US2037202A - Variable vacuum tube resistor - Google Patents

Description

April 14, 1936. F. E. TERMAN VARIABLE VACUUM TUBE RESISTOR 2 Sheets$heet l Filed May 25, 1951 Wm WM NT 5 a H m m ATTORNEY April 14, 1936. F, TERMAN' 2,037,292

VARIABLE VACUUM TUBE RESISTOR Filed May 25, 1931 2 Sheets-Sheet 2 INVENTOR FREDERICK E. TERM/4N. w r

ATTORNEY Patented Apr. 14, 1936 UNITED STATES PATENT OLFFICE 1 Claim.

Myinvention relates to variable resistance elements, and particularly to elements whose resistance may be varied cyclically at any desired frequency.

In my copendingapplication, Serial No. 489,917, filed October 20, 1930, I have shown a method of producing side-band frequencies, in the absence of a carrier frequency, by passing a modulating current through an impedance element which varies cyclically at a carrier frequency. In said application I have shown and claimed one type of vacuumtube resistance element which may be varied in the manner described. This application is directed to another general method of obtaining the desired result.

Among the objects ofthis invention are: first, to provide a cyclically variable resistance element which is bilaterally symmetrical; second, to provide a method of operating a multi-element vacuum tube of ordinary construction to provide a resistance element of the typedescribed; third, to provide specific circuits wherein such resistance elements may be used with maximum advantage; and fourth, to provide a general method 5 whereby tubes of ordinary construction may be utilizedas symmetrical variable resistors in alternating current circuits.

Other objects of my invention will be apparent or will be specifically pointed out in the description forming a part of this specification, but I do not limit myself to the embodiment of my invention, herein described, as various forms may be adopted within the scope of the claim.

Referring to the drawings:

Figure 1 is acircuit diagram showing the use of a special three electrode vacuum tube as a cyclicall y variable resistance element which is symmetrical with respect to current flow.

Figure 2 is a. circuit diagram showing the. use of the ordinary three electrode vacuum tube, as a. variable. asymmetric resistance element.

Figures: 3 and 4 are. characteristic curves of different three electrode vacuum tubes, illustrating the portion of the characteristics wherein such tubes: may be used as. variable resistors.

Figure- 5. isa circuit diagram illustrating the use. of a. tetrode as, a variable resistor.

Figure 6 illustrates another circuit by which a tetrode may be used .in accordance with this invention. 9

Figure 7 illustrates a, push-pull circuit, wherein two triodes are used to give. asymmetrical variable resistance.

. 5 Figures. 8. and, 9.- show two. circuits utilizing triodes in a, back to-back connection for. giving a symmetrical variable resistance.

Figure 10 illustrates a circuit wherein a, single double filament. tube. is utilized to give substantially the same characteristic as the backto- 5 back connection of Figure 8.

Figure 11 illustrates. the use of a single, tube having two. separate plates ina circuit similar to that of Figure '7, wherein two tubes are used.

Under the conditions in. which it is, ordinarily 10 used, a vacuum tube cannot truly be said to have any fixed value which may be assigned as its plate resistance. That is, although there is a value of plate current corresponding to any given value of plate and grid voltages, ii the 15 plate voltage be varied the plate current does not alter in linear relation, or, if plate and grid voltages be varied simultaneously, the tube does not act as a simple resistor whose. plate circuit varies in resistance in accordance. with the vary- 20 ing grid potential.

I have found, however, that if. the plate or other output electrode be operated at a potential such that it is not the determining factor in establishing the total space current within 5 the tube, the latter being, at any control electrode potential, substantially independentot the potential on the output electrode, and if another electrode within the tubebe varied to change the total space current, the distribution of'current 30 output. electrode current.

In general, this resistance is asymmetric. That is, it. isinfmite for potentials in one direction and finite for potentials in the opposite direction. This condition is not necessarily undesirable, but where a symmetrical resistor is. desired it may be achievedby connecting two tubes-in opposite relation, either push-pull? or bacletc-back so as to give equivalent results on both halvesof the waves. Furthermore, tubes may be constructed having either two: filaments; or two plates, which .may; be. symmetrically connected tota circuit. in

substantially the same manner as two tubes. 5 5

The various figures show in detail various methods of carrying out the invention thus described in general terms. In each of the circuits thus indicated a source I of alternating current power is connected through the primary 2 of a transformer to what would ordinarily be consirdered the output terminal of a vacuum tube which acts as a cyclically varying resistance element. There will then be induced in a transformer secondary 3 a current of side-band frequency which may be carried by the output leads 4 and applied to any desired amplifier or other utilization device.

