An effort has been made to include sufficient analysis of the operation of the circuits to indicate clearly the operation and the various factors on which the operation depends. This has a twofold purpose, one of which is to indicate the procedure that must be adopted in effecting an analysis and the second of which is to indicate the factors on which the operation depends.
This is considered to be very important, since in some instances the tube plays a direct part in the operation of the circuit, whereas in others it may serve simply in the capacity of a switch. However, the mathematical developments are only a part of the analysis, since the discussion attempts to introduce the physical aspects of the problem and then to incorporate the mathematical results into the complete analysis.
A rather regrettable situation will be found to exist in the matter of notation. However, such single-subscript notation in electron-tube circuits is often inadequate, and double-subscript notation is employed, except for those particular cases where no confusion is likely to arise. The result is a mixed single-sub- script and double-subscript system of notation, the single-subscript terms generally conforming to the IRE notation.
A controversial matter is also to be noted. Throughout the text the symbols a-c and d-c are used as adjectives. Purists might object that the word current in a-c current is redundant and that the phrase a-c voltage is fundamentally meaningless. However, the use of the symbols a-c and d-c as descriptive adjectives is becoming increasingly widespread and does provide a clear and convenient abbreviation. A number of problems have been included at the end of each chapter.
These have been formulated in a way that requires an understanding of the subject matter. Problems which entail nothing more difficult than the substitution of numbers into equations have been kept to a minimum. Wherever possible, the problems are based on practical data in order to familiarize the student with such practical details. To provide proper acknowledgment of the source of much of this material proves to be an impossible task.
Much of the material that is principally of a radio-engineering character has appeared in one form or another in a wide variety of sources over many years, and the significant original sources seem to have been generally neglected. The principal source of many of the circuits which were extended for use in radar applications Avas the M. Radiation Laboratory, of which the author was a staff member during the war.
However, it is known that many of these circuits Avere adapted from existing circuits of diverse origin, whereas some Avere developed at other laboratories, including British laboratories. In only a feAV cases is the identity of the groups who did some of this work known. Millman and S. Certain of the material closely parallels that in the earlier book. The author wishes to acknoAvledge many helpful discussions with a number of his colleagues. He is particularly indebted to Professors David K. Cheng and Clenn M. Clasford, both for such discussions and for their assistance in proofreading portions of the text.
Thanks are also due to the General Electric Co. Syracuse, N. There are two important basic questions that relate to such tubes. One relates to the actual source of the electrons and their liberation, and the second relates to the control of the electron beam. A brief discussion of these matters will be included. According to modern theory, all matter is electrical in nature. The atom, which is one of the fundamental building blocks of all matter, consists of a central core or nucleus which is positively charged and which carries nearly all the mass of the atom.
Enough negatively charged electrons surround the nucleus so that the atom is electrically neutral in its normal state. Since all chemical substances consist of groups of these atoms which are bound to each other, then all matter, whether it is in the solid, the liquid, or the gaseous state, is a potential source of electrons. All three states of matter do, in fact, serve as sources of electrons. These processes will be considered in some detail in what follows.
With the release of the electrons, a means for their control must be provided. Such control is effected by means of externally controlled electric or magnetic fields, or both. These fields perform one or both of the following functions: 1 control of the number of electrons that leave the region near the emitter; 2 control of the paths of the electrons after they leave the emitter.
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Control method 1 is the more common, and such a control method is incorporated in almost all electron tubes, except those of the field-deflected variety. The cathode-ray tube is a very important example of a field-deflected tube. However, even in this latter case, a control of type 1 is incorporated to control the electron-tube current, even though the subsequent motion is controlled by means of an electric or a magnetic field, or both.
Thermionic Emission. Consider matter in the metallic state. Metals are most generally employed in the form of a wire or ribbon filament. This does not occur, however. Consider what happens to an electron as it seeks to escape from a metal. The escaping, negatively charged electron will induce a positive charge on the metal. There will then be a force of attraction between the induced charge and the electron. Unless the escaping electron possesses sufficient energy to carry it out of the region of influence of this image force of attraction, it will be returned to the metal.
