The simplest transistor amplifier can be a good tool for studying the properties of devices. The schemes and designs are quite simple, you can independently manufacture the device and check its operation, measure all parameters. Thanks to modern field-effect transistors, it is possible to make a miniature microphone amplifier literally from three elements. And connect it to a personal computer to improve the sound recording parameters. And the interlocutors during conversations will hear your speech much better and more clearly.

Frequency characteristics

Low-frequency (sound) frequency amplifiers are available in almost all household appliances - music centers, televisions, radios, radios, and even personal computers. But there are also high-frequency amplifiers on transistors, lamps and microcircuits. Their difference is that ULF allows you to amplify the signal only audio frequency which is perceived by the human ear. Transistor audio amplifiers allow you to reproduce signals with frequencies in the range from 20 Hz to 20,000 Hz.

Therefore, even the simplest device is able to amplify the signal in this range. And it does it as evenly as possible. The gain depends directly on the frequency of the input signal. The graph of the dependence of these quantities is almost a straight line. If, on the other hand, a signal with a frequency outside the range is applied to the input of the amplifier, the quality of work and the efficiency of the device will quickly decrease. ULF cascades are assembled, as a rule, on transistors operating in the low and medium frequency ranges.

Classes of operation of audio amplifiers

All amplifying devices are divided into several classes, depending on what degree of current flow through the cascade during the period of operation:

1. Class "A" - the current flows non-stop during the entire period of operation of the amplifying stage.
2. In the class of work "B" current flows for half the period.
3. Class "AB" indicates that the current flows through the amplifying stage for a time equal to 50-100% of the period.
4. In "C" mode, the electric current flows for less than half of the operating time.
5. Mode "D" ULF has been used in amateur radio practice quite recently - a little over 50 years. In most cases, these devices are implemented on the basis of digital elements and have a very high efficiency - over 90%.

The presence of distortion in various classes of low-frequency amplifiers

The working area of ​​a class "A" transistor amplifier is characterized by rather small non-linear distortions. If the incoming signal emits pulses with more than high voltage, this causes the transistors to saturate. In the output signal, higher harmonics (up to 10 or 11) begin to appear near each harmonic. Because of this, a metallic sound, characteristic only for transistor amplifiers.

With an unstable power supply, the output signal will be modeled in amplitude near the mains frequency. The sound will be on the left frequency response more rigid. But the better the power stabilization of the amplifier, the more complex the design of the entire device becomes. ULF operating in class "A" have a relatively low efficiency - less than 20%. The reason is that the transistor is constantly on and current flows through it constantly.

To increase (albeit insignificant) efficiency, you can use push-pull circuits. One disadvantage is that the half-waves of the output signal become asymmetrical. If you transfer from class "A" to "AB", the non-linear distortion will increase by 3-4 times. But the efficiency of the entire circuit of the device will still increase. ULF classes "AB" and "B" characterizes the increase in distortion with a decrease in the signal level at the input. But even if you turn up the volume, it will not help to completely get rid of the shortcomings.

Work in intermediate classes

Each class has several varieties. For example, there is a class of amplifiers "A +". In it, the transistors at the input (low-voltage) operate in the "A" mode. But high-voltage, installed in the output stages, work either in "B" or in "AB". Such amplifiers are much more economical than those operating in class "A". A noticeably smaller number of non-linear distortions - no higher than 0.003%. Better results can be achieved using bipolar transistors. The principle of operation of amplifiers on these elements will be discussed below.

But still there are a large number of higher harmonics in the output signal, which makes the sound characteristic metallic. There are also amplifier circuits that work in the "AA" class. In them, non-linear distortion is even less - up to 0.0005%. But the main drawback of transistor amplifiers is still there - a characteristic metallic sound.

"Alternative" designs

It cannot be said that they are alternative, just some specialists involved in the design and assembly of amplifiers for high-quality sound reproduction are increasingly preferring tube designs. Tube amplifiers have the following advantages:

1. Highly low value the level of non-linear distortion in the output signal.
2. There are fewer higher harmonics than in transistor designs.

But there is one huge minus that outweighs all the advantages - you must definitely install a device for coordination. The fact is that the tube cascade has a very high resistance - several thousand ohms. But the speaker winding resistance is 8 or 4 ohms. To match them, you need to install a transformer.

