Electrovacuum device- a device designed to generate, amplify and convert electromagnetic energy, in which the working space is freed from air and protected from the surrounding atmosphere by an impermeable shell.
Such devices include both vacuum electronic devices, in which the electron flow passes in a vacuum, and gas-discharge electronic devices, in which the electron flow passes in a gas. Incandescent lamps also belong to electric vacuum devices.
In electrovacuum devices, conductivity is carried out by means of electrons or ions moving between the electrodes through a vacuum or gas.
The beginning was laid by the discovery of thermoelectrons. In 1884, the famous American inventor Thomas Alva Edison, in search of a rational design for an incandescent lamp, discovered an effect named after him. Here is its first description: “Between the branches of the filament” of an incandescent bulb, at the same distance from both, a platinum plate is placed, which is an insulated electrode ... If you turn on a galvanometer between this electrode and one of the ends of the filament, then when the lamp burns, a current is observed that changes its direction depending on whether the positive or negative end of the carbon filament is attached to the tool. In addition, its intensity increases with the strength of the current passing through the thread.
An explanation follows: “apparently, in this lamp, particles of air (or coal) scatter from the filament in straight lines, carry away an electric charge.”
Edison is an inventor, he does not analyze the phenomenon. Quoted phrases, in essence, are limited to the content of the note. It's nothing more than a priority claim. Edison's attempts to find practical use had no effect.
Thus, the phenomenon of thermionic emission was discovered and the first radio tube, an electrovacuum diode, was created.
Thermionic emission (Richardson effect, Edison effect) - the phenomenon of the emission of electrons by heated bodies. The concentration of free electrons in metals is quite high, therefore, even at medium temperatures, due to the distribution of electrons in terms of velocities (in terms of energy), some electrons have enough energy to overcome the potential barrier at the metal boundary. As the temperature rises, the number of electrons whose kinetic energy of thermal motion is greater than the work function increases, and the phenomenon of thermionic emission becomes noticeable.
The study of the patterns of thermionic emission can be carried out using the simplest two-electrode lamp - a vacuum diode, which is an evacuated balloon containing two electrodes: cathode K and anode A.
Fig.3.1 Vacuum diode construction
In the simplest case, a filament of a refractory metal (for example, tungsten) heated by an electric current serves as the cathode. The anode is most often in the form of a metal cylinder surrounding the cathode. The designation of the diode in the electrical circuit diagrams is shown in Figure 3.2.
Rice. 3.2. Designation of a vacuum diode in electrical circuit diagrams.
If the diode is included in the circuit, then when the cathode is heated and a positive voltage is applied to the anode (relative to the cathode), a current appears in the anode circuit of the diode. If you change the polarity of the voltage, then the current stops, no matter how strongly the cathode is heated. Consequently, the cathode emits negative particles - electrons.
If the temperature of the heated cathode is kept constant and the dependence of the anode current on the anode voltage is removed - the current-voltage characteristic, then it turns out that it is not linear, that is, Ohm's law is not fulfilled for a vacuum diode. The dependence of the thermionic current on the anode voltage in the region of small positive values is described by the law of three second
where B is a coefficient depending on the shape and size of the electrodes, as well as their relative position.
With an increase in the anode voltage, the current increases to a certain maximum value, called the saturation current. This means that almost all electrons leaving the cathode reach the anode, so a further increase in the field strength cannot lead to an increase in thermionic current. The dependence of thermionic current on the anode voltage is shown in Figure 3.3.
Rice. 3.3. Dependence of thermionic current on the anode voltage
Therefore, the saturation current density characterizes the emissivity of the cathode material. The saturation current density is determined by the Richardson - Deshman formula, derived theoretically on the basis of quantum statistics:
where A is the work function of electrons leaving the cathode,
T - thermodynamic temperature,
C is a constant, theoretically the same for all metals (this is not confirmed by experiment, which, apparently, is explained by surface effects). A decrease in the work function leads to a sharp increase in the saturation current density. Therefore, radio tubes use oxide cathodes (for example, nickel coated with alkaline earth metal oxide), the work function of which is 1–1.5 eV.
The operation of many vacuum devices is based on the phenomenon of thermionic emission. electronic appliances.
Electrovacuum triode, or simply triode, - an electronic lamp with three electrodes: a thermionic cathode (direct or indirect heating), an anode and one control grid. Invented and patented in 1906 by American Lee de Forest. The design of the vacuum triode is shown in Fig. 3.4
Fig.3.4 Vacuum triode design
Triodes were the first devices used to amplify electrical signals in the early 20th century. The electrical circuit diagram of the triode is shown in fig. 3.5
Rice. 3.5 Symbol for a triode in electrical circuit diagrams
Volt-ampere characteristics triode is shown in Figure 3.6
Rice. 3.6 Volt-ampere characteristic of triode
The current-voltage characteristic of the triode has a high linearity. Due to this, vacuum triodes introduce minimal non-linear distortions into the amplified signal.
At present, vacuum triodes have been replaced by semiconductor transistors. The exceptions are areas where it is required to convert signals with a frequency of the order of hundreds of MHz - GHz of high power with a small number of active components, and the dimensions and weight are not so critical, for example, in the output stages of radio transmitters, as well as induction heating for surface hardening. Powerful radio tubes have a comparable powerful transistors efficiency; their reliability is also comparable, but the service life is much less. Low-power triodes have a low efficiency, since a significant part of the power consumed by the cascade is spent on heating, sometimes more than half of the total lamp consumption.
