Electrovacuum devices: principle of operation, examples. Thomas Edison's incandescent light bulbs

Electrovacuum devices have become widespread. With the help of these devices, it is possible to convert electrical energy of one type into electrical energy of another type, differing 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 of Gorreklama Voronezh.

it is possible to regulate various electrical, light and other quantities smoothly or in steps, at high or low speed and with low energy consumption for the regulation process itself, i.e. without a significant decrease in efficiency, characteristic of many other methods of regulation and control.

These advantages of electrovacuum devices have 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 of vacuum or is filled with a special medium (vapors or gases) and the action of which 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 ionic devices (gas discharge), which are characterized by an electric discharge in a gas or vapor.

In electronic devices, ionization is practically absent, and if observed to a small extent, it does not have a noticeable effect on the operation of these devices. The gas rarefaction in these devices is estimated at a residual gas pressure of 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 this pressure, a significant part of the moving electrons collides with gas molecules, leading to ionization, and, therefore, in these devices the processes are electron-ionic.

The operation of conductor (discharge-free) electric vacuum devices is based on the use of phenomena associated with electric current in solid or liquid conductors located in a rarefied gas. In these devices there is no electrical discharge in gas or vacuum.

Electrovacuum devices are divided according to various criteria. A special group consists of vacuum tubes, i.e. electronic devices designed for various transformations of electrical quantities. These lamps, according to their purpose, are generators, amplifiers, rectifiers, frequency converters, detectors, measuring lamps, etc. Most of them are designed to operate in continuous mode, but lamps are also produced for pulsed mode. They create electrical impulses, i.e. short-term currents, provided that the duration of the impulses is much shorter than the intervals between impulses.

Electrovacuum devices are also classified according to many other criteria: by the type of cathode (hot 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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Using electrovacuum devices (EVDs), it is possible to convert electrical quantities, such as current or voltage, in shape, value and frequency, as well as radiation energy and vice versa. It is possible to carry out complex transformation of an optical image into electricity special shape or vice versa (in television and oscilloscope tubes). It is possible to regulate electrical, light and other quantities smoothly or in steps at high or low speed and with low energy consumption for the regulation process itself, i.e. without a significant decrease in efficiency. The low inertia characteristic of EVPs allows them to be used in a huge frequency range from zero to 1012 Hz.

These advantages of electronic devices have led to their use for rectification, amplification, generation, frequency conversion, 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 of vacuum or is filled with a special medium (vapors or gases) and the action of which is based on the use of electrical phenomena in a vacuum or gas.

Vacuum should be understood as the state of a gas, in particular air, at pressures below atmospheric. In relation to EVP, the concept of “vacuum” is defined based on the nature of the movement of electrons. If electrons move freely in space, without colliding with the molecules remaining after pumping out the gas, then they speak of a vacuum. And if electrons collide with gas molecules, then we should simply talk about a rarefied gas.

Electrovacuum devices are divided into electronic, in which a purely electronic current passes in a vacuum, and ionic (gas-discharge), which is characterized by an electric discharge in a gas (or vapor).

In electronic devices, ionization is practically absent, and gas rarefaction by pressure less than 100 μPa, characteristic of high vacuum.

In ion devices the pressure is 133 * 10 -3 Pa and higher. In this case, a significant part of the moving electrons collides with gas molecules and ionizes them.

There is another group of conductive (discharge-free) EVPs. Their action is based on the use of phenomena associated with electric current in solid or liquid conductors located in a discharged gas. In these devices electric charge not in gas or vacuum. These include incandescent lamps, current stabilizers, vacuum capacitors, etc.

A special group of EVPs consists of electronic tubes intended for various transformations of electrical quantities. These lamps are generator, amplifier, rectifier, frequency converter, detector, measuring, etc.

Depending on the operating frequencies, vacuum tubes are divided into low-frequency, high-frequency and ultra-high-frequency.

In all EVPs, the electron flow can be regulated by influencing it with an electric or magnetic field. Electronic tubes that have two electrodes - a cathode and an anode - are called diodes. Diodes for rectifying alternating current in power supplies are called kenotrons. Lamps that have control electrodes in the form of grids come with the number of electrodes from three to eight and are respectively called: triode, tetrode, pentode, hexode, heptode and octode. In this case, lamps with two or more grids are classified into the group of multi-electrode lamps. If the lamp contains several systems of electrodes with independent electron flows, then it is called combined (double, diode, double triode, triode-pentode, double diode-pentode, etc.).

