US2000304A - Thermionic tube - Google Patents

Description

May 7, 1935.

F. E, TERMAN THERMIONIF; TUBE Filed April 25, 1951 IOOOOO OV/VAM/C SCREEN GRID RES/STANCE.

0 Linn /'0 2'0 0'0 4'0 50 0'0 7'0 SCREEN 09/0 VOLTAGE.

a7 l i l l INVENTOR FREDERICK E. TERMA/V.

ATTORNEY.

Patented May 7, 1935 2 000 304 UNITED STATES PATENT OFFICE THERMIONIC TUBE Frederick E. Terman, Stanford University, Calif. Application April 25, 1931, Serial No. 532,742

6.Claims. (01. 250-27) This invention relates to vacuum tubes and resistance component of the input impedance is vacuum tube amplifier circuits therefor, and parnot, in general, linear, and 'ifthis component be ticularly to tubes wherein the input or control small, i. e., if the conductance be large, serious electrode is operated at a positive potential. distortions are introduced. For these reasons it 5 Among the objects of this invention are: First, is Customary to operate the control grid of a'vac- '5 to provide a tube combining a high amplificauum tube at a negative biasing potential such tion constant with high mutual conductance and that the grid is never permitted to becomeposilow output resistance; second, to provide a tube tive. Under these circumstances, the electronic which combines a high input impedance with current flowing from filament grid is sensibly low output impedance; third, to provide a tube zero, and the resistive component of the imped- 10 which may be operated with a positive bias upon ance may be taken as fin V a control electrode Without the introduction of The high' p impedance 1711115 gained, appreciable distortion; fourth, to provide a tube however, obtai at the eXpeIlSe of low utput wherein secondary emission from the control elecped ce a d high mutual Conductance. y

trade exercises an advantageous instead of a disinvention relates to a meth d of operating vac- T5 advantageous effect; and fifth, to provide a methmun tube amplifiers wherein the grid ismainod of operating vacuum tubes of ordinary contained at ai'positive potential, thereby obtaining struction wherein many of the advantages 010- the advantages of high mutual conductance, tainable from the special tube above mentioned low output resistance, and high amplification may be realized. constant, without so reducing the input imped- .120

Other objects of this invention will be apparent ance of the tube as to introduce distortion and or will be specifically pointed out in the descriploss of amplificationthrough absorption of altertion forming a part of this specification, but I hating current power by the input circuit of'the do not limit myself to the embodiment of this tube.

invention herein described, as various forms may Broadly considered, this invention comprises 125 V be adopted within the scope of the claims. the method of cperationof a vacuum tubewhere- Referring to the drawing: 'in the potentials of the electrodes are so ad- Figure 1 is an axial sectional view, and Figure justed that secondary emission of electrons oc- 2 is a transverse sectional view of a triode emcurs from the control grid, this secondary emisbodying this invention. sion being such that the rate of change-in thee-30 Figure 3 shows graphs illustrating the input number of electrons reaching'the gridfrcm the impedance of a tetrode of standard construction filament, with change in grid potential, is subwhen operated in accordance with my invention, stantially equal to the rate of change in the andiwhen operated in the usual manner., number of secondary electrons drawn from the Figure 4 is a schematic diagram of an ampli grid with the same potential changes. The-i35 fier circuit embodying a triode and connected'in achievement of this end is aided by coating-the I accordance with this invention. control grid with'a substance facilitating the Figure 5 S a Similar C cu t d agram Showi g emission of secondary electrons, such as a subthe use of a tetrode in the same manner. stantially monatomic layer of barium or tho- 40 Almost all problem of amplifier design av rium, and the use of such a coating ona control 40 two phases; first, gain, or amountof amplificaelectrode is one aspect of my broad invention. tion, and Second, linearity, f e d m fIOm d It is; to be noted that it is the rate ofchange tortion. The relative importance of these two of the grid current which is considered; the sophases may differ with the particular amplifier called static input resistance of thetube may and the service to which it is assigned, but whatbe relatively low, but if the rates of change of 45 i ever the service both phases must be considered. current entering and leaving the grid are sub- Both the requirements for high gain, and that stantially equal the dynamic resistance of the l for linearity, require that the vacuum tube which grid will be high. Since the power absorbed in is used in the circuit should have a high input the grid circuit due to low static resistance is impedance anda low output impedance. Of-these furnished by a local D. C. source, it is relatively 5 secondary requirements high input impedance is unimportant, falling into the same category as usually the more important, since the higher the power absorbed by the'D. 0. component of the input impedance the greater the control poplate current. The signal or alternating potentential which can be built up across it with a tial which is to be amplified need therefore suplimited amount of input power, and because the ply negligible power to the tube input, and hence 55 Such a tube is shown in Figures 1 and 2, wherein the usual glass envelope l is provided with a stem 2 having leads 3, 5, 6 and 1 sealed through the press 8. The leads 3 and Ears connected to a filament II], which is preferably-short and heavy so as to present a surface which is large in comparison with its length. The object of this is to obtain a large emission of electrons together with as high a potential gradient as possible adjacent the filament.