A tetrode is a two-grid electron tube designed to amplify the voltage and power of electrical signals. The electrical circuit diagram of the tetrode is shown in fig. 3.7
Rice. 3.7 Symbol of the tetrode in electrical circuit diagrams
Unlike the triode, the tetrode has a shielding grid between the control grid and the anode, which weakens the electrostatic effect of the anode on the control grid. Compared to the triode, the tetrode has a large gain, a very low anode-control grid capacitance, and a large internal resistance.
According to their purpose, they are divided into tetrodes for amplifying low-frequency voltage and power and broadband tetrodes for amplifying video signals. The beam tetrode, like an ordinary one, is a two-grid lamp, but differs from the latter in the absence of a dynatron effect, which is achieved by using beam-forming plates located between the screening grid and the anode and connected inside the balloon with the cathode. Beam tetrodes are mainly used to amplify low-frequency power in the final stages of receivers, televisions and other equipment.
Pentode(from other Greek πέντε five, according to the number of electrodes) - a vacuum electron tube with a screening grid, in which a third (protective or antidynatron) grid is placed between the screening grid and the anode. By design and purpose, pentodes are divided into four main types: low-power high-frequency amplifiers, output pentodes for video amplifiers, output pentodes of amplifiers low frequencies, and powerful generator pentodes.
Shielded tubes, the tetrode and pentode, outperform the triode at high frequencies. The upper operating frequency of the pentode amplifier can reach 1 GHz. The efficiency of a power amplifier based on pentodes (about 35%) is significantly higher than that of an amplifier based on triodes (15%-25%), but somewhat lower than that of an amplifier based on beam tetrodes.
The disadvantages of pentodes (and in general of all shielded lamps) are higher than that of a triode, nonlinear distortion, in which odd harmonics predominate, a sharp dependence of the gain on the load resistance, a higher level of intrinsic noise ..
More complex are multi-electrode lamps with two control grids - heptodes, which appeared in connection with the invention of superheterodyne reception.
The content of the article
ELECTROVACUUM AND GAS DISCHARGE DEVICES, vacuum tubes used to generate, amplify or stabilize electrical signals. An electronic tube is essentially a sealed ampoule in which electrons move in a vacuum or gaseous medium. The ampoule is usually made of glass or metal. The electronic flow is controlled by electrodes inside the lamp.
Although semiconductor devices have replaced vacuum tubes in most applications, tubes still find use in video terminals, radar, satellite communications and many other electronic devices.
The lamp has several conductive elements called electrodes. The emission of electrons in the lamp is carried out by the cathode. This emission is caused either by heating the cathode, as a result of which the electrons “boil” and evaporate from its surface, or by the action of light on the cathode. The movement of the emitted electrons is controlled by electric fields created by other electrodes inside the lamp. In most cases, the lamp electrodes are isolated from each other and connected to external circuits by means of wire leads. The electrodes that serve to control the movement of electrons are called grids; the electrodes where the electrons are collected are called anodes.
In a vacuum tube, it is relatively easy to control the magnitude, duration, frequency, and other characteristics of the electron flow. This simplicity and ease of operation make it a valuable instrument in numerous applications.
Thermionic emission.
Electrons do not spontaneously go beyond the surface layer of the metal due to the action of attractive forces, the source of which is the metal itself. The potential energy of an electron at any point of the metal near its surface can be represented in the form of a graph (Fig. 1), from which it can be seen that in order to go beyond the metal surface, the electron must increase its energy T 0 , which it has at absolute zero temperature, additionally by the value W. At room temperature, a very small number of electrons have the energy needed to escape, but as the temperature rises, the energy of the electron increases and approaches the level required for emission. AT electronic tubes ah, the necessary thermal energy is provided by an electric current passed through a wire filament (heater) located in the lamp.
Diode.
After the electrons have left the cathode, their movement is determined by the forces of the electric fields acting on them in a vacuum. In the simplest electronic lamp - a diode - electrons are attracted by the positive potential of the second electrode - the anode, where they are collected and passed into the circuit of the corresponding circuit (Fig. 2). The diode is thus a device that passes current in only one direction - from the anode to the cathode - and, therefore, is a rectifier. A simple illustration of the use of a diode is the circuit shown in fig. 3, where the diode is used to charge the capacitor with voltage from the source alternating current. When the cathode potential is below the anode potential, current flows through the diode so that eventually the capacitor charges up to the peak voltage of the AC source. Diagram options fig. 3 are used for signal detection audio frequency from radio frequency wave and to obtain power direct current from AC sources.
Triode.
A triode is an electronic tube in which there is a third (control) electrode installed between the cathode and the anode (Fig. 4). This electrode is usually a grid of fine wires, mounted very close to the cathode, so that with a small potential difference between the grid and the cathode, a relatively high electric field acts in the region between these two electrodes. In this case, the grid potential will have a strong effect on the electrons.
A typical triode amplifier circuit is shown in fig. 5. Connected to the grid is a negative bias battery, labeled Egg. Since the grid has a negative potential with respect to the cathode, it will not attract electrons from the stream moving from the cathode to the anode. The anode is maintained at a positive potential relative to the cathode, which is provided by the battery E pp. Parameter values Egg, E pp, resistor resistance Rg in the grid circuit and load resistor R L choose so that a certain current flows through the lamp. The anode potential, therefore, turns out to be somewhat smaller than the potential E pp its power source, due to the flow of current through R L.
If a positive signal is applied to the grid through the capacitor, it will affect the electrons leaving the cathode. Since such a grid presents a weak physical barrier to electrons, they will pass through the grid to the anode. Therefore, when changing the grid potential in positive side the current through the triode increases and the voltage across the anode decreases. (This decrease is due to an increase in the voltage drop across R L associated with an increase in current.) If the input signal coming to the grid changes its potential in a negative direction, then the opposite process occurs; the voltage at the anode increases. In many vacuum tubes, the change in grid voltage essentially determines the change in anode current; it follows that the voltage changes at the anode are determined by the choice R L. As a result, a small change in the grid voltage can, at a sufficiently large R L cause a much larger voltage change across the anode.
multi-electrode lamps.