The main ion devices are thyratrons, zener diodes, lamps with sign indication, mercury valves (controlled and uncontrolled), ion arresters, etc.

A large group consists of cathode ray devices, which include picture tubes (television receiving tubes), transmitting television tubes, oscillographic and storage tubes, electron-optical image converters, cathode ray switches, indicator tubes of radar and hydroacoustic stations, etc.

The group of photoelectronic devices includes electrovacuum photocells (electronic and ionic) and photoelectronic multipliers. Electric lighting devices include incandescent lamps, gas-discharge light sources and fluorescent lamps.

A special place is occupied by X-ray tubes, counters of elementary particles and other special devices.

Electrovacuum devices are also classified according to other criteria: by the type of cathode (hot or cold), by the material and design of the cylinder (glass, metal, ceramic, combined), by the type of cooling (natural or radiant, and forced - air, water, steam ).

Electrovacuum devices (EVDs) are devices in which an electric current is created by a flow of electrons or ions moving in a high vacuum or inert gas environment. EVPs are divided into electronically controlled lamps (ECL), cathode ray tubes (CRT), gas discharge devices (GD) and photoelectric (photoelectronic) devices.

In an EUL, an electric current is created by the movement in a high vacuum (gas pressure is only 1.33 () Pa (mm Hg)) of electrons 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. The electrons receive this energy when the cathode is heated with an electric current. The emission of electrons by a heated cathode is called thermionic emission.

The negative space charge formed by electrons escaping from the cathode creates an electric field at its surface, which prevents electrons from leaving the cathode, forming a potential barrier on their path.

A voltage positive relative to the cathode is applied to the anode, which reduces the potential barrier at the cathode surface. Electrons, the energy of which 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. As the anode voltage increases, 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 no current flows through the diode. This is an important feature of the diode - its one-way electrical conductivity.

In Fig. Figure 3.1 shows the symbols of diodes and diagrams 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). The 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 a certain negative grid voltage, called the turn-off voltage U CK .з 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 it creates 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 much more strongly 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 voltage.

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 = f (U ak), taken at different grid voltages (Fig. 3.3, b).

The disadvantages of the triode are the large feed-through capacitance (capacitance between the grid and the anode) and low static gain. These disadvantages 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 a 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 effect of the second grid, the throughput capacity of the lamp and the influence 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 shielding one, which is equal to or slightly less than the anode voltage. In this case, part of the electrons hits the shielding grid and creates a current I c2 of this grid.

Electrons hitting the anode knock out secondary electrons from it. When (and such cases occur during tetrode operation), secondary electrons are attracted by the shielding grid, which leads to an increase in the shielding grid current and a decrease in the anode current. This phenomenon is called the dynatron 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 shielding mesh. 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, which, as a rule, has zero potential, between the screening grid and the anode.

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 ensured by electrically connecting it to the cathode.

The main characteristics of tetrodes and pentodes are the 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 EUL are:

slope of the anode-grid characteristic

internal (differential) resistance

static gain

The parameters S, and , called differential, are related to each other by the relation.

Cathode ray tubes

Cathode ray tubes (CRTs) are electronic vacuum devices that use a stream of electrons concentrated in the form of a beam. These devices have the shape of a tube extended in the direction of the beam. The main elements of a CRT are a glass cylinder, or bulb, an electronic spotlight, a deflection system and a screen (Fig. 3.6).

Cylinder 7 serves to maintain the required vacuum in the CRT and protect the electrodes from mechanical and

climate impacts. Part of the inner surface of the cylinder is covered with graphite film 8, called aquadag. A voltage positive relative to the cathode is applied to the aquadag.

An electronic spotlight is designed to create a focused electron beam (beam) with the required current density. It consists of a thermionic cathode 2, inside of 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 a cylindrical cathode.

The modulator is connected to a source of negative voltage, 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.

A non-uniform electric field is formed between the modulator and the first anode, which focuses all the electrons emitted from the cathode and passing through the modulator hole at a certain point on the CRT axis in the cavity of the first anode. This 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 CRT axis 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. Consequently, the modulator voltage determines the density of the electron beam and the brightness of the luminous spot on the CRT screen. Focusing the beam 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 deflection 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 an electron beam is deflected by an electric field (electrostatic deflection), deflection 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 higher potential. The plates, the electric field between which deflects the electron beam in the horizontal direction, are called horizontal deflection or X-plates, and in the vertical direction - vertical deflection or Y-plates.