Of course, this is not a very big drawback - there are also transistor devices that use transformers to match the output stage and the speaker system. Some experts argue that the most effective scheme is a hybrid - which uses single ended amplifiers not covered by negative feedback. Moreover, all these cascades operate in the ULF class "A" mode. In other words, a transistorized power amplifier is used as a repeater.

Moreover, the efficiency of such devices is quite high - about 50%. But you should not focus only on efficiency and power indicators - they do not speak of the high quality of sound reproduction by the amplifier. Much more important are the linearity of the characteristics and their quality. Therefore, you need to pay attention first of all to them, and not to power.

Scheme of a single-ended ULF on a transistor

The simplest amplifier, built according to the common emitter circuit, operates in class "A". The circuit uses a semiconductor element with an n-p-n structure. A resistance R3 is installed in the collector circuit, which limits the flowing current. The collector circuit is connected to the positive power wire, and the emitter circuit is connected to the negative. When using semiconductor transistors with p-n-p structure the circuit will be exactly the same, only you need to change the polarity.

With the help of a coupling capacitor C1, it is possible to separate the AC input signal from the DC source. In this case, the capacitor is not an obstacle to the flow alternating current along the base-emitter path. The internal resistance of the emitter-base junction, together with resistors R1 and R2, is the simplest supply voltage divider. Typically, resistor R2 has a resistance of 1-1.5 kOhm - the most typical values ​​\u200b\u200bfor such circuits. In this case, the supply voltage is divided exactly in half. And if you power the circuit with a voltage of 20 Volts, you can see that the value of the current gain h21 will be 150. It should be noted that HF ​​amplifiers on transistors are made according to similar circuits, only they work a little differently.

In this case, the emitter voltage is 9 V and the drop in the “E-B” circuit section is 0.7 V (which is typical for transistors based on silicon crystals). If we consider an amplifier based on germanium transistors, then in this case the voltage drop in the “EB” section will be 0.3 V. The current in the collector circuit will be equal to that which flows in the emitter. You can calculate by dividing the emitter voltage by the resistance R2 - 9V / 1 kOhm = 9 mA. To calculate the value of the base current, it is necessary to divide 9 mA by the gain h21 - 9mA / 150 \u003d 60 μA. ULF designs usually use bipolar transistors. The principle of its work is different from the field.

On the resistor R1, you can now calculate the drop value - this is the difference between the base and supply voltages. In this case, the base voltage can be found by the formula - the sum of the characteristics of the emitter and the "E-B" transition. When powered by a 20 Volt source: 20 - 9.7 \u003d 10.3. From here, you can calculate the resistance value R1 = 10.3V / 60 μA = 172 kOhm. The circuit contains capacitance C2, which is necessary for the implementation of the circuit through which the alternating component of the emitter current can pass.

If you do not install capacitor C2, the variable component will be very limited. Because of this, such a transistor audio amplifier will have a very low current gain h21. It is necessary to pay attention to the fact that in the above calculations the base and collector currents were assumed to be equal. Moreover, the base current was taken to be the one that flows into the circuit from the emitter. It occurs only when a bias voltage is applied to the output of the base of the transistor.

But it must be borne in mind that absolutely always, regardless of the presence of bias, the collector leakage current necessarily flows through the base circuit. In circuits with a common emitter, the leakage current is increased by at least 150 times. But usually this value is taken into account only when calculating amplifiers based on germanium transistors. In the case of using silicon, in which the current of the "K-B" circuit is very small, this value is simply neglected.

MIS transistor amplifiers

Amplifier on field effect transistors, shown in the diagram, has many analogues. Including using bipolar transistors. Therefore, we can consider as a similar example the design of a sound amplifier assembled according to a common emitter circuit. The photo shows a circuit made according to a circuit with a common source. R-C connections are assembled on the input and output circuits so that the device operates in the class “A” amplifier mode.

Alternating current from the signal source is separated from the DC supply voltage by capacitor C1. Be sure the field-effect transistor amplifier must have a gate potential that will be lower than that of the source. In the presented diagram, the gate is connected to a common wire through a resistor R1. Its resistance is very large - resistors of 100-1000 kOhm are usually used in designs. Such a large resistance is chosen so that the signal at the input is not shunted.