It is logical to ask the question: what could be the effect of increasing the number of grids in a vacuum tube? Usually the second grid, which is called the screen grid and is maintained at a positive potential, is located between the control grid and the anode. Its role is to shield the control grid from the anode, thus reducing the capacitance between them, which in some cases can lead to undesirable effects. feedback. A lamp with two grids (four electrodes) is called a tetrode. In some cases, another grid is added between the screen grid and the anode - an antidynatron one, resulting in a five-electrode lamp, or a pentode. In a tetrode, electrons that reach the anode surface knock out secondary electrons when they hit it. Some of them can move in the opposite direction and be collected by a screen grid, usually having a potential close to that of the anode. Such a process causes losses in the total flow of electrons passing through the anode (in the anode current). The anti-dynatron grid located between the screen grid and the anode is maintained at a negative potential with respect to both adjacent electrodes, so that the returning electrons are repelled by it back to the anode. On fig. 6 shows a typical pentode switching circuit.
In some cases, in order to save space and money, two separate vacuum tube structures are combined in a single sealed package.
cathode ray tubes.
A cathode ray tube (CRT) uses a beam of electrons from a heated cathode to reproduce an image on a fluorescent screen. This beam is carefully focused into a beam that creates a small spot on the screen and excites the electrons of the screen's phosphor, which leads to the emission of light. This beam is deflected under the action of an electric or magnetic field, while describing the trajectory on the screen, and the intensity of the beam can be changed by means of a control electrode, thereby changing the brightness of the spot. The part of the CRT in which the focused electron beam is created is called the electron spotlight. Although the electronic projector is the main part of the CRT, due to its complexity, it will be considered after others.
Beam deflection systems.
At the output of the electron projector, a narrow electron beam is obtained, which, on its way to the screen, can be deflected by an electric or magnetic field. Electric fields are commonly used in CRTs with small screens, such as those found in oscilloscopes. Magnetic fields are required to deflect the beam in television CRTs with large screens.
In electric field deflection systems, the field vector is oriented perpendicular to the initial beam path (which is usually denoted by the direction z). The deflection is carried out by applying a potential difference to a pair of deflecting plates, as shown in fig. 7. Usually the deflection plates deflect in the horizontal direction (direction x) proportional to time. This is achieved by applying a voltage to the deflecting plates, which increases uniformly as the beam travels across the screen. Then this voltage quickly drops to its original level and again begins to increase evenly. The signal to be investigated (usually a periodic oscillation) is applied to the plates deflecting in the vertical direction ( y). As a result, if the duration of a single horizontal sweep is equal to the period or corresponds to the signal repetition frequency y, the screen will continuously display one period of the wave process. In cases where a large deflection is required, the use of an electric field to deflect the beam becomes inefficient.
In order for the beam to create a sufficiently bright spot on the screen, and for the deflecting potential not to reach the breakdown voltage between the deflecting plates, the electrons must be greatly accelerated. Moreover, the CRT should not be too long, so that the device in which it is intended to be used does not become unacceptably bulky. Finally, the length of the deflecting plates is also limited. When using magnetic fields to deflect the beam at large angles, the CRT turns out to be short (Fig. 8).
Luminescent screen.
The luminescent screen is formed by applying a thin layer of phosphor on the inner surface of the end wall of the conical part of the CRT. The kinetic energy of the electrons bombarding the screen is converted into visible light.
Electronic projector.
An electronic searchlight is placed in the narrow neck of the CRT bulb. One of the many possible designs of an electronic searchlight is shown schematically in Fig. 9, a. The cathode and a number of closely spaced cylindrical electrodes are aligned along their common axis. On fig. 9, b magnification shows the focusing area of the beam (i.e. the "lens" of the electron projector), in which an inhomogeneous, but axisymmetric electric field acts. The electric field vectors are everywhere perpendicular to the equipotential surfaces and directed to the left in the figure, since the second anode is at a higher potential than the first one. In this case, the electrons are formed into a converging beam, which, due to proper adjustment of the shape of the electrodes and their relative potentials, is precisely focused when they reach the screen surface. In some cases, focusing is carried out by means of a magnetic field directed parallel to the axis of the CRT. On fig. 9, in the principle of such focusing is explained.
The electrical potential that determines top speed electrons at the output of the electronic projector ranges from a few hundred to 10,000 V. In operation, the last accelerating electrode (second anode) is usually grounded. The electrodes have diaphragms with round holes, which cut off the peripheral electrons from the beam, thereby preventing spot blurring. In addition, they trap secondary emission electrons returning from various surfaces of the CRT's internal components.
Photoelectronic devices.
A photoelectronic electrovacuum device (photocell) is an electronic lamp having a cathode that emits electrons when visible light or infrared or ultraviolet radiation hits it. Changes in the intensity of the radiation cause corresponding changes in the electron flow in the lamp, and hence the current in the external circuit.
In scientific research and technology, photoelectronic devices are used to measure illumination. They are also used in street lighting control devices, for color equalization in television and color matching in printing, for counting objects in production. Photoelectronic devices are used to read sound when showing movies. Sound is recorded on film as a continuous track of variable density, which modulates a light beam directed at a photoelectronic device. The output signal of this device is proportional to the density of the sound track recorded on the film.
On fig. ten, a the volt-ampere characteristics of a typical electrovacuum photocell are shown, and in fig. ten, b are the relative spectral characteristics of a typical photoelectronic device and the human eye at constant light intensity and varying radiation wavelength. The absolute values of the amplitudes of the spectral characteristics depend on the choice of material for the sensitive surface of the photocathode.