The main parameter of an 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 a current passing through two pairs of coils located mutually perpendicular to the neck of the CRT.

The screens of 9 cathode ray tubes, used to convert electrical signals into light, are coated with a special composition - a phosphor, which glows when a focused stream of electrons 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 produce color images contain phosphor grains with blue, red and green luminescence - triads arranged in a certain order. In the neck of the tube there are three autonomous electronic spotlights. They are located in such a way that their electron beams intersect at some distance from the screen. A shadow mask is installed in the plane of intersection of the rays, in which there is a large number of holes. After passing through the holes in the mask, each of the electron beams hits its element of the triad (Fig. 3.7).

By mixing three colors of different brightness, a glow of the desired color is obtained.

In addition to fluorescent ones, there are dielectric screens. An electron beam, moving across such a screen, creates various charges in its sections, i.e., a kind of potential relief that can persist for a long time. Dielectric screens are used in storage CRTs, called potentialoscopes.

GAS DISCHARGE DEVICES

The operating principle of gas discharge devices (GD) is based on electrical phenomena occurring in a gaseous environment.

Hydraulic fracturing cylinders are filled with inert gases (neon, argon, helium, etc.), their mixtures, hydrogen or mercury vapor. Under normal conditions, most atoms and molecules of a gas are electrically neutral and the gas is a good dielectric. An increase in temperature, exposure to strong electric fields or high-energy particles causes ionization of the gas. Gas ionization that occurs when fast-moving 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 highly ionized gas is called electron-ion plasma or simply plasma.

Along with the process of gas ionization, there is also a reverse process called recombination. Since the total energy of an electron and a positive ion is greater than the energy of a neutral atom, during recombination a portion of the energy is released, which is accompanied by the glow of the gas.

The process of passing an electric current through a gas is called an electrical discharge in a gas. The current-voltage characteristic of the gas-discharge gap is shown in Fig. 3.8.

At voltage U 3 , called the ignition voltage, gas ionization takes on an avalanche-like character. The resistance of the gas-discharge gap anode - cathode decreases sharply, and a glow discharge appears in the gas discharge (section CD). The combustion voltage U r, which supports the glow discharge, is somewhat less than the ignition voltage. During 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

no electron emission. Since an external ionizer is not required, the glow discharge is called self-sustaining, in contrast to the discharge in the AB section, 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 in the hydraulic fracturing zone, an arc discharge occurs (section EF). If the arc discharge is supported by thermionic emission of the cathode due to its heating by positive ions striking the surface, the discharge is called self-sustaining. If thermionic emission of the cathode is created by its heating from an external voltage source, then the arc discharge is called non-self-sustaining.

A glow discharge, accompanied by a gas glow, is used in neon lamps, gas-discharge sign and linear indicators, zener diodes and some other hydraulic fracturing devices.

Gas discharge indicators. Significant 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 symbols. 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. Segmental sign indicators are more universal. Thus, the IN-23 segment glow discharge indicator, consisting of 13 segments, allows, with appropriate switching of the segment cathodes, to highlight any number from 0 to 9, a letter of the Russian or Latin alphabet.

Linear gas-discharge indicators (LGI) display information about voltage or current in a circuit in the form of luminous dots or lines. The position of the point 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 axis. 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 combustion 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 voltage in DC circuits.

Thyratron. A more complex hydraulic fracturing device is the thyratron. It contains a cathode, an anode, and one or more control electrodes called grids. A thyratron can be in two stable states: non-conducting and conducting. In 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 voltage, positive 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 greater the auxiliary discharge current in the grid circuit, the lower the thyratron ignition voltage. After a discharge occurs between the cathode and anode, changing 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 less than the combustion voltage.

Glow discharge thyratrons consume very little energy, operate over a wide temperature range, are not sensitive to short-term overloads, and are ready for instant action. Due to these qualities they are used in pulse devices, generators, some units of computers, relay equipment, display devices, etc.

PHOTOVOLTAIC DEVICES

Electrovacuum and gas-discharge photoelectric devices include photocells and photomultipliers, the operating principle of which is based on the use of an external photoelectric effect.