The minimum amount of energy that is required to release the electron against this attractive force is known as the work function of the metal. This requisite minimum amount of energy may be supplied by any one of a number of different methods. One of the most important methods is to heat the metal to a high temperature. In this way, some of the thermal energy supplied to the metal is transferred from the lattice of the heated metal crystals into kinetic energy of the electrons. An explicit expression relating the thermionic-emission current density and the temperature of the metal can be derived.
It follows from Eq. Unfor- tunately, however, the low-work-function metals melt in some cases and boil in others, at the temperatures necessary for appreciable thermionic emission. The important emitters in present day use are pure tungsten, thoriated-tungsten, and oxide-coated cathodes. The thermionic-emis- sion constants of these emitters are contained in Table Superior numbers refer to references at the end of each chapter. In fact, this material is particularly impor- tant because it is virtually the only material that can be used successfully as the filament in high-voltage tubes.
It is used in high-voltage X-ray tubes, in high-voltage rectifier tubes, and in the large power-amplifier tubes that are used in radio and communication applications.
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It has been found that the application of a very thin layer of low-work- function material on filaments of tungsten will materially reduce the work function of the resulting surface. A thoriated-tungsten filament is obtained by adding a small amount of thorium oxide to the tungsten before it is drawn. It is found desirable to carbonize such an emitter, since the rate of evaporation of the thorium layer from the filament is thus reduced by about a factor of 6. Thoriated-tungsten filaments are limited in application to tubes that operate at intermediate voltages, say 10, volts or less.
Higher voltage tubes use pure tungsten filaments. It consists of a metal sleeve of konal an alloy of nickel, cobalt, iron, and titanium or some other metal, which is coated with the oxides of barium and strontium. These cathodes are limited for a number of reasons to use in the lower voltage tubes, say about 1, volts or less.
They are used almost exclusively in receiving- type tubes and provide efficient operation with long life. Curves showing the relative cathode efficiencies of tungsten, thoriated- tungsten, and oxide-coated cathodes are illustrated in Fig. It will be seen that tungsten has a considerably lower eflficiency than either of the other two emitters.
Cathode efficiency curves of an oxide-coated, a thoriated-tungsten, and a pure tungsten filament. Typical fila- mentary cathodes are illustrated in Fig. These filamentary cath- odes may be of the pure tungsten, thoriated-tungsten, or oxide-coated type. Typical directly heated cathodes. The indirectly heated cathode for use in vacuum tubes is illustrated in Fig. A cathode assembly of this type has such a high heat capacity that its tem- Fig.
Typical indirectly heated Fig. Different types of heat- cathodes. Heat-shielded cathodes, which can be used only in gas-filled electron tubes for reasons to be discussed in Chap. This mate- rially increases the efficiency of the cathode. Several different types of heat-shielded cathodes are illustrated in Fig. Photoelectric Emission.
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The energy that is required to release an electron from a metal surface may be supplied by illuminating the surface with light. There are certain restrictions on the nature of the surface and the frequency of the impinging light for such electron emission to take place. That is, electron emission is possible only if the frequency Sec. For response over the entire visible region, to A, the work function of the photosensitive surface must be less than 1. The essential elements of a photo- tube are the photosensitive cathode surface and a collecting electrode, con- tained in a glass envelope that either is evacuated or contains an inert gas at low pressure.
A photograph of such a phototube is shown in Fig. The number of photoelectrons per square millimeter of area of a photo- cathode is small, and it is customary to use photocathodes of large area, as 1. The current characteristics of such phototubes for different collecting potentials between the cathode and the collecting anode, with light Fig.
The volt-ampere characteristics of a type PJ vacuum phototube, with light intensity as a parameter. Figure shows the curves of a vacuum phototube with light intensity as a parameter. The presence in the glass envelope of an inert gas, such as neon or argon, at low pressure materially alters the volt-ampere curves. A set of char- acteristic curves for a gas phototube are given in Fig. The presence of the gas in a phototube increases the sensitivity of the phototube, the Fig.
Illumincition, footccmdles Fig. A significant comparison of the output from two phototubes, one of the vacuum type and the other of the gas-filled type, other characteristics of the tubes being the same, is contained in Fig. Note that the photocurrent for the vacuum phototube is a linear function of the illumi- nation, whereas that for the gas-filled cell shows deviations from the linear at the higher illuminations.