This resistance almost does not pass electric current, as a result of which the potential of the gate (in the absence of a signal at the input) is the same as that of the ground. At the source, the potential is higher than that of the ground, only due to the voltage drop across the resistance R2. From this it is clear that the potential of the gate is lower than that of the source. And that is exactly what is required for normal functioning transistor. It should be noted that C2 and R3 in this amplifier circuit have the same purpose as in the design discussed above. And the input signal is shifted relative to the output signal by 180 degrees.

ULF with output transformer

You can make such an amplifier with your own hands for home use. It is carried out according to the scheme that works in class "A". The design is the same as discussed above - with a common emitter. One feature - it is necessary to use a transformer for matching. This is a disadvantage of such a transistor audio amplifier.

The collector circuit of the transistor is loaded primary winding, which develops the output signal transmitted through the secondary to the speakers. A voltage divider is assembled on resistors R1 and R3, which allows you to select the operating point of the transistor. With the help of this circuit, a bias voltage is supplied to the base. All other components have the same purpose as the circuits discussed above.

push-pull audio amplifier

This is not to say that this is a simple transistor amplifier, since its operation is a little more complicated than that of those discussed earlier. In push-pull ULF, the input signal is split into two half-waves, different in phase. And each of these half-waves is amplified by its own cascade, made on a transistor. After each half-wave has been amplified, both signals are combined and sent to the speakers. Such complex conversions can cause signal distortion, since the dynamic and frequency properties of two, even of the same type, transistors will be different.

As a result, the sound quality at the output of the amplifier is significantly reduced. When a push-pull amplifier is operating in class “A”, it is not possible to reproduce qualitatively complex signal. The reason is that the increased current flows constantly through the arms of the amplifier, the half-waves are asymmetrical, and phase distortions occur. The sound becomes less intelligible, and when heated, signal distortion increases even more, especially at low and ultra-low frequencies.

Transformerless ULF

The low-frequency amplifier on a transistor, made using a transformer, despite the fact that the design may have small dimensions, is still imperfect. Transformers are still heavy and bulky, so it's best to get rid of them. A much more efficient circuit is made on complementary semiconductor elements with various types conductivity. Most of the modern ULFs are performed exactly according to such schemes and work in class "B".

Two powerful transistors, used in the design, work according to the scheme of the emitter follower (common collector). In this case, the input voltage is transmitted to the output without loss and amplification. If there is no signal at the input, then the transistors are on the verge of turning on, but still turned off. When a harmonic signal is applied to the input, the first transistor opens with a positive half-wave, and the second one is in the cutoff mode at this time.

Therefore, only positive half-waves can pass through the load. But negative ones open the second transistor and completely block the first one. In this case, only negative half-waves are in the load. As a result, the signal amplified in power is at the output of the device. Such a transistor amplifier circuit is quite effective and is able to provide stable work, high-quality sound reproduction.

ULF circuit on one transistor

Having studied all the above features, you can assemble an amplifier with your own hands on a simple element base. The transistor can be used domestic KT315 or any of it foreign analogue- for example BC107. As a load, you need to use headphones, the resistance of which is 2000-3000 ohms. A bias voltage must be applied to the base of the transistor through a 1 MΩ resistor and a 10 µF decoupling capacitor. The circuit can be powered from a source with a voltage of 4.5-9 Volts, current - 0.3-0.5 A.

If the resistance R1 is not connected, then there will be no current in the base and collector. But when connected, the voltage reaches a level of 0.7 V and allows a current of about 4 μA to flow. In this case, the current gain will be about 250. From here, you can make a simple calculation of the transistor amplifier and find out the collector current - it turns out to be 1 mA. Having assembled this transistor amplifier circuit, you can test it. Connect the load - headphones to the output.

Touch the input of the amplifier with your finger - a characteristic noise should appear. If it is not there, then most likely the design is assembled incorrectly. Recheck all connections and element ratings. To make the demonstration clearer, connect a sound source to the ULF input - the output from the player or phone. Listen to music and appreciate the sound quality.