The photocell (Fig. 3.10) has a glass flask 2 in which a vacuum is created (electric vacuum 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 bulb 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 applied. Anode 4 is made in the form of a ring so as not to interfere with the light flow. The anode and cathode are equipped with leads 6 passing through a plastic holder 5 of the flask.

When the photocathode is illuminated by a light flux, electrons are knocked out of it. If a voltage positive relative to the cathode is applied to the anode, the electrons knocked out from the photocathode will be attracted to the anode, creating a photocurrent I f in its circuit. The dependence of the photocurrent on the luminous flux Ф is called the luminous cha-

characteristics of the photocell. The photocurrent also depends on the voltage U applied between the photocathode and the anode. This dependence is called the anodic current-voltage characteristic. It has a pronounced saturation region, in 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 low value of the photocurrent (up to several tens of microamps for vacuum photocells and several units of microamps for gas-discharge photocells), photocells are usually used with lamp or transistor amplifiers.

A photomultiplier tube (PMT) is an EVP in which the photoelectron emission current is amplified due to secondary electron emission. In the glass container of the photomultiplier (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 photomultiplier can reach 14. Positive voltages are applied to the dynodes, and the dynode voltages increase with distance from the photocathode. 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 electrical removal field of the first

dynode 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 influence of an electric field between the first and second dynodes, electrons emitted from the first dynode enter the second dynode D2, knocking out secondary electrons from it. The number of secondary electrons knocked out of dynode D2 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 PMTs, the photocurrent of the cathode is multiplied, which makes it possible to use them for measuring very low light fluxes. The output current of the PMT reaches several tens of milliamps.

Test questions and assignments

1. Explain the principle of controlling the anode current in the EUL using the control grid voltage.

2. Name the main parts of an electrostatic beam control CRT and explain their purpose.

3. Name the main types of gas-discharge devices and areas
their applications.

4. Give a brief description of the external photoelectric effect. What
How is this phenomenon used in photocells and photomultipliers?

Related information.

Definition . Electric vacuum devices are devices whose operating principle is based on the use of electrical phenomena in gases or vacuum occurring in a working space isolated from the environment by a gas-tight shell (cylinder).

Electrovacuum and gas-discharge devices are made in the form of a glass, ceramic or metal cylinder, inside of which electrodes are placed under conditions of high vacuum or inert gas: cathode, anode, grids. The cathode is a radiator (emitter) of free electrons, the anode is a collector (collector) of charge carriers. The anode current is controlled using grids or control electrodes.

In order to get an idea of ​​the electric vacuum and gas discharge devices used in aviation electronic equipment, let’s consider their classification.

Classification and symbolic graphic designation

1. Based on the number of electrodes, electronic devices are divided into two-electrode (vacuum diode), three-electrode (vacuum triode), and multi-electrode lamps.

Rice. 1.

Electrovacuum diode - This is a two-electrode lamp consisting of a cathode and an anode. If the voltage at the anode is positive relative to the cathode, then the electrons emitted by the cathode move towards the anode, creating an anode current. When the voltage is negative, there is no current at the anode, therefore the diode conducts in only one direction. This property of the diode determines its main purpose - rectifying alternating current. The symbolic graphic designation of an electric vacuum diode is shown in Fig. 1.

Electrovacuum triode- This is a three-electrode lamp in which a grid is located between the anode and cathode. The grid is designed to regulate the anode current. The grid voltage changes the field between the anode and cathode and thus affects the anode current. If the voltage on the grid is negative relative to the cathode, then it has a inhibitory effect on the electrons emitted by the cathode, as a result of which the anode current decreases. When the grid voltage is positive, it has an accelerating effect on electrons, increasing the anode current. In this case, part of the electrons hits the grid creating a grid current. Consequently, the grid is a control electrode, the voltage on which allows you to change the anode current.

The conventional graphic designation of an electric vacuum triode is shown in Fig. 2.

Rice. 2.

To increase the effect on the anode current, the grid is located closer to the cathode. When the voltage on the grid is negative, there is practically no current in it.

Rice. 3. Conventional graphic designation of triodes: a - with a cathode grid; b - with screen grid

TO multigrid lamps relate: tetrodes- with two grids, pentodes- with three grids, hexodes- with four grids, heptodes- with five grids and octodes- with six grids. The most common are tetrodes and pentodes.

U tetrodes one of the grids is called the control grid and has a negative voltage. The other grid is located either between the control and the anode or between the control and the cathode. In the first case, such a grid is called shielding, in the second - cathode.

The conventional graphic designation of electric vacuum tetrodes is shown in Fig. 3.

In tetrodes with a screening grid, the cathode current is distributed between the screening grid and the anode. The main advantage of such a tetrode is the reduction in capacitance between the anode and the control grid. The shielding mesh reduces this capacitance to fractions of a picofarad and reduces the permeability of the anode.

However, the proximity of the shielding grid to the anode has the disadvantage that at low voltage the anode appears dynatron effect- reduction in anode current due to secondary emission (dip in the anode characteristic (Fig. 3.4)). In this case, secondary electrons do not return back to the cathode, but are captured by the screening grid.

Pentode called a lamp with three grids. The introduction of the third grid is due to the need to eliminate the dynatron effect characteristic of the tetrode. This grid is called protective (or antidynatron) and is located between the shielding grid and the anode. The voltage on this grid is usually made equal to the voltage on the cathode; for this purpose, it is sometimes connected to the cathode inside the flask. The dynatron effect is eliminated due to the potential barrier formed in the space between the anode and the screening mesh. At the same time, this potential barrier does not pose a significant obstacle to electrons moving towards the anode at high speed.

2. According to the design features of the filament circuit, electronic tubes are divided into lamps with directly heated cathodes and lamps with indirectly heated cathodes.

Direct filament cathode is a metal filament made of a material with high resistance (tungsten or tantalum), through which an incandescent current passes. This cathode is characterized by low heat losses, simplicity of design and low thermal inertia. The disadvantage of such a cathode is that it must be powered with direct current. When powered by alternating current with a frequency of 50 Hz, the emission current changes with twice the frequency of the supply voltage, which creates an unwanted low-frequency background noise.

Indirect filament cathode represents a tube containing a filament inside. The filament is isolated from the cathode. As a result, the temperature and emission current pulsations when powering the filament with alternating current are practically smoothed out.

• 3. By purpose lamps are divided into receiver-amplifiers, generator, frequency converter, detector, measuring and so on.
• 4. Depending on operating frequency range distinguish between lamps low ( from 1 - 30 MHz), high(from 30 to 600 MHz) and ultra-high(over 600 MHz) frequencies.
• 5. By type of electronic emission distinguish lamps with thermionic, secondary And photoelectronic emissions.

Electron emission is necessary to create an electron flow inside an electric vacuum device between the electrodes.

Thermionic emission is the process of electrons leaving solid or liquid bodies into a vacuum or gas.

Secondary electron emission refers to the emission of electrons by a body due to bombardment by electrons emitted by another body.

Photoelectron emission refers to the emission of electrons by a body located in a flow of radiant energy.

2.1.2 Characteristics and parameters

The characteristics of the lamp express the dependence of currents on voltages in its various circuits. The properties of electron tubes are assessed by anodic or anode-grid static characteristics.

Anode a static characteristic is a graphically expressed dependence of the anode current I a from the voltage at the anode U a. Addiction I a = f(U a) is removed for several constant voltage values U With(the exception is the anode characteristics of the diode). Appearance the anode characteristic is determined by the number of electrodes in the lamp (Figure 4).

Rice. 4. Anode characteristics of electronic tubes: a - diode; b - triode; c - tetrode; g - pentode

Anode-grid static characteristics are graphically expressed dependences of the anode current I A from grid voltage U c at fixed values ​​of anode voltage U A. Same as for anodic dependence characteristics I A = f(U With ) taken for several constant values ​​of the anode voltage Ua. (Figure 5).

The higher the anode voltage U A, the higher and to the left the anode-grid characteristics are located I A = f(U With ) . This is explained by the fact that at a higher anode voltage, a greater negative voltage must be applied to the grid so that the resulting electric field in the space between the cathode and the grid remains unchanged in magnitude.

TO basic electrical parameters vacuum diodes include the following: vacuum gas discharge device

1. Internal DC resistance:

Where U A- constant component of the anode voltage, I A- constant component of the anode current.

Rice. 5. Anode-grid characteristics of electron tubes: a - triode; b - pentode

2. Internal differential resistance R d A diode represents the resistance of the space between the anode and cathode for alternating current. It is the reciprocal of the slope and is determined using the anode static characteristics (Fig. 3.4, a):

and usually amounts to hundreds and sometimes tens of ohms.

Usually resistance R 0 more R d .

3. Slope S shows how the anode current changes when the anode voltage changes and is expressed by the following dependence:

• 4. Filament voltage U n- voltage supplied to the heater. This value is a passport value. When the lamp is underheated, the cathode temperature decreases, and therefore the emission current. When the filament voltage increases sharply U n the service life of the cathode is sharply reduced, so the filament voltage should not deviate by more than 10% from the nominal one.
• 5. Emission current I e - the maximum current that can be obtained as a result of the emission of electrons by the thermionic cathode. It is represented by the total charge of electrons that left the thermionic cathode in one second.
• 6. Acceptable reverse voltage diode U arr max- the maximum negative voltage at the anode that the diode can withstand without violating the properties of one-way conductivity.

The parameters of some serial vacuum diodes are given in Table. 1.

Table 1. Main parameters of serial vacuum diodes

The main electrical parameters of electronic tubes consisting of three or more electrodes include:

1. The internal (output) resistance of the lamp is the resistance of the the anode-cathode gap of the lamp for the alternating component of the anode current is determined by the formula:

Where U A - change in voltage at the anode, V; I A- change in anode current, mA. For vacuum diodes, the internal resistance is called alternating current resistance and is defined as:

2. Slope of characteristic S shows how many milliamps the lamp anode current will change when the voltage on the control grid changes by 1 V at constant voltages on the anode and other grids:

Where U With - change in grid voltage, V.

It should be noted that the greater the steepness, the stronger the control action of the grid and the higher the lamp gain can be obtained, all other things being equal.

3. Static gain shows how many times a change in voltage on the first grid has a stronger effect on the anode current than a change in the anode voltage. The gain is determined by the ratio of the change in the anode voltage to the change in the grid voltage, which equally affect the anode current:

4. The power dissipated at the anode is determined by the formula:

5. Output power Pout characterizes the useful power supplied by the lamp to the external circuit.

The parameters of some serial triodes, tetrodes and pentodes are given in table. 2.

Table 2. Basic parameters of serial triodes, tetrodes and pentodes

Electrovacuum devices.

1. Electrovacuum are devices in which electrical conductivity is carried out by electrons or ions moving between electrodes through a vacuum or gas. Electrovacuum devices are divided into electronically controlled lamps, electron beam And gas discharge devices.

The basic structural elements of any electric vacuum device are electrodes placed inside a cylinder (gas-tight shell). The electrode of an electric vacuum device is a conductor that emits (emit) or collects electrons (ions) or controls their movement from electrode to electrode using an electric field. Depending on the purpose, the following electrodes of an electric vacuum device are distinguished: cathode, anode and control ones.

^ Cathode– is a source of electrons in an electric vacuum device.

Anode– accelerating electrode - usually serves as both the output electrode and the main collector (collector) of electrons.

Managers called an electrode designed to control the main flow of electrons. If the control electrode is made in the form of a grid, it is often called a control grid. Electrodes are made in the form of threads, flat plates, hollow cylinders and spirals; they are fixed inside the cylinder on special holders - traverses and mica or ceramic insulators. The ends of the holders are soldered into the glass base of the cylinder.

Cylinders Electrovacuum devices are gas-tight shells made of glass, metal or ceramics. In cylinders of electronically controlled lamps a vacuum of 10 -8 ... 10 -4 Pa is created, and in cylinders of gas-discharge devices - 10 -1 ... 10 4 Pa.

^ The world's first electrovacuum device – the incandescent lamp was invented in 1873 by the Russian scientist A.N. Lodygin. In 1883, American inventor T.A. Edison discovered the effect of a one-way flow of electrons in a vacuum from a heated filament to a metal plate if a certain potential difference is applied to them, for example, by connecting it to a galvanic cell. This is how the prototype of the electron tube appeared. At that time, such a lamp could not find practical application, but work on studying its properties and the conditions for the passage of electrons in a vacuum continued.
^ 2. Physical basis of the operation of electronically controlled lamps.

Electronically controlled lamp is called an electrovacuum device, the operation of which is based on the control of a current limited by a space charge using electrode potentials. Depending on their purpose, electronically controlled lamps are divided into generator, modulator, control, amplification, and rectifier lamps. By type of work, continuous and pulsed lamps are distinguished, and by frequency range - low-frequency, high-frequency and ultra-high-frequency. Based on the number of electrodes, lamps are divided into diodes, triodes, tetrodes, pentodes, hexodes, heptodes, octodes, ennodes and decodes.

^ Electronic emission called the emission of electrons from the surface of substances into the surrounding space. In the metals from which the cathodes of electric vacuum devices are made, free electrons are in a state of chaotic continuous thermal motion and have a certain kinetic energy, depending on the temperature of the cathode.

Thermionic called the emission of electrons caused only by heating the cathode (electrode). As a result of heating the metal, the kinetic energy of electrons and their speed increase. The principle of operation of thermionic cathodes, which are widely used in electronically controlled lamps, is based on the phenomenon of thermionic emission.
^ 3. Electron beam devices.

Electron beam are called such electrovacuum devices that use a stream of electrons concentrated into a narrow beam - an electron beam controlled both in intensity and position in space. One of the most common cathode ray devices is a receiving cathode ray tube (CRT).

CRT transforms electrical signal into an optical image. There are several types of receiving CRTs: projection, oscillographic, indicator, sign-printing, color, monochrome, light valve and picture tubes.

Modern picture tubes use mixed beam control. An electric field is used to focus, and a magnetic field is used to deflect the beam.

^ CRT designation. The first element of the CRT designation is a number that indicates the size of the screen - its diameter or diagonal (for picture tubes with a rectangular screen). The second element is two letters indicating the type of tube (for example, LO - oscillographic with an electrostatic beam control system, LC - picture tubes with magnetic beam deflection). After the letters there is a number by which tubes of the same type with different parameters are compared. At the end of the designation there is a letter that determines the color of the screen (B - white, C - colored, I - green, A - blue, etc.). For example, 40LK6B is a kinescope with a screen size of 40 cm diagonally, the 6th design option, having White color screen glow. Typically, foreign manufacturing companies indicate the diagonal size of the kinescope in inches (1 inch equals 2.54 cm).
^ 4. Gas discharge devices. Physical principles of operation of gas-discharge devices.

An electric discharge in gases (or vapors) is a set of phenomena that occur in them during the passage of an electric current. Electrovacuum devices, the electrical characteristics of which are determined mainly by the ionization of intentionally introduced gas or steam, are called gas-discharge.

These include, for example, ion and mercury valves, thyratrons, ion arresters, glow discharge indicators.

Unlike electronically controlled lamps, in these devices not only electrons, but also charged particles (atoms, molecules) of gas or vapor - ions - participate in the creation of current.

^ Gas discharge devices They consist of a gas-tight cylinder (usually glass) filled with an inert gas, hydrogen or mercury vapor, and a system of metal electrodes. The gas pressure in the cylinder, depending on the type of device, ranges from 10 -1 to 10 3 Pa and sometimes reaches 10 4 Pa.

In the absence of exposure to ionization sources, gases consist of neutral atoms and molecules, so they practically do not conduct electric current. Current flows through a gas (as through any medium) only if there are free electrically charged particles - charge carriers - in this medium. In a gas, they can be formed if electrons are “torn off” from neutral atoms (or molecules) due to the action of some energy source. In this case, charge carriers of different signs are formed: electrons - negative charges and positive ions - gas atoms that have lost electrons - positive charges.

In real conditions, any gas is always affected (even if very weakly) by ambient temperature, cosmic and radioactive radiation industrial installations, etc., contributing to the formation of charged particles. Therefore, in any volume of gas there are always electrons and ions that can cause an electrical discharge. In an electric discharge, three processes are distinguished: excitation of atoms, their ionization and recombination of charge carriers of different signs.

Excitation of atoms is the process of transition of one of its outer electrons to an orbit more distant from the nucleus due to the energy acquired as a result of a collision with a free electron. This state of the atom is unstable and does not last long: from a few to tens of nanoseconds. The electron then returns to its original orbit, and the atom radiates the energy received during the collision into outer space. This energy is released in the form of electromagnetic radiation, often accompanied by a visible glow from the gas.

Atomic ionization is the process of formation of ions and free electrons from electrically neutral atoms.