In some cases, a gas is introduced into the device to increase its current sensitivity. However, this sensitivity becomes strongly dependent on the anode potential, while in a vacuum photocell the output signal remains unchanged over a wide range of anode potentials (Fig. 11).
Photomultiplier.
The action of a photomultiplier is based on the use of secondary electrons, which are released when an electron possessing high speed hitting the metal surface. The device works as follows. Electrons emitted by an ordinary photocathode are attracted by the electric field of a dynode - an electrode whose potential is slightly higher than the potential of the cathode. When an electron hits a dynode, several secondary electrons fly out of it. They accelerate towards the second dynode, which is at a higher potential than the first, and as a result of the collision, an even greater number of secondary electrons are formed. After several such stages of cascade "multiplication" of electrons, the process finally reaches the anode, which collects electrons. The greatly increased number of electrons collected by the anode creates a much larger current compared to the current of the photocathode. If every electron hitting a dynode knocks out n secondary electrons, then with the number of dynodes equal to k, the current gain will be nk. The position of the dynodes is carefully calculated so that most of the electrons leaving one dynode end up on the other, and so on. On fig. 12, a it is shown how this process is realized in a relatively limited volume of an electron tube. On fig. 12, b the connection diagram of a typical photomultiplier is presented. The resistors of all dynodes usually have the same resistance. On fig. 12, in the current characteristic of the photomultiplier is given. In this case, the potential difference between adjacent dynodes is 100 V, and the resulting current amplification factor is 10 6 .
Discharge lamps.
A discharge lamp is a vacuum tube that contains enough gas to significantly affect its performance. The pressure of this gas is below atmospheric pressure. Usually, inert gases (neon, argon, etc.) or mercury vapor are used to fill gas-discharge lamps. The characteristics of a lamp are determined by both the properties of the gas used and its pressure inside the lamp.
Collisions and ionization.
The presence of gas molecules in an electron tube can cause two effects. Collisions with molecules can cause deceleration of the electron flow in the lamp (such collisions can lead to an increase in space charge with the formation of an electron cloud around the cathode, which causes a decrease in current), and if the electrons are accelerated by a sufficiently large potential difference, they can knock electrons out of the gas molecules, leaving positively charged ions behind them. This process is called ionization. If the accelerating potential in the lamp is even higher, then the primary electron and the electron released from the molecule during the ionization process can be accelerated to such a high speed that they cause further ionization. Such a process leads to a discharge - the propagation of ionization in the space between the anode and cathode of the lamp. Education a large number positive ions and electrons released during ionization increases the current flowing through the lamp, and the resistance of the lamp during discharge becomes very small.
Discharge diodes and gas-filled lamps.
A gas-discharge diode (gastron) is a diode in which the presence of gas creates a high conductivity in the forward direction. The electrons emitted by the cathode are accelerated towards the anode, and as a result, a discharge occurs. The discharge continues until the anode potential falls below a certain cutoff potential. But as soon as the anode becomes negative, the lack of electrons is no longer able to initiate the discharge again. If, however, the anode potential drops to a large negative value (for example, more than -100 V), then the discharge is triggered by the electrons emitted by the anode. In other words, the anode emits electrons more easily when its potential is not zero, but negative. Electrons can be released by thermal emission even at room temperature due to their thermal motion. They can also appear due to photoelectric processes caused by bombardment with photons. In any case, the emitted electrons will cause ionization in the lamp, followed by a discharge. Therefore, large negative voltages are usually not applied to the anodes of gas-discharge diodes. However, such diodes find use in low-voltage rectifier circuits, particularly in battery chargers where high forward current is required.
A neon lamp is a gas-discharge diode with two identical electrodes without heaters. On fig. 13 shows the current-voltage characteristic of such a lamp. It is easy to see that the voltage drop across the lamp remains almost unchanged after the lamp is "lit" by applying a voltage to it slightly higher than the starting one. This characteristic of gas discharge lamps operating in the self-sustaining glow discharge region makes them useful devices for maintaining a constant voltage in a circuit with a varying load current. Typically, specially designed lamps are used for such voltage stabilizers (zener diodes), but a simple neon lamp is also suitable. The lamps must be connected to the voltage source through a series resistor to prevent too much current rise, which could damage the lamp or the voltage source.
Thyratron.
Thyratron is a gas-discharge triode, usually with a heated cathode. The thyratron anode is typically held at a high enough potential to initiate a discharge when the grid is at cathode potential. (A negative potential is maintained on the grid in order to prevent electrons from escaping from the near-cathode region and initiating a discharge.) At the right moment, on a signal, the grid potential rises enough to start the discharge. After the discharge occurs, the grid does not control it until the anode voltage drops to a level at which the discharge goes out.
A small positive pulse applied to the grid allows you to initiate the passage of a large current through the lamp. This control function determines the usefulness of the thyratron. The "starting potential" of the grid - the voltage at which the discharge is initiated - depends on the potential of the anode and the temperature of the gas in the lamp.
In ionic (gas-filled) photovoltaic cells, gas is used to obtain current amplification due to the ionization of gas molecules by photoelectrons. The anode potential is never brought to a level at which the discharge becomes self-sustaining and does not require the emission of photoelectrons from the cathode.
Electrovacuum devices (EVD) are devices in which an electric current is created by a stream of electrons or ions moving in a high vacuum or inert gas medium. EVP are divided into electronically controlled lamps (EUL), cathode ray tubes (CRT), gas discharge devices (GDP) and photoelectric (photoelectronic) devices.
In the EUL, an electric current is generated by the movement of electrons in a high vacuum (gas pressure is only 1.33 () Pa (mm Hg)) from one electrode to another. The simplest EUL is a diode.