However, the greater sensitivity of the gas-filled cell is clearly evident. Secondary Emission. It is possible for a particle, either an electron or a positive ion, to strike a metallic surface and transfer all or a part of its kinetic energy in this collision to one or more of the internal Sec. If the energy of the incident particle is sufficiently high, some of the internal electrons may be emitted. Several tubes have been designed which incorporate secondary-emission surfaces as part of the device, and highly sensitive phototubes have such auxiliary elements in them.
Frequently the secondary emission that exists is of a deleterious nature. This matter will be discussed in explaining certain features of the characteristics of tetrodes. High Field Emission. The presence of a very strong electric field at the surface of a metal will cause electron emission. Ordinarily the field in the average electron tube is too small to induce such electron emission.
This process has been suggested to account for the electron emission from a mercury-pool cathode in a mercury rectifier. The process in which an atom loses an electron is known as ionization. The atom that has lost the electron is called a positive ion. The process of ionization may occur in several ways. Electron Bombardment. Consider a free electron, which might have been released from the envelope or from any of the electrodes within the tube by any of the processes discussed above.
Suppose that this free electron has acquired enough energy from an applied field so that upon collision with a neutral atom, it removes an electron. Following this action, two electrons and a positive ion exist. Since there are now two electrons available, both may collide with gas particles and thus induce further ionization. Such a process as this may become cumulative, with consequent large electron release. This process is very important and accounts for the successful operation of gas- and vapor-filled rectifier tubes. It is also the basis of the gas amplification in gas-filled phototubes.
If the gas is exposed to light of the proper frequency, then this radiant energy may be absorbed by the atom, with resulting electron emission. This process is important in initiating certain discharges. Positive-ion Bombardment. The collision between a positive ion and a neutral gas particle may result in electron release, in much the same manner as by electron bombardment.
This process is very inefficient and is usually insignificant in normal gas tubes. Thermal Emission. If the temperature of the gas is high enough, some electrons may become dislodged from the gas particles. However, the gas temperature in electron tubes is generally low, and this process is normally unimportant.
MiUman, J. A tungsten filanaent, 0. What is the temperature-limited current? The filament of an FP tungsten-filament tube is 1. If the total emission current is 30 ma, at what temperature is the filament operating? A simple inverted-V oxide-coated cathode is made of tungsten ribbon 0.
What is the thermionic-emission current? Calculate the relative thermionic-emission currents if bo has the value 12,; the value 11, Monochromatic light of wave length A falls on the following surfaces : a. Cesium, with a work function 1. Platinum, with a work function 5. Is photoelectric emission possible in both cases?
A PJ vacuum photocell is to be used to sound an alarm when the light at a given region of a room falls below 40 ft-c or increases above ft-c. What are the corresponding photocurrents? A collecting potential of 45 volts is used. This cathode ivill emit electrons, most of which have very little energy when they emerge.
Those electrons that first escape will diffuse throughout the space within the envelope. An equilibrium condition will soon be reached when, because of the mutual repulsion between electrons, the free electrons in the space mil prevent any additional electrons from leaving the cathode. The equilibrium state will be reached when the space charge of the electron cloud produces a strong enough electric field to prevent any subsequent emission.
The inclusion of a collecting plate near the thermionic cathode will allow the collection of electrons from the space charge when this plate is maintained at a positive potential with respect to the cathode ; the higher the potential, the higher the current. Of course, if the thermionic emis- sion is limited, then the maximum current possible is the temperature- saturated value. In addition to such a simple two-element device, which is the diode, grids may be interposed between the cathode and plate.
If a single grid is interposed, the tube is a triode. If two grids are present, the tube is a tetrode; three grids yields a pentode, etc. Details of the characteristics and operation of such devices will be considered in some detail in the following pages. The Potential Distribution between the Electrodes. Consider a simple diode consisting of a plane cathode and a collecting plate, or anode, which is parallel to it. It is supposed that the cathode can be heated to any desired temperature and that the potential between the cathode and anode may be set at any desired value.