Scheme No. 1

Amplifier class selection . We will immediately warn the radio amateur - we will not make a class A amplifier on transistors. The reason is simple - as mentioned in the introduction, the transistor amplifies not only the useful signal, but also the bias applied to it. In other words, it enhances D.C.. This current, together with the useful signal, will flow through acoustic system(AC), and the speakers, unfortunately, are able to reproduce this direct current. They do this in the most obvious way - pushing or pulling the diffuser from the normal position to the unnatural.

Try to press the speaker cone with your finger - and you will see what a nightmare this sound will turn into. Direct current in its action successfully replaces your fingers, so it is absolutely contraindicated for a dynamic head. To separate the direct current from the alternating signal is possible only by two means - a transformer or a capacitor - and both options, as they say, one is worse than the other.

circuit diagram

The diagram of the first amplifier that we will assemble is shown in fig. 11.18.

This is a feedback amplifier whose output stage operates in mode B. The only advantage of this circuit is its simplicity, as well as the uniformity of the output transistors (no special complementary pairs are required). However, it is widely used in low power amplifiers. Another plus of the scheme is that it does not require any configuration, and with serviceable parts it will work right away, and this is very important for us now.

Let's take a look at how this circuit works. The amplified signal is fed to the base of the transistor VT1. The signal amplified by this transistor from the resistor R4 is fed to the base of the composite transistor VT2, VT4, and from it to the resistor R5.

Transistor VT3 is turned on in emitter follower mode. It amplifies the positive half-waves of the signal on the resistor R5 and feeds them through the capacitor C4 to the AC.

The negative half-waves are enhanced by the composite transistor VT2, VT4. In this case, the voltage drop across the diode VD1 closes the transistor VT3. The signal from the output of the amplifier is fed to the circuit divider feedback R3, R6, and from it - to the emitter of the input transistor VT1. Thus, the transistor VT1 plays the role of a comparison device in the feedback circuit.

It amplifies the direct current with a gain equal to unity (because the resistance of the capacitor C to direct current is theoretically infinite), and the useful signal - with a coefficient equal to the ratio R6 / R3.

As you can see, the value of the capacitance of the capacitor in this formula is not taken into account. The frequency starting from which the capacitor can be neglected in the calculations is called the cutoff frequency of the RC chain. This frequency can be calculated using the formula

F = 1 / (R×C).

For our example, it will be about 18 Hz, i.e. more low frequencies the amplifier will amplify worse than it could.

Pay . The amplifier is assembled on a board made of one-sided fiberglass with a thickness of 1.5 mm and dimensions of 45 × 32.5 mm. Wiring printed circuit board in a mirror image and the layout of parts can be downloaded. Video of the amplifier in operation MOV format you can download to view. I want to immediately warn the radio amateur - the sound reproduced by the amplifier was recorded in the video using the microphone built into the camera, so speaking about the sound quality, unfortunately, would not be entirely appropriate! Appearance amplifier is shown in fig. 11.19.

Element base . In the manufacture of the amplifier, transistors VT3, VT4 can be replaced by any ones designed for a voltage of at least the supply voltage of the amplifier, and a permissible current of at least 2 A. The VD1 diode must also be designed for the same current.

The rest of the transistors are any allowable voltage not less than the supply voltage, and the permissible current is not less than 100 mA. Resistors - any with a permissible power dissipation of at least 0.125 W, capacitors - electrolytic, with a capacity not less than indicated on the diagram, and an operating voltage less than the amplifier supply voltage.

Amplifier Heatsinks . Before trying to make our second design, let's, dear radio amateur, dwell on the radiators for the amplifier and give here a very simplified method for calculating them.

First, we calculate the maximum power of the amplifier using the formula:

P = (U × U) / (8 × R), W,

where U- amplifier supply voltage, V; R- AC resistance (usually it is 4 or 8 ohms, although there are exceptions).

Secondly, we calculate the power dissipated on the collectors of transistors, according to the formula:

P race = 0.25 × P, W.

Thirdly, we calculate the radiator area required to remove the corresponding amount of heat:

S \u003d 20 × P races, cm 2

Fourthly, we select or manufacture a radiator, the surface area of ​​\u200b\u200bwhich will be at least calculated.