Diode. A diode contains only two electrodes: a cathode and an anode. The cathode is a source of free electrons. For electrons to leave the cathode, they need to be given additional energy, called the work function. Electrons receive this energy when the cathode is heated by electric current. The emission of electrons from a heated cathode is called thermionic emission.
The negative space charge formed by the electrons emitted from the cathode creates an electric field near its surface, which prevents the electrons from leaving the cathode, forming a potential barrier on their way.
A positive voltage relative to the cathode is applied to the anode, which reduces the potential barrier at the cathode surface. Electrons whose energy is sufficient to overcome the potential barrier leave the space charge region, enter the accelerating electric field of the anode voltage and move towards the anode, creating an anode current. With an increase in the anode voltage, the anode current of the diode also increases.
With a negative anode voltage, the potential barrier at the cathode surface increases, the electron energy is insufficient to overcome it, and the current does not flow through the diode. This is an important feature of the diode - its one-sided electrical conductivity.
On fig. 3.1 showing conventions diodes and the scheme of their connection to the anode voltage source.
Triode. Unlike a diode, a triode has three electrodes: a cathode, an anode, and a grid (Fig. 3.2, a, b). Grid is located
between the cathode and the anode in the immediate vicinity of the cathode. If a negative voltage is applied to the grid (Fig. 3.2, c), then the potential barrier at the cathode will increase, and the anode current will decrease. At some negative grid voltage, called the turn-off voltage U CK .z an , the anode current will decrease to zero. If a positive voltage is applied to the grid (Fig. 3.2, d), then the electric field formed by it between the cathode and the grid will lead to a decrease in the potential barrier and an increase in the anode current.
Due to the fact that the grid is located closer to the cathode than the anode, the voltage applied to it affects the potential barrier and the anode current of the triode is much stronger than the anode voltage of the same value. Therefore, in a triode, the anode current is controlled by changing the grid voltage, and not the anode one.
The main characteristics of the triode are families of static anode-grid (transfer) characteristics, taken at different anode voltages U a k (Fig. 3.3, a), and anode (output) characteristics I a \u003d f (U ak), taken at different grid voltages (Fig. 3.3, b).
The disadvantages of the triode are a large throughput capacitance (the capacitance between the grid and the anode) and a low static gain. These shortcomings are eliminated by introducing a second grid into the EUL.
Tetrode. This is a four-electrode electronically controlled lamp containing a cathode, an anode and two grids (Fig. 3.4, a). The first grid, located near the cathode, is used, as in the triode, to control the anode current and is called the control grid. The second grid, located between the first grid and the anode, is a kind of screen between these electrodes. As a result of the shielding action of the second grid, the throughput capacitance of the lamp is significantly reduced and the effect of the anode voltage on
Potential barrier at the cathode surface. Therefore, to create a directed movement of electrons from the cathode to the anode, a positive voltage U c 2 k is applied to the second grid, called the screening grid, which is equal to or slightly less than the anode voltage. In this case, part of the electrons enters the screening grid and creates a current I c2 of this grid.
Electrons hitting the anode knock secondary electrons out of it. When (and such cases take place during the operation of the tetrode), secondary electrons are attracted by the screening grid, which leads to an increase in the current of the screening grid and a decrease in the anode current. This phenomenon is called the dinatron effect. To eliminate the dynatron effect, which limits the working area of the EUL, a potential barrier for secondary electrons is created between the anode and the screening grid. Such a barrier is formed by increasing the electron flux density due to its focusing in beam tetrodes (Fig. 3.4, b) or by introducing a third grid between the screening grid and the anode, which, as a rule, has zero potential.
Pentode. A five-electrode EUL is called a pentode (Fig. 3.4, i). The zero potential of the third grid, which is called antidynatron or protective, is provided by electrical connection her with the cathode.
The main characteristics of tetrodes and pentodes are families of static anode (output) at and grid-anode at characteristics, which are taken at a constant voltage U c 2k and plotted on the same graph (Fig. 3.5).
The parameters characterizing the amplifying properties of the EUL are:
steepness of the anode-grid characteristic
internal (differential) resistance
static gain
Parameters S, and , called differential, are interconnected by the relation .
CATHONY RAY TUBE
Cathode ray tubes (CRTs) are electronic electrovacuum devices that use a stream of electrons concentrated in the form of a beam. These instruments are in the form of a tube extended in the direction of the beam. The main elements of a CRT are a glass cylinder, or flask, an electronic searchlight, a deflecting system and a screen (Fig. 3.6).
Cylinder 7 serves to maintain the necessary vacuum in the CRT and protect the electrodes from mechanical and
climatic influences. Part of the inner surface of the cylinder is covered with a graphite film 8, called aquadag. A positive voltage relative to the cathode is applied to the aquadag.
An electronic searchlight is designed to create a focused electron beam (beam) with the required current density. It consists of a thermionic cathode 2, inside which there is a heater 1, a control electrode 3, called a modulator, the first 4 and second 5 anodes. The modulator and anodes are made in the form of hollow cylinders coaxial with the cylindrical cathode.
The modulator is connected to a negative voltage source, adjustable from zero to several tens of volts. Positive voltages are applied to the anodes: several hundred volts for the first and several kilovolts for the second.
An inhomogeneous electric field is formed between the modulator and the first anode, which focuses all the electrons that have flown out of the cathode and passed through the modulator hole, at a certain point on the CRT axis in the cavity of the first anode. Such an electric field is called an electrostatic lens.
A second electrostatic lens is formed between the first and second anodes. Unlike the first, short-focus, it is long-focus: its focus is located on the axis of the CRT in the plane of the screen 9.
