US2247316A - Variable gain amplifier - Google Patents

Description

June 24, 1941. D. a. SMITH VARIABLE GAIN AMPLIFIER Filed may e, 1937 I I I Patented June 24, 1h41 Eiwit VAREABLE GAIN AMPLIFIER David B. Smith, Philadelphia, Pa., assigner to Philco Corporation, a corpcration of Pennsylvania Application May 6, 1937, Serial No. 141,151

13 Claims.

This invention relates to the control of the gain or amplication of variable gain ampliners and, more particularly, to a novel gain control device by means of which an input signal may be amplified to an extent which is governed by a second electrical signal. The invention is 'capable of general application and may be used in any instance Where it is desired to control the amplification of a signal in the manner above mentiond. For example, the invention is Well adapted for use in television systems, and finds several different applications therein. In order to illustrate the invention, a specific example of its use for one particular purpose in a television system will be given. It will be understood, of course, that speciiic embodiments of the invention other than those disclosed herein may be used and are, therefore, to be considered within the scope of the invention.

One object of the invention is to provide an amplifier having a high impedance input circuit, which when energized by one or more input signals, will form in its output circuit, an output signal proportional to the input signal or to the difference between the input signals.

Another object of the invention is to provide a variable gain amplier adapted to transfer signals over a Wide frequency range, and comprising a plurality of groups of space discharge devices, the devices of one group being adapted to supply input signals of opposite phase to the devices of another group which are so arranged as to obtain an output signal that is proportional to the difference between the input si-gnais. Consequently, a control signal applied in the saine phase to the devices of said one group balances out in the system and does not appear in the output circuit but serves only to control the gain of the devices of said one group.

A further object of the invention is to provide a system of this character wherein the space discharge devices of said one group comprise a cathode load feature and wherein there is provision for loading the anode circuits of said devices to the full extent to obtain full advantage of the cathode load feature.

Still another object of the invention is to provide an ampliiier having one input circuit which may be adapted for push-pull input signais, and another input circuit for a control signal, the amplifier being so arranged that, first, the control signal is definitely established in such a manner as to control the mutual conductance and thus the gain of the amplifier tubes without introducing degeneration of the input signals and, second, the output signal of the amplifier comprises only the input signal amplified by an amount dependent upon the control signal.

Other objects and features of the invention will appear hereinafter as the description proceeds.

In the accompanying drawings:

Fig. 1 is a circuit diagram of a device embodying the invention;

Fig. 2 shows the characteristic curves of certain tubes which may be used in the device of the invention; and

Fig. 3 is a schematic illustration of a scan ning signal generator employing an embodiment of the invention which may be used to actuate the scanning means in a conventional television system.

The generaI principles of the invention may be understood from a consideration of Fig. l wherein the output signal En appearing in the output circuit corresponds to the amplified input signal E1, the degree of amplification depending upon. a second or auxiliary input signal E2. The device comprises two tubes V1 and V2, which may be supplied with input signals by the phase-inversion tube V3. Such phase-inversion circuits are well known in the art and need not be described in detail. It suices to state