Surrounding the filament, and spaced .therefrom a considerably greater distance than is customary with tubes used for similar purposes, is a grid H, which is preferably formed of a large number of turns of fine wire, and which connects to the lead 6. The device is operative if the grid be made of nickel, but its operation is greatly improved if it be coated with a thin layer of material having a very high coefficient of secondary electron emission. Of such materials, the best which has yet been discovered is a monatomic layer of barium, which may be deposited thereon after the tube has beenevacuated by distillation from the filament. Other materials such as thorium may also be used for a similar purpose with slightly decreased efficiency. Owing to the extreme minuteness of the secondary emitting layer, no attempt has been made to show it in the drawing, as it is far 'below microscopic thickness.

A plate electrode I2 is connected to the lead '1, and surrounds the-grid. The spacing of this electrode from grid and filament is also preferably greater than that in tubes as ordinarily designed for similar service.

Such a tube is connected in an amplifier'circuit in accordance with Figure 4, "whereinthe signal voltage is indicated as generated by a source l3 which is connected to the filament I9 and grid H, a biasing battery 15 being supplied in series with the grid toprovide a positive charge thereon. The plate I2 is connected through the suitable output impedance I 6 through a plate current source I! and thence back to the filament. For simplicity, the filament supply is omitted in the drawing.

With a barium coated grid H' of the type described, the positive potential from the source 15 may be of the order of 10 volts. age, the ratio of secondary electrons to in-falling primary electrons is approximately 1:1.

Obviously, the secondary electrons emitted from the control grid will not all be drawn to the plate of the tube, since their general direction of emission is toward the filament. Some of them will return 'to the grid and the net-effect of these upon the current flowing in the grid circuit will be nil. The design of the grid should 1 be such that as many as possible of the secondary electrons will'be projected into the field from the plate, and thus drawn'to the plate instead of returning to the grid. This effect is most easily accomplished by using very fine wire in the grid structure. This will result in the greater number At this volttron emission or the portion thereof drawn to the plate be in the 1:1 ratio. Both of these quantities will vary, not only with the potential of the grid, but also with that of the plate. What is required in order to limit distortion and power absorption in the grid circuit is that the rates of change of the electron current falling onto the grid from the filament, and secondary emission current drawn from the grid be approximately equal. This can easilybe achieved, the necessary potentials for any given tube structure being easily found by experiment.

It is not even necessary that the dynamic resistance, representing the ratio of change in grid voltage to change in net grid current be constant, if it be at all timeshso high as to introduce but a small portion of the total loss in the circuit. The relatively great emitting area of the cathode, and the wide spacing between cathode and grid, permit the establishment of a space charge at :the grid potentials used.

Assuming that a space charge is established in the vicinity of the cathode, the plate current of the tube is given an equation of the form where Egg and Ep are the grid and plate voltages respectively, K is a constant depending onq-the tube construction, and u is the amplification factor of the tube (1. e., the relative effectiveness upon the electrostatic field produced at the surface of the cathode by voltages applied to the gridand plate). Differentiation of this equation with respect to grid voltage gives the mutual conductance which is therefore It is seen from this equation that in order to have a high mutual conductance the quantity (E -l-fip/u) should be as large as possible.

Since the grid potential 'Eg in an ordinary tube is negative, a high mutual conductance canbe obtained only by making the quantity E /u large, with the result that at a given plate voltage 'the mutual conductance can be made large only by i using a small amplification factor u. This point is well brought out by the usual power tubes used in radio receivers, which gain their high mutual conductance by having a low amplification factor,

whereas tubes with higher u and operating at the Y same plate voltage always have a lower mutual conductance.

In my positive grid tube it is simultaneously possible to obtain a large value (Eg-i-E /u) and a large amplification factor because the grid poten- I tial Eg is positive. It is therefore possible to make a positive grid tube having the same mutual conductance as an ordinary power tube but with a much higher amplification factor. Thus analysis of the characteristics of commercial power" tubes of the 245 and 250 types show that with the substitution of closely wound high amplification grids, adapted for secondary emission and positive bias, these tubes will supply the same power output for which they are now adapted with only one third of the input voltage.