A change in the modulator voltage leads to a change in the number of electrons that can overcome the potential barrier at the cathode and enter the accelerating electric field of the first anode. Therefore, the modulator voltage determines the density of the electron beam and the brightness of the luminous spot on the CRT screen. Beam focusing on the CRT screen is achieved by changing the non-uniform electric field of the second electrostatic lens by changing the voltage of the first anode.
The deflecting system serves to direct the focused electron beam to any point on the screen. This is achieved by exposing the electron beam to a transverse electric or magnetic field.
When the electron beam is deflected by an electric field (electrostatic deflection), the deflecting voltages are applied to two mutually perpendicular pairs of parallel plates 6. The electron beam, passing between the plates, is deflected towards the plate with a high potential. Plates, the electric field between which deflects the electron beam in the horizontal direction, are called horizontally deflecting or X-plates, and in the vertical - vertically deflecting or Y-plates.
The main parameter of the electrostatic deflection system is the deflection sensitivity S, defined as the ratio of the deflection of the luminous spot on the CRT screen to the deflection voltage. For modern CRTs S E = 0.1 ... 3 mm / V.
Along with electrostatic, magnetic deflection of the electron beam is also used. The deflecting magnetic field is created by the current passing through two pairs of coils located mutually perpendicular on the neck of the CRT.
Screens 9 cathode ray tubes, used to convert electrical signals into light, are covered special composition- a phosphor that glows when a focused electron beam hits it. Zinc and zinc-cadmium sulfides, zinc silicate (willemite), calcium and cadmium tungstates are used as phosphors. Such screens are called fluorescent.
Only part of the energy of the electron beam is spent on the glow of the phosphor. The rest of the beam energy is transferred to the screen electrons and causes secondary electron emission from the screen surface. The secondary electrons are attracted by the aquadag, which is usually electrically connected to the second anode.
CRT screens used to obtain a color image contain grains of phosphors with blue, red and green glow - triads arranged in a certain order. In the neck of the tube there are three autonomous electronic searchlights. They are arranged in such a way that their electron beams intersect at some distance from the screen. In the plane of intersection of the rays, a shadow mask is installed, in which there are a large number of holes. After passing through the holes in the mask, each of the electron beams hits its own element of the triad (Fig. 3.7).
As a result of mixing three colors of different brightness, a glow of the desired color is obtained.
In addition to luminescent, there are dielectric screens. An electron beam, moving along such a screen, creates various charges on its sections, i.e., a kind of potential relief that can be maintained for a long time. Dielectric screens are used in memory CRTs, called potentialoscopes.
GAS DISCHARGE DEVICES
The principle of operation of gas discharge devices (GDP) is based on electrical phenomena occurring in a gaseous medium.
Hydraulic fracturing cylinders are filled with inert gases (neon, argon, helium, etc.), their mixtures, hydrogen or mercury vapor. Under normal conditions, most of the atoms and molecules of a gas are electrically neutral and the gas is a good insulator. An increase in temperature, exposure to strong electric fields or particles with high energies causes ionization of the gas. Ionization of a gas that occurs when fast-flying electrons collide with neutral gas atoms is called impact ionization. It is accompanied by the appearance of free electrons and positive ions, which leads to a significant increase in the electrical conductivity of the gas. A strongly ionized gas is called an electron-ion plasma or simply plasma.
Along with the process of gas ionization, there is also an inverse process called recombination. Since the energy of an electron and a positive ion is greater than the energy of a neutral atom, during recombination, a part of the energy is released, which is accompanied by a glow of the gas.
Passing process electric current through a gas is called an electrical discharge in the gas. The current-voltage characteristic of the gas-discharge gap is shown in fig. 3.8.
At a voltage U 3 , called the ignition voltage, the ionization of the gas becomes an avalanche. The resistance of the gas-discharge gap between the anode and the cathode decreases sharply, and a glow discharge appears in the HF (section CD). The burning voltage U r supporting the glow discharge is somewhat less than the ignition voltage. In a glow discharge, positive ions move towards the cathode and, hitting its surface, increase the number of electrons emitted from it due to heating and secondary
noah electron emission. Since an external ionizer is not required in this case, the glow discharge is called self-sustained, in contrast to the discharge in section AB, which requires an external ionizer (cosmic radiation, thermionic emission, etc.) for its appearance and is called non-self-sustaining. With a significant increase in current, an arc discharge occurs in the hydraulic fracturing (section EF). If the arc discharge is supported by thermionic emission of the cathode due to its heating by positive ions hitting the surface, the discharge is called self-sustaining. If the thermionic emission of the cathode is created by heating it from an external voltage source, then the arc discharge is called non-self-sustaining.
Glow discharge, accompanied by gas glow, is used in neon lamps, gas-discharge sign and linear indicators, zener diodes and some other hydraulic fracturing.
gas discharge indicators. iconic gas discharge indicators consist of a gas-filled cylinder, ten cathodes and one common anode. Cathodes are in the form of numbers, letters, or other characters. Voltage is applied to the anode and one of the cathodes through a limiting resistor. A glow discharge occurs between these electrodes, which has the shape of a cathode. By switching different cathodes, different signs can be displayed. Segment sign indicators are more versatile. Thus, the segment glow discharge indicator IN-23, consisting of 13 segments, allows, with appropriate switching of cathodes-segments, to highlight any number from 0 to 9, a letter of the Russian or Latin alphabet.
Linear gas discharge indicators (LGI) display information about the voltage or current in the circuit in the form of luminous dots or lines. The position of the dot and the length of the line are proportional to the voltage or current in the circuit. The LGI electrode system has an elongated cylindrical shape.