It is desired to examine the potential distribution between the tube elements for various cathode temperatures and fixed anode-cathode applied potential. Suppose that the temperature of the cathode is high enough to allow some electrons to be emitted. An electron space-charge cloud will be formed in the envelope. The potential distribution between plane-parallel electrodes, for several values of cathode temperature. This follows from Eq. A study of this expression will yield significant information. It is supposed that the electrons that are emitted from the cathode have zero initial velocities.
Under these conditions, the general char- acter of the results will have the forms illustrated in Fig. At the higher temperature Ts, the charge density p is not zero. Clearly, the anode-cathode potential, which is externally controlled, will be independent of the temperature, and all curves must pass through the fixed end points. All curves must be concave upward, since Eq. Moreover, the curvature is greater for larger values of p, corresponding to the higher temperatures. It is possible to justify that the maximum current that can be drawn from the diode for a fixed plate voltage and any temperature is obtained under the condition of zero electric field at the surface of the cathode.
Equations of Space Charge. An explicit relation between the current collected and the potential that is applied between the anode and cathode is possible. In general, the current density is a measure of the rate at which the electrons pass through unit area per unit time in the direction of the field. It relates the current den- sity, and so the current, with the applied potential and the geom- etry of the tube. It shows that the space-charge current is inde- pendent of the temperature and the work function of the cathode. The volt-ampere characteristics of a typical diode.
If the electron supply from the cathode is restricted, the current may be less than the value predicted by Eq. The conditions are somewhat as represented graphically in Fig. For ratios rjrk of 8 or more, may be taken as unity. Experimental results to verify the three-halves power law for tubes with oxide-coated, thoriated tungsten, and pure tungsten filaments. Attention is called to the fact that the plate current depends upon the three-halves power of the plate potential both for the plane parallel and also for a diode possessing cylindrical symmetry.
The specific value of the constant k that exists in this expression cannot be analytically determined unless the geometry of the system is specified. The dependence of the current on the potential for any tube may be determined by plotting the results obtained experimentally on a loga- rithmic scale. The type 10 tube is a triode and was converted into a diode by connecting grid and plate together.
The other tubes are diodes. It will be observed that the logarithmic plots are straight lines, although the slopes of these lines are all slightly less than the theoretical 1. Rating of Vacuum Diodes. The current and potential ratings of a diode, i. A limit is set to the tube current by the cathode efficiency of the emitter. Thus, for a given input power to the filament, a maximum cur- rent is specified. There is a maximum temperature limit to which the glass envelope of the tube may be safely allowed to rise. This is the temperature to which the tube was raised during the outgassing process.
For higher temperatures, the gases adsorbed by the glass walls may be liberated. Owing to this limitation, glass bulbs are seldom used for vacuum tubes of more than about 1 kw capacity. A very important limitation is set by the temperature to which the anode may rise. In addition to the fraction of the heat radiated by the cathode that is intercepted by the anode, the anode is also heated by the energy carried by the anode current. The instantaneous power carried by the anode current and supplied to the anode is given by et,ib, where is the anode-cathode potential and 4 is the anode current.
The temperature to which the anode rises will depend upon the area of the anode and the material of its construction. The most common metals used for anodes are nickel and iron for receiving tubes and tantalum, molybdenum, and graphite for transmit- ting tubes. The surfaces are often roughened or blackened in order to increase the thermal emissivity. The anodes of many transmitting tubes may be operated at a cherry-red heat without excessive gas emission. To allow for forced cooling of the anode, cooling coils may be provided, or the tube may be immersed in oil.
The newer type of transmitting tubes are frequently provided with radiator fins for forced-air cooling. Several different types of transmitting tubes are illustrated in Fig. The voltage limitation of a high-vacuum diode is also dependent on the type of its construction.
If the filament and anode leads are brought out side by side through the same glass press, some conduction may take place between these leads through the glass. This effect is particularly marked if the glass is hot, and the resulting electrolysis wall cause the glass to deteriorate and eventually to leak. The highest voltage permissible between adjacent leads in glass depends upon the spacing and upon the type of glass but is generally kept below 1, volts.