As used throughout this specification the term side-band frequency refers to a. frequency (ftp). where f is a relatively high carrier frequency, and p is the relatively low modulating frequency supplied by the source 1.

The circuits shown differ in the details of the vacuum tube resistance element, perhaps the simplest being that shown in Figure 2, wherein the leads 6 and I connect from the plate 8 of the tube to the power source I and from the cathode 9 to the transformer secondary 2, re-

spectively. V

The tube III, is, in this case, the ordinary triode or three electrode vacuum tube. A positive potential is supplied by the battery I I to the control electrode or grid I2 of the tube, and in series with the-battery is a source I3 of alternating current potential of carrier frequency. A similar source I3 of carrier frequency potential is common to all of the circuit diagrams, and exercises a similar function in every case.

In order to maintain the conditions set forth above in the three electrode vacuum tube, the potential from the power source I must be low,

i. e., in the neighborhood of zero. Moreover, the

battery II must be arranged to place a positive potential upon the grid. Under these circumstances, the total space current flowing depends almost wholly upon the grid potential, whereas the proportion of this space current which flows to the plate is a function of the plate potential. These results may be seen from the characteristic curves of Figs. 3 and 4, which were taken with different types of tubes operating under the conditions set forth.

It will be noted that in each of these figures the characteristic curves corresponding to different grid voltages meet or coincide at a point I4, at which zero plate current flows. With certain tubes, this point will also correspond to zero plate voltage; with other tubes, a biasing battery I5 may be supplied in the plate circuit, to render the point of zero plate current coincident with zero voltage of the power source I.

In Figure 4 the curves also intersect at the point I I, and this also is a suitable operating point,

although a direct current component may be present in this case which must be taken into consideration. This D. C. component introduces a carrier frequency component in the output circuit, which may be balanced out if undesirable.

The curves shown in these two figures are characteristic, and will differ little in form as between tubes of varying size, although the magnitudes of the quantities will vary considerably. The voltage corresponding to the point I4 will vary little with the type of tube, since the negative potential corresponding to zero plate current is a function of the initial velocity of emission from the cathode, and depends on the nature of its surface and the temperature at which it is operated. The current values, and corresponding 'vention utilizing a tetrode 25.

grid potentials will vary with the type of tube and particularly with the dimensions and spacing of the electrodes. Thus, a positive grid bias of 5 to 10 volts may be the maximum permissible with a small tube, whereas, a large tube with widely spaced elements may require volts or more positive bias for best operation.

In the circuit of Figure 2, the effective resistance of the tube is infinite on one-half of the cycle. This is a disadvantage in some services, which may be overcome by utilizing a special tube as is shown in Figure 1. In this case, the'leads I5 and 'I' each connect to a filament I6, a single grid II being arranged between the two filaments. A carrier potential source I3, in series with the battery I8, connects to the center of a network which is arranged to by-pass the carrier frequency to both sides of the modulating frequency circuit, i. e., the leads 6' and I. This network is shown as comprising two inductors I9,-I9', which offer a high impedance to the modulating frequency from the source I when bridged by the condensers 2U, 20', the latter elements serving to by-pass the carrier frequency.

It will be seen that in this arrangement each of the filaments in turn acts as a cathode while the other filament performs the anode function. Positive potential on the grid I'I causes a substantially constant space current to flow from each filament at any given grid potential, but the with the tube In of Figure 2.

In Figure 5 is shown a modification of my in- In this case the leads 6 and I connect to the filament and plate of the tube respectively as before. The screen grid 26 of the tube is maintained at a positive potential by a battery 21, but in this case the grid 28 is maintained negative by a battery 29. The total space current is independent of the potential of the-plate 30, being a resultant of the potential of the grid 28 and the screen grid 26, and

is controlled by variations of potential of the con.-

trol grid 28.

The effect of this arrangement is very similar to that of Figure 2, but considerably higher potentials may be used on the plate, and therefore more power may be absorbed in the circuit with a greater amount of side-band frequency power withdrawn from the leads 4.