This calculation is very approximate, but for amateur radio practice it is usually sufficient. For our amplifier, with a supply voltage of 12 V and an AC resistance of 8 ohms, the “correct” radiator would be an aluminum plate measuring 2 × 3 cm and at least 5 mm thick for each transistor. Keep in mind that a thinner plate does not transfer heat well from the transistor to the edges of the plate. I would like to immediately warn you - the radiators in all other amplifiers must also be of “normal” size. Which ones - count for yourself!

Sound quality . After assembling the circuit, you will find that the sound of the amplifier is not entirely clear.

The reason for this is the “pure” class B mode in the output stage, the characteristic distortions of which even the feedback cannot fully compensate for. For the sake of experiment, try replacing the VT1 transistor in the circuit with the KT3102EM, and the VT2 transistor with the KT3107L. These transistors have a much higher gain than KT315B and KT361B. And you will find that the sound of the amplifier has improved significantly, although some distortion will still be noticeable.

The reason for this is also obvious - a larger gain of the amplifier as a whole provides greater accuracy of the feedback, and its greater compensating effect.

Continue reading

Hi all! In this article I will describe in detail how to make a cool amplifier for home or car. The amplifier is easy to assemble and set up, and has good quality sound. Below you will find circuit diagram the amplifier itself.

The circuit is made on transistors and has no scarce parts. The power supply of the amplifier is bipolar +/- 35 volts, with a load resistance of 4 ohms. When connecting an 8 ohm load, the power can be increased to +/- 42 volts.

Resistors R7, R8, R10, R11, R14 - 0.5 W; R12, R13 - 5W; the rest 0.25 W.
R15 trimmer 2-3 kOhm.
Transistors: Vt1, Vt2, Vt3, Vt5 - 2sc945 (usually c945 is written on the case).
Vt4, Vt7 - BD140 (Vt4 can be replaced by our Kt814).
Vt6 - BD139.
Vt8 - 2SA1943.
Vt9 - 2SC5200.

ATTENTION! The c945 transistors have different pinouts: ECB and EBK. Therefore, before soldering, you need to check with a multimeter.
The LED is ordinary, green, exactly GREEN! He's not here for beauty! And it should NOT be super bright. Well, the rest of the details can be seen in the diagram.

And so, let's go!

To make an amplifier, we need tools:
- soldering iron
-tin
- rosin (preferably liquid), but you can get by with the usual
- metal scissors
-cutters
-awl
- medical syringe, any
- drill 0.8-1 mm
- drill 1.5 mm
- drill (preferably some kind of mini drill)
-sandpaper
-and a multimeter.

Materials:
- one-sided textolite board measuring 10x6 cm
- sheet of notebook paper
-a pen
- varnish for wood (preferably dark color)
- small container
-baking soda
-lemon acid
-salt.

I will not list the list of radio components, they can be seen on the diagram.
Step 1 We are preparing a fee
And so, we need to make a board. Because laser printer I don’t have (in general, I don’t have any), we will make the board “the old fashioned way”!
First you need to drill holes on the board for future parts. Who has a printer, just print this picture:

if not, then we need to transfer the markings for drilling to paper. How to do this you will understand in the photo below:

when you translate, do not forget about the fee! (10 by 6 cm)

something like that!
We cut off the size of the board we need with metal scissors.

Now we apply the sheet to the cut out board and fix it with adhesive tape so that it does not move out. Next, we take an awl and outline (by points) where we will drill.

Of course, you can do without an awl and drill right away, but the drill can move out!

Now you can start drilling. We drill holes 0.8 - 1 mm. As I said above: it is better to use a mini drill, since the drill is very thin and breaks easily. For example, I use a screwdriver motor.

Holes for transistors Vt8, Vt9 and for wires are drilled with a 1.5 mm drill. Now we need to clean our board with sandpaper.

Now we can start drawing our paths. We take a syringe, grind off a needle so that it is not sharp, we collect varnish and go!

It is better to trim the jambs when the varnish has already hardened.

Step 2 We charge a fee
For board etching, I use the simplest and cheapest method:
100 ml of peroxide, 4 teaspoons of citric acid and 2 teaspoons of salt.

We stir and immerse our board.

Next, we clean the varnish and it turns out like this!

It is advisable to immediately cover all the tracks with tin for the convenience of soldering parts.

Step 3 Soldering and tuning
It will be convenient to solder according to this picture (view from the side of the parts)

For convenience, from the beginning we solder all the small parts, resistors, etc.