Higher voltage tubes are usually provided mth filament leads at one end of the glass envelope, with the anode at the other end. The glass envelope must be long enough so that flashover on the outside of the tube will not occur. In a diode as a rectifier, no current will flow during the time that the anode is negative with respect to the cathode. Commercial vacuum diodes are made which will rectify current at high voltages, up to , volts.
Such units are used with X-ray equip- ment, with high-voltage cable-testing equipment, and with the high- Fig. Photographs of two transmitting tubes. The dimensions and shape of the glass envelope will depend upon the current capacity of the tube and the type of cooling to be used, oil-cooled tubes being generally smaller than air-cooled types.
The Grid. The introduction of a third element between the cathode and plate of the diode by DeForest in was the start of the extensive developments involving vacuum tubes. This new elec- trode, called the control grid, consists of a wire mesh, or screen, which surrounds the cathode and is situated close to it. The potential applied Sec. Clearly, the electric field resulting from the potential of the grid tends to maintain a large space-charge cloud, whereas the field of the plate tends to reduce the space charge.
However, owing to its proximity to the cathode, a given potential on the grid will exercise a greater effect on the space charge than the same poten- tial on the plate. This would seem to imply that a strict proportionality should exist between the relative effectiveness of the grid and plate potentials on the space charge and that the plate current should be represented approximately by the equation ib — k cc where Cb is the plate-cathode potential, is the grid-cathode potential, and the factor is a measure of the relative grid-plate potential effective- ness on the tube current.
The validity of Eq. No simple, rigorous theoretical deriva- tion of this equation is possible, even for a triode of relatively simple geometry. However, the value of the amplification factor p can be calculated with a fair degree of accuracy from equations that are based on electrostatic considerations. By maintaining the grid at some negative potential with respect to the cathode, it will repel electrons and will, in part, neutralize the attractive field of the anode, thus reducing the anode current.
If the grid potential is made positive, the electron stream will increase because of the combined action of both the grid and the plate potentials. But, with a positive potential on the grid, some of the space charge will be attracted to it, and a current in the grid will result. The grid structure must be designed to dis- sipate the grid power if the grid potential is to be maintained posi- tive; otherwise the grid structure may be seriously damaged. Generally the grid is maintained negative, although positive-grid triodes for power-amplifier applications are available.
The variations of the plate and grid currents with variations of grid Fig. Total space, plate, and grid current in a triode, as a function of grid voltage, with fixed plate voltage. In this diagram, the plate potential is maintained constant. For sufficiently negative grid potential, cutoff of the plate current occurs. As the grid potential is made less negative, the plate current follows a smooth curve, the variation being expressed analytically by Eq.
As the grid potential is made positive, grid current flows, the magnitude of this current increasing rapidly with increasing grid potential. For positive grid potentials, and with the consequent grid current, Eq. With increasing grid potentials, the grid current increases, and the plate current decreases. Triode Parameters. In view of Eq. If Eq. The projec- tions of these surfaces on the three co- ordinate planes give three families of characteristic curves.
These curves are given in Figs. The curves of Fig. The constant-current characteristics of a triode. The main effect of making the grid more negative is to shift the curves to the right, without changing the slopes appreciably. This is in accord with what would be expected from consideration of Eq. If the grid potential is made the independent variable, the mutual, or transfer, characteristics of Fig. The effect of making the plate potential less positive is to shift the curves to the right, the slopes again remaining substantially unchanged. The simultaneous variation of both the plate and the grid potentials so that the plate current remains constant gives rise to a third group of characteristics illustrated in Fig.
These show the relative effects of the plate and grid potentials on the plate current of the tube. But from the discussion of Sec. Consequently, the amplification factor is defined as the ratio of the change in plate voltage to the change in grid voltage for a constant plate current. Consider the variation in the plate current. This is obtained by expanding Eq. But it is here assumed that the variation is small and that it is adequately represented by the first two terms of the expansion. Subject to this limitation, the expres- sion has the form This expression indicates simply that changes both in the plate voltage Ae6 and in the grid voltage Acc will cause changes in the plate current.