In Figure 6, the tube 25' acts in much the same way as the tube 25 of the preceding figure. In

this case, however, the carrier frequency potential source I3 is connected in series with the screen grid 26 and battery 21, while the inner grid 28' is held at a constant negative potential by means of the battery 29'. V

Figure 7 illustrates a push-pull method of obtaining a symmetrical resistance circuit. In this case the leads 6' and I connect to the primary of a modulating frequency transformer 35. The two ends of the secondary 36 of this transformer connect with the plates 31, 31' of the two triodes 38, 38, while the center tap on the secondary connects to the filament 40, 40. The battery 4| places a positive potential upon both grids 42, 42, superposed upon which is the carrier frequency potential from the source I3. The two tubes 38, 38' vary in' resistance together, each tube presenting a finite resistance during one-half of the cycle of the modulator or power source I. The

changes in impedance vary the effective impedance of the transformer as seen from the source I, to effect the desired results. A biasing battery 43 may be used, like the battery I5 of Figure 2, to bring the plate potential to the optimum operating point. I

In Figure 8 each of the two tubes. 45 and 45 is connected to the modulating power source circuit in substantially the same manner as the single tube of Figure 2, the difference being that the two tubes have their filaments connected to opposite sides of the circuit. The carrier frequency potential is applied to the grids from the source I3 through a transformer having a single primary 46 and separate secondaries 41, 4?. Separate batteries 48, 48 are also provided for keeping the grids 49, 49' positive. The transformer arrangement is used to keepthe potential of the modulating source I from the two grids.

Figure 9 shows a modification of the back-toback circuit wherein the modulating frequency is kept from the grid of the tube and the carrier frequency is kept from the plate, without actual physical separation of the circuits by means oi a transformer, as in the case of Figure 8. In this case high frequency choke coils 55, are inserted in the circuit between the tubes 56, 56' and the carrier potential source I3. Blocking networks 51 and 51' are inserted between each side of the circuit 6', I and the control grids 58, 58' to prevent the modulating frequency potential from reaching the grid. A similar blocking network 60 is inserted in series with the carrier potential source I3 and the filaments of the two tubes. Except for the'method of separating the potentials, this circuit behaves in the same manner as that of Figure 8. The constants of the blocking networks 51, 51' and 60 depend of course upon the modulating frequencies used. The ba teries (SI and 6| keep the two grids at positive potentials as in the preceding cases.

A peculiarity of this circuit is that the resistances of tubes 56 and 56 vary in opposite senses with the potential from the carrier frequency source. The result is that there is a change in phase in current in the resistor circuit, as compared with the output of the other circuits shown, as the current passes through zero. The same effect may be obtained from the circuit of Figure 8, and also from thecircuit next to be described, if the transformer secondaries connecting source I3 to the tubes be connected to swing the grids in opposite phase. Whether this is to be avoided or not depends upon the use to be made of the resultant current; in some cases the phase reversal may be advantageous.

Figure 10 shows another modification of the circuit of Figure 8 wherein the two tubes are replaced by a single tube somewhat similar to that shown in Figure 1, but having two grids 66 and 66 positioned between the two filaments 61, 61'. As in the modification of Figure 1, each filament acts alternately as a cathode and as an anode. The carrier potential source I3 supplies the two grids through a transformer comprising one primary 58 and two secondaries 69, 69, as in Figure 8, and separate positive biasing batteries I0, III are used.

Figure 11 shows a single tube modification of the push-pull circuit shown in Figure '7. The modulating power source leads to primary 15 of a transformer whose effective resistance changes with changes in the load upon its secondary. The single filament I6 is connected to a center tap on the transformer secondary TI. The two ends of the secondary are connected to two separate plates I8, 18 within the single tube. The grid I9 may either be a single structure surrounding the filament, or two separate grids, one on either side of the filament, connected together. The source I3 is connected through the positive biasing battery to the filament.

It will be understood that the three electrode structures shown in most of these circuit modifications may be replaced in practically any of the circuits by tetrode or pentode tubes operating upon the same principle. Three electrode tubes, 7

or at least tubes operating upon the ordinary three electrode principle, are shown in the various circuit modifications merely because these lead to the simpler structures, but the method of using four or five element tubes in similar structures will at once be apparent, the resultant circuits being related to those shown in the same manner in which the circuits of Figures 5 and 6 are related to the simple circuit of Figure 2.

I claim:

The method of operating a tetrode having an anode, cathode and two grids as a resistor whose value is determined by the potential of one of the electrodes thereof which comprises applying an alternating potential between the cathode and plate of said tetrode, applying a fixed negative potential to the inner grid of said tetrode, and varying the positive potential of the outer grid to vary the effective resistance between said'cathode and said plate.

FREDERICK E. TERMAN.

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