Although the tube just described is especially adapted to the practice of this invention, ordinary commercial tubes such as are now on the market may also be utilized for this purpose. shows schematically a circuit utilizing a tetrode with a positive control grid to give high input impedance by secondary emission.

The tetrodeZO has the usual cathode 2|, inner g-rid 22, outer grid 23, and plate 25. The comm Figure 5 potential is represented as generated by a source 26, in series with which is a positive biasing potential source 21 connected in series with outer grid 23 and filament 2|. The plate is connected through the output impedance 28 and plate supply 30 back to the filament in the usual manner. The inner grid 22 may be operated at the same potential as the cathode, or it may have either a positive or negative bias placed upon it by a battery 3|. In the usual tetrode, the spacing of the elements is such that were both grids and the plate operated at positive potentials the electrons from the cathode would be drawn off too rapidly to permit the establishment of a space charge. It is to prevent this effect that the wide spacing between grid and filament is utilized in the form of this invention first described.

Figure 3 shows a curve 35 illustrating the variation in dynamic resistance to the screen grid or control grid of a standard tube in such a circuit. The inner grid in this case was operated at cathode potential. It will be seen that for all positive bias voltages between 35 volts and 65 volts, the dynamic input resistance to the tetrode is in excess of a half mcgohm.

Although the dynamic resistance varies over this range, it is still greatly in excess of any resistance across which it would ordinarily be connected, so that any distortions introduced into the circuit by the resistance variations are extremely small.

The high order of the input resistance obtained in this manner in comparison with the resistances usually obtained with positive grid voltages is shown by comparison of the curve 35 with the curve 36, which shows the input impedance of a tube operated in a similar circuit, but without secondary emission. It will be noted that while the resistance of the secondary emitting tube is in excess of 200,000 ohms for all values of positive grid potential up to '75 volts, the screen grid tube Without positive emission shows high impedance only for very small control voltages, the impedance falling rapidly to about 80,000 ohms at voltages where a secondary emitting grid gives ten times this input impedance.

The curve of Figure 3 is given to show the possibilities of my method with standard tubes. Were the grid 23 specially treated to facilitate secondary emission, the dip at about 10 volts would disappear, thus giving very high input impedance at all input voltages up to about 65 volts.

Where this condition obtains it is possible to operate the tube at zero bias without appreciable distortion, the input conductance being zero for the negative half cycles and negligible for the positive. This is of great advantage in eliminating the necessity of any biasing means, such means being extremely troublesome, particularly in power tubes where the biasing voltage must be high.

I am aware that secondary emission has been used in vacuum tubes in order to give negative resistive effects, but such effects are not the ones required for the operation of my invention, which, it will be seen, operates upon a wholly different principle. Moreover, with the exception of such tubes as utilize the negative resistance effect above referred to, it has usually been assumed that secondary emission was a deleterious effect, which was to be avoided from all electrodes, whereas I have here shown that by its proper utilization greatly increased efficiency may be obtained from electrons, an anode, an output circuit for saidtube connected to said anode, an input circuit for applying a variable potential to said control electrode and separate from said output circuit, and means for applying a positive biasing potential to said electrode such that the rate of change of current to said control electrode and the rate of change of secondary electron current drawn therefrom with changes of potential thereof are substantially equal.

2. In a vacuum tube circuit, a tube having a control electrode capable of emitting secondary electrons, and an anode, an output circuit for said tube connected to said anode, an input circuit for applying avariable potential to said control electrode and separate from said output circuit, and means for applying a positive biasing potential to said electrode such that the rate of change of current in the control electrode circuit with changes of potential thereof is materially reduced by secondary emissionfrom said control electrode.

3. In a vacuum tube circuit, a tube having a control electrode capable of emitting secondary electrons, and an anode, an output circuit for said tube connected to said anode, an input circuit for applying a variable potential to said control electrode and separate from said output circuit, and

means for applying a positive biasing potential to said electrode such that the rate of change of current in the control electrode circuit with changes in potential thereof is reduced by secondary emission to less than 5x 10 amperes per volt.

4. In a vacuum tube circuit, a tube having a control electrode capable of emitting secondary electrons, and an anode, an output circuit for said tube connected to said anode, an input circuit for applying a variable potential to said control electrode and separate from said output circuit, and means for applying a positive biasing potential to said electrode such that the rate of change of current in the control electrode circuit with changes in potential thereof is reduced by secondary emission to less than 2 x 10- amperes per volt.

5. The method of increasing the effective input impedance of a vacuum tube wherein the variations of potential of a control electrode cause variations of current flowing to a separate anode in an output circuit separate from the input circuit supplying said control electrode, which comprises causing said control electrode to emit secondary electrons at a rate which varies substan- FREDERICK E. TERMAN.

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