This ratio has the units of resistance, is known as the plate resistance of the tube, and is designated by the symbol r-p. Clearly, Vp is the slope of the plate characteristics of Fig. The variations of these parameters for a fixed value of plate potential for the 6C5 tube are shown in Fig. The transconductance I j varies from a very small value at X. The a 6C5 triode as a iunction of plate current. High-power triodes are used extensively in transmitters.
The grid of such a tube is driven positive with respect to the cathode during part of the cycle, and the current is cut off during part of the cycle. The characteristics of importance of such tubes are the plate curves and the constant- current curves. The variations over normal operating limits are as illustrated in Figs. In the tetrode a fourth electrode is interposed between the grid and the plate. The plate characteristics of a power-triode type A. The constant-current characteristics of the power triode of Fig.
Because of its design and disposition, the screen grid affords very complete electrostatic shield- ing between the plate and the control grid. This shielding is such that the grid-plate capacitance is reduced by a factor of about 1, or more. However, the screen mesh does not interfere appreciably with the electron flow. The reduction of the grid-plate capacitance is a very important improvement over the triode, and this matter will be considered in some detail in Chap. Because of the electrostatic shielding of the plate by the screen, the potential of the plate has almost no effect in producing an electric field at the cathode.
Since the total space current is determined almost wholly by the field near the cathode surface, the plate exerts little or no effect on the total space charge drawn from the cathode. There is, therefore, a significant difference between the triode and the tetrode. In a triode, the plate performs two distinct functions, that of controlling the total space current, and that of collecting the plate current.
In a tetrode, the plate serves only to collect those electrons that have passed through the screen. The passive character of the plate makes the tetrode a much better voltage amplifier than the triode. This follows from the fact that in the triode Avith a resistance load an increase in load current is accompanied by a decreased plate-cathode potential, which results in a decreased space current. In the tetrode, the decreased plate-cathode potential still exists, but owing to the secondary role of the plate the space current is not materially affected.
The disposition of the cathode and the control grid is nearly the same in both the tetrode and the triode, and therefore the grid-plate trans- conductance is nearly the same in both tubes. Also, the plate resistance of the tetrode is considerably higher than that of the triode.
This fol- lows from the fact that the plate voltage has very little effect on the plate current. Thus, with the high plate resistance and with a gm that is about the same as for the triode, the tetrode amplification factor is very high. Tetrode Characteristics. In the tetrode with fixed control-grid and screen-grid potentials, the total space current is practically constant.
Hence, that portion of the space current which is not collected by the plate must be collected by the screen; where the plate current is large, the screen current must be small, and vice versa. The general character of the results is illustrated in Fig. Although the plate potential does not affect the total space current to a very great extent although a slight effect is noted in the curve at the lower plate potentials , it does determine the division of the space current between plate and screen.
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As the plate potential is increased, a rapid rise occurs in the plate current, with a corresponding reduction of the screen current. When the plate potential is larger than the screen potential, the plate collects almost the entire space current and the screen current approaches zero or a very small value. An inspection of the curves of Fig.
This region is one of negative plate resistance, since an increasing plate potential is ac- companied by a decreasing plate current. The kinks, or f olds, in the curves are caused by the emission of electrons from the plate by the process of secondary emission. This results from the impact of the pri- mary electrons with the plate. That is, secondary electrons will be released from the anode, and if this is the electrode with the highest posi- tive potential, the electrons will be collected by the anode, without any noticeable effect. If, however, secondary electrons are liberated from the anode, and if these electrons are collected by some other electrode, then the anode current will decrease, whereas the current to the collecting elec- trode will increase.
It is this latter situation which exists in the tetrode when the plate potential is low and the screen is at a high potential. When the plate potential is higher than the screen potential, the secondary elec- trons from the plate are drawn back, with- out appreciable effect. If under these potential conditions secondary electrons are liberated from the screen, these will be collected by the anode. The correspond- ing plate current will be greater than that in the absence of secondary emission from the screen.
Transfer Characteristics. Since the plate of a tetrode has no appreciable influence on the space current, it is expected that the cathode, the control grid, and the screen grid should possess characteristics not unlike those of a triode.
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This is actually the case, as illustrated in Fig. These curves show the effect of varia- acteristics of a tetrode, for a fixed screen potential, and with the plate potential as a parameter. Because of the slight influence of the plate, the transfer curves are bunched together.