Gas-discharge zener diode. The zener diode (Fig. 3.9, a) has two electrodes - cathode 1, made in the form of a hollow cylinder, and anode 3 in the form of a thin rod located along the cathode OSB. To reduce the ignition voltage, a small pin 2, called the ignition electrode, is welded on the inside of the cathode.
The operation of a glow discharge zener diode is based on maintaining an almost constant burning voltage on its electrodes when the current flowing through the zener diode changes within significant limits (section CD in Fig. 3.8).
Zener diodes are used to stabilize the voltage in DC circuits.
Thyratron. A more complex hydraulic fracturing is a thyratron. It contains a cathode, an anode, and one or more control electrodes called grids. The thyratron can be in two stable states: non-conductive and conductive. On fig. 3.9, b shows the device of a thyratron with a cold cathode of the MTX-90 type. The thyratron consists of a cylindrical cathode 1, a rod metal anode 2 and a metal mesh 3 made in the form of a washer. When a small positive voltage relative to the cathode is applied to the grid, an auxiliary “quiet” discharge occurs between the grid and the cathode. When a positive voltage is applied to the anode, the discharge is transferred to the anode. The higher the auxiliary discharge current in the grid circuit, the lower the ignition voltage of the thyratron. After the occurrence of a discharge between the cathode and the anode, a change in the grid voltage does not affect the current strength of the thyratron, and the current through the thyratron can be stopped by reducing the anode voltage to a value lower than the burning voltage.
Glow discharge thyratrons consume very little energy, operate in a wide temperature range, are not sensitive to short-term overloads, and are ready for instantaneous action. Due to these qualities, they are used in pulse devices, generators, some nodes of calculating devices, in relay equipment, display devices, etc.
PHOTOELECTRIC DEVICES
Electrovacuum and gas-discharge photovoltaic devices include photocells and photomultipliers, the principle of operation of which is based on the use of an external photoelectric effect.
The photocell (Fig. 3.10) has a glass bulb 2, in which a vacuum is created (electrovacuum photocell
ment) or which is filled with an inert gas (gas-discharge photocell) It consists of an anode and a photocathode. The photocathode is the inner surface of the flask 3 (with the exception of a small area - window 1), covered with a layer of silver, on top of which a layer of cesium oxide is deposited. The anode 4 is made in the form of a ring so as not to interfere with the light flux. The anode and cathode are provided with terminals 6 passing through the plastic holder 5 of the flask.
When a photocathode is illuminated with a light flux, electrons are knocked out of it. If a positive voltage relative to the cathode is applied to the anode, the electrons knocked out of the photocathode will be attracted to the anode, creating a photocurrent I f in its circuit. The dependence of the photocurrent on the light flux Ф is called the light ha-
characteristics of the photocell. The photocurrent also depends on the voltage U applied between the photocathode and the anode. This dependence is called the anode CVC. It has a pronounced saturation region, on which the photocurrent depends little on the anode voltage (Fig. 3.11, a)
In gas-discharge photocells, an increase in voltage U causes gas ionization and an increase in photocurrent (Fig. 3.11, b).
Due to the small value of the photocurrent (up to several tens of microamperes for vacuum photocells and several microamperes for gas-discharge photocells), photocells are usually used with tube or transistor amplifiers.
A photomultiplier tube (PMT) is called an EEW, in which the photoelectron emission current is enhanced by secondary electron emission. In the PMT glass container (Fig. 3.12), in which a high vacuum is maintained, in addition to the photocathode K and anode A, there are additional electrodes that are emitters of secondary electrons and are called dynodes. The number of dynodes in a PMT can reach 14. Positive voltages are applied to the dynodes, and as the distance from the photocathode increases, the dynode voltages increase. The voltage between adjacent dynodes is about 100 V. When the photocathode is illuminated, electrons fly out from its surface, which are accelerated by the electric removal by the field of the first
dynodes and fall on the first dynode, knocking out secondary electrons from it. The number of the latter is several times greater than the number of electrons emitted from the photocathode. Under the action of an electric field between the first and second dynodes, the electrons that have flown out of the first dynode fall on the second dynode D2, knocking out secondary electrons from it. The number of secondary electrons knocked out of the D2 dynode is several times greater than the number of electrons that hit it. Thus, an increase in the number of secondary electrons occurs at each dynode. Consequently, in the PMT there is a multiple amplification of the photocurrent of the cathode, which allows them to be used to measure very low light fluxes. The PMT output current reaches several tens of milliamps.
Control questions and tasks
1. Explain the principle of controlling the anode current in the EUL using the control grid voltage.
2. Name the main parts of a CRT with electrostatic beam steering and explain their purpose.
3. Name the main types of gas discharge devices and areas
their applications.
4. Give brief description external photoelectric effect. How
How is this phenomenon used in photocells and photomultipliers?
Similar information.
Electrovacuum devices are widely used. With the help of these devices, it is possible to convert electrical energy of one type into electrical energy of another type, which differs in shape, magnitude and frequency of current or voltage, as well as radiation energy into electrical energy and vice versa.
With help electrovacuum devices Press wall birthday Gorreklama Voronezh.
it is possible to carry out the regulation of various electrical, light and other quantities smoothly or in steps, at high or low speed and with low energy costs for the regulation process itself, i.e. without a significant reduction in efficiency, characteristic of many other methods of regulation and control.
These advantages of electrovacuum devices led to their use for rectification, amplification, generation and frequency conversion of various electric currents, oscillography of electrical and non-electrical phenomena, automatic control and regulation, transmission and reception of television images, various measurements and other processes.
Electrovacuum devices are devices in which the working space, isolated by a gas-tight shell, has a high degree rarefaction or filled with a special medium (vapours or gases) and whose action is based on the use of electrical phenomena in a vacuum or gas.