And then everything else.

After soldering, the board must be washed from rosin. You can wash it with alcohol or acetone. On kraynyak it is possible even gasoline.

Now you can try to turn it on! With proper assembly, the amplifier works immediately. When you first turn on the resistor R15 must be turned in the direction of maximum resistance (we measure it with a device). Do not connect the column! The output transistors are MANDATORY on the radiator, through insulating gaskets.

And so: turn on the amplifier, the LED should be on, we measure the output voltage with a multimeter. There is no standing, so everything is fine.
Next, you need to set the quiescent current (75-90mA): to do this, close the input to ground, do not connect the load! On the multimeter, set the mode to 200mV and connect the probes to the collectors of the output transistors. (marked with red dots in the photo)

Readers! Remember the nickname of this author and never repeat his schemes.
Moderators! Before you ban me for insults, think that you "let an ordinary gopnik to the microphone", who should not even be allowed close to radio engineering and, moreover, to teaching beginners.

Firstly, with such a switching scheme, a large direct current will flow through the transistor and speaker, even if variable resistor will be in the right position, that is, music will be heard. And with a large current, the speaker is damaged, that is, sooner or later, it will burn out.

Secondly, in this circuit there must be a current limiter, that is, a constant resistor, at least 1 KΩ, connected in series with a variable one. Any do-it-yourselfer will turn the variable resistor regulator all the way, it will have zero resistance and a large current will go to the base of the transistor. As a result, the transistor or speaker will burn out.

A variable capacitor at the input is needed to protect the sound source (this should be explained by the author, because immediately there was a reader who removed it just like that, considering himself smarter than the author). Without it, only those players in which such protection is already installed at the output will work normally. And if it is not there, then the player's output may be damaged, especially, as I said above, if you unscrew the variable resistor "to zero". In this case, the output of an expensive laptop will be energized from the power source of this penny trinket and it can burn out. Homemade ones are very fond of removing protective resistors and capacitors, because "it works!" As a result, the circuit can work with one sound source, but not with another, and even an expensive phone or laptop can be damaged.

The variable resistor, in this circuit, should only be a trimmer, that is, it should be adjusted once and closed in the case, and not brought out with a convenient handle. This is not a volume control, but a distortion control, that is, it selects the operating mode of the transistor so that there is minimal distortion and that smoke does not come out of the speaker. Therefore, it should never be accessible from the outside. It is IMPOSSIBLE to adjust the volume by changing the mode. For this you need to "kill". If you really want to adjust the volume, it's easier to turn on another variable resistor in series with the capacitor, and now it can already be output to the amplifier case.

In general, for the simplest circuits - and in order to work right away and not to damage anything, you need to buy a TDA-type chip (for example, TDA7052, TDA7056 ... there are many examples on the Internet), and the author took a random transistor that was lying around in his desk. As a result, gullible amateurs will look for just such a transistor, although its gain is only 15, and admissible current as much as 8 amperes (it will burn any speaker without even noticing).

Scheme simple amplifier sound on transistors, which is implemented on two powerful composite transistors TIP142-TIP147 installed in the output stage, two low-power BC556B in the differential path and one BD241C in the signal pre-amplification circuit - only five transistors for the entire circuit! This UMZCH design can be freely used, for example, as part of a home music center or for the buildup of a subwoofer installed in a car, at a disco.

The main attraction of this sound power amplifier lies in the ease of assembly even by novice radio amateurs, there is no need for any special settings, there are no problems in purchasing components at an affordable price. The PA circuit presented here has electrical characteristics with high linearity of operation in the frequency range from 20Hz to 20000Hz. p>

When choosing or self-manufacturing a transformer for a power supply, the following factor must be taken into account: - the transformer must have a sufficient power margin, for example: 300 W per one channel, in the case of a two-channel version, then naturally the power doubles. You can use your own separate transformer for each, and if you use the stereo version of the amplifier, then you will generally get a “double mono” type device, which will naturally increase the efficiency of sound amplification.

The operating voltage in the secondary windings of the transformer should be ~ 34v alternating, then the constant voltage after the rectifier will be in the region of 48v - 50v. In each power supply arm, it is necessary to install a fuse rated for a working current of 6A, respectively, for a stereo when working on one power supply - 12A.