These curves should be compared with those of the triode in Fig. The transfer curves for plate voltages below the screen potential, and this is the region of operation which is generally avoided in practice, become separated. This anomalous behavior is directly the result of the secondary-emission effects discussed above. Tube Parameters. It is expected, on the basis of the foregoing discussion, that the plate current may be expressed as a function of the potential of the various electrodes by an expression of the form ih ,f cb,eci,Cc2 where Cd is the potential of the first, or control, grid, Cd is the potential of the second, or screen, grid, and cj is the potential of the plate, all with respect to the cathode.
This functional relationship is just a natural extension of that which applies for triodes. In fact, an approximate explicit form of the dependence is possible. This form, which is an extension of Eq. The third term in the expansion may be omitted under these conditions. The partial-differential coefficients appearing in this expression furnish the basis for the definitions of the tube parameters. Although the insertion of the screen grid between the control grid and the anode in a triode serves to isolate the plate circuit from the grid circuit, the range of operation of the tube is limited owing to the effects of secondary emission.
This limitation results from the fact that, if the plate-potential swing is made too large, the instantaneous plate potential may extend into the region of rapidly falling plate current, with a resulting marked distortion in the output. The transfer curves of a pen- tode for a fixed screen potential and with the plate potential as a parameter.
The kinks, or folds, that appear in the plate-characteristic curves and that limit the range of operation of the tetrode may be removed by inserting a coarse sttppressor-grid structure between the screen grid and the plate of the tetrode. Tubes that are provided with this extra grid are known as pentodes. The suppressor grid must be maintained at a lower potential than the instantaneous potential reached by the plate at any time in its potential excursions.
Usually the suppressor is connected to the cathode, either externally or internally. Now since both the screen and the anode are positive with respect to the suppressor grid, secondary electrons from either electrode will be returned to the emitting electrode.
The main electron stream will not be materially affected by the presence of the suppressor grid. The effects of the insertion of the suppressor grid are shown graphically in Fig. The pentode has displaced the tetrode in radio-frequency r-f voltage amplifiers, because it permits a somewhat higher voltage amplification at moderate values of plate potential.
Likewise it permits a greater plate- voltage excursion without distortion. For those seeking additional cooling, radiators are available on many types to allow for forced-air cooling. Description Richardson Electronics is a leading manufacturer and distributor of rectifiers for industrial RF heating, motor controls, and welding applications. Our extensive inventory can meet the requirements of most every application Description For high linearity, high gain and greater isolation, the tetrode is the tube of choice. Many manufacturers offer tetrode design transmitters to meet increasing power needs.
In fact, tetrodes are the primary means for achieving power levels above 1 MW. Shortwave transmitters, including those broadcas While a mainstay in AM, FM or TV broadcast transmitters; tetrodes are being used today in high power RF heating systems where the designer uses a separate oscillator driver to attain tight frequency control. Air-cooling is used with plate dissipation ratings of up to 40 kW; while water, vapor-phase and multi-phase cooling is used above 40 kW.
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Stringent quality control and selection ensure the best possible imaging performance. Description Cold cathode tubes are filament-less, gas-filled devices, used in both voltage regulation and switching applications. Rely on Richardson Electronics for these hard-to-find products and the support you need to make the right selection Description Amperite has been solving circuit timing requirements since Richardson Electronics offers Amperite's full line of high-quality time delay relays and flashers.
If we don't carry the part you need, Amperite has the flexibility to custom design a delay relay or flasher to meet your specific need If we don't carry the part you need, Amperite has the flexibility to custom design a delay relay or flasher to meet your specific needs. It also features ease of adjustment and increased stability thanks to the associated RF circuit TH and TH cavities and simplified cooling requirements.
Due to the r Due to the relatively low operating voltage, the power supply design is simplified. Description Geiger-Mueller counters are small, robust nuclear radiation detectors used to determine the presence of alpha, beta or gamma radiation. They are capable of detecting, measuring, and with simple circuitry, providing sufficient output to drive displays and indicators.