Electrovacuum devices are divided into electronic devices, in which a purely electronic current passes in a vacuum, and ion devices (gas-discharge), which are characterized by an electric discharge in a gas or vapor.
In electronic devices, ionization is practically absent, and if it is observed to a small extent, it does not have a noticeable effect on the operation of these devices. The rarefaction of gas in these devices is estimated by the pressure of residual gases less than 10-6 mm Hg. Art., characteristic of high vacuum.
In ion devices, the pressure of residual gases is 10-3 mm Hg. Art. and higher. At such a pressure, a significant part of the moving electrons collide with gas molecules, leading to ionization, and, therefore, in these devices, the processes are electron-ion.
The action of conductive (non-discharge) electrovacuum devices is based on the use of phenomena associated with electric current in solid or liquid conductors in a rarefied gas. In these devices, there is no electric discharge in a gas or in a vacuum.
Electrovacuum devices are divided according to various criteria. A special group is vacuum tubes, i.e. electronic devices designed for various transformations electrical quantities. According to their purpose, these lamps are generator, amplifying, rectifier, frequency converter, detector, measuring, etc. Most of them are designed to operate in continuous mode, but they also produce lamps for pulsed mode. They create electrical impulses, that is, short-term currents, provided that the duration of the impulses is much less than the intervals between the impulses.
Electrovacuum devices are also classified according to many other criteria: by the type of cathode (heated or cold), by the design of the cylinder (glass, metal, ceramic or combined), by the type of cooling (natural, i.e. radiant, forced air, water).
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Electrovacuum devices include electrical devices, the operation of which is based on the use of flow electric charges in vacuum or in a rarefied gas medium.
By vacuum is meant the state of a gas, in particular air, at a pressure below atmospheric pressure. If the electrons move freely in space, without colliding with the molecules remaining after pumping out the gas, then
talk about high vacuum.
Electrovacuum devices are divided into electronic, in which the flow of electric current in vacuum is observed, and ionic (gas-discharge), which are characterized by an electric discharge in a gas (or vapor). In electronic devices, ionization is practically absent, and the gas pressure is not less than 100 µPa (10-6-10-7 mm Hg).
In ion devices, the pressure is 133×10-3 Pa (10-3 mm Hg) and higher. At
In this case, a significant part of the moving electrons collides with gas molecules and ionizes them.
Electronic devices are called vacuum tubes.
The classification of electronic devices is carried out according to the following criteria:
Purpose and scope,
Number of electrodes,
Type of cathode (direct or indirect heating),
Electronic flow control method.
Electronic devices are divided into:
1. Rectifier lamps (kenotrons) designed to convert
alternating current to direct current.
2. Receiving-amplifying lamps designed to amplify and convert
the formation of high-frequency oscillations in receivers and to improve the oscillation
low frequency bans in receivers and amplifiers.
Depending on the number of electrodes, receiving-amplifying lamps are divided into:
Two-electrode (diodes), having two electrodes - a cathode and an anode (diodes are used to detect (rectify) high-frequency currents, convert low-frequency currents and various automatic control
Three-electrode (triodes), which, in addition to the cathode and anode, have a third electrode, a control grid (triodes are used to amplify low-frequency oscillations and in many special circuits);
Four-electrode (tetrodes) having a cathode, an anode and two grids (tetrodes are used to powerfully amplify low-frequency oscillations);
Five-electrode (pentodes) having a cathode, an anode, and three grids (pentodes are used to amplify high and low frequency oscillations, powerful pentodes are used to amplify the power of low frequency oscillations);
Multielectrode (four grids - hexodes, five grids - heptodes, six grids - octodes) are used for frequency conversion in receivers;
Combined, containing two or more systems of electrodes with independent
my streams of electrons. There are the following types of combined vacuum tubes: double diode, double triode, double tetrode, double
diode - triode, double diode - tetrode, diode - tetrode, diode - pentode, double
diode - pentode, triode - pentode, double beam tetrode, etc.
3. Generator and modulating lamps. These lamps are more powerful than the receiver-amplifiers. They are used to generate high frequency oscillations, amplify these oscillations in power and for modulation.
Generator and modulator lamps are three-electrode, four-
electrode and five-electrode.
4. Ultra-high frequency lamps designed specifically for operation in the ultra-short wave (VHF) range. Some of these lamps work on the same principle as conventional lamps, and differs from them only in size. Another part of the VHF band lamps has a special design. Finally,
In the VHF range, klystrons and magnetrons are used, the operation of which is based on completely different principles than the operation of a conventional electron tube.
Rice. 1.1 Appearance some types of lamps:
a and b - receiving-amplifying glass lamps; c - baseless mini-
tyurnaya lamp; g - metal receiving-amplifying lamp; e -
high power glass baseless lamp; e - cermets -
calic pulse
5. Electron-beam devices. These include kinescopes (receiving television tubes), transmitting television tubes, oscilloscope and memory tubes, image intensifier tubes, cathode beam switches, indicator tubes of radar and hydroacoustic stations, etc.
The appearance of lamps of some types is shown in fig. 1.1.
Electrovacuum devices are also classified:
1. According to the material and design of the cylinder:
Glass;
Metal;
Ceramic;
Combined.
2. By type of cooling:
Natural, or radiant;
Forced - air, water, steam.
Classification of gas-discharge devices is made according to the type of discharge occurring in the gas. Three types of gas-discharge devices are used in radio engineering equipment:
a) Glow discharge devices. These devices have a cold, not heated
the cathode is used and is used mainly for voltage stabilization.
b) Arc discharge devices with a liquid or solid non-heated cathode.
c) Arc discharge devices with an artificially heated cathode. These devices are used to rectify AC to DC and
various control and automation schemes.