECTOR RECEIVER