In addition to producing quantita In addition to producing quantitative data they are often used for personnel safety devices in nuclear power installations, laboratories and medical facilities. Description Richardson Electronics offers a broad range of Bayard-Alpert type ionization gauge tubes to meet your needs. The positive pulse applied between point iii: and the: grid produces a change in bias on tube a which causes its anode current to increase through the winding l and thereby initiates the blocking osciilator action.
The grid is very rapidly driven sufficiently far positive so that grid current flows, charging capacitor iii in such a manner that point 3! The positive pulse onthe grid also causes the anode current to increase very rapidly in the typical blocking oscillator regenerative action. Capacitor I8 is also charged, with the same polarity, by current flowing from the cathode through the capacitor, through resistors M, 2 and i5. The anode current of tube 9 will continue to increase very rapidly until saturation takes place.
The saturation may be either in the tube or the transformer or both. When saturation takes place the current remains relatively constant for a period of approximately 2 to 25 microseconds, depending on the tube, the value of capacitor I 8 and the value of resistor it. At the expiration of this time, the degenerative action of the blocking oscillator takes place and the current very rapidly decreases.
The duration of the pulses can be varied by changing the value of capacitor i8 and resistor H. The output voltage prlse from the blocking oscillator is taken across resistor it. This pulse may be applied directly to the cathode of another tube without an intermediate blocking capacitor. This permits almost instantaneous recovery of the blanked tule after the "lan"'ng pulse. The use of resistor iii in i 1 the cathode to provide the: pulse substantially ehrninates overshoot in the output pulse. The cathode of tube connected to point l9 to receive the blanking pulse.
The grid of tube 2a is connected to an adjustable potentiometer ii to provide a variable source of negative bias for the tube. The positive pulse on the cathode of tube 2e makes the bias on the tube sufllciently negative to completely out off the tube. Tube 22 is coupled to tube by a con ventional resistance-capacitance coupling. The input signal is applied to the grid of tube 26 through blocking capacitor The type of tube used is not critical.
Two tube types which are well suited for use in this circuit. These tubes are mentioned merely by way of example and it is not intended that the invention should be considered limited to these specific tubes or even to the class of twin triodes in general. The following typical values are given for the components of the blocking oscillator portion of the circuit using a type 7F8 tube:. Resistor H ohms 1, Resistor i2.
Using the above listed values with volts as the source of negative potential and volts as the source of positive potential, a pulse of approximately 9 microseconds was obtained. The aforementioned values are given solely for illustrative purposes and are not intended to limit the invention in any Way. It will be apparent that there may be deviations from the invention as described. For example, the output pulse of the blocking oscillator need not be used for blanking a tube but could be used any place where a pulse of relatively long duration is desired.
Accordingly, I claim all deviations which fall fairly within the spirit and scope of the invention as identified in the hereinafter appended claims. A blocking oscillator comprising a vacuum tube having an anode, a cathode and at least one grid, a transformer, a first winding of said transformer for electrically connecting the anode of said tube to a source of positive potential, a second winding of said transformer for electrically connecting a grid of saidtube to a source of negative potential, a third winding of said transformer electrically connected to the cathode of r said tube, a capacitor electrically connecting the cathode of said tube to said second winding, and resistor means electricallyconnecting.
A blocking oscillator as set forth in claim 1 in which the common junction of said third winding and said resistor means is electrically connected to the cathode of a second tube. In a blocking oscillator, a vacuum tube having an anode, a cathode and at least one grid, a transformer having a primary winding for connecting said anode to a source of positive po-ten tial and at least two secondary windings, a first secondary winding for connecting a grid of said tube to a source of negative potential, a second secondary winding connecting the cathode of said tube to a source of reference potential, 2.
A circuit for producing a blocking pulse comprising at least two vacuum tubes each having an anode, a cathode and at least one grid, a transformer, a first winding of said transformer for electrically connecting the anodes of said tubes to a source of positive potential, a second winding of said transformer for electrically connecting a grid of the first tube to a source of negative potential, a third winding of said transformer electrically connected to the cathode of the first tube, a capacitor electrically connecting the cathode of said first tube to said second winding, and resistor means electrically connecting said third winding to a source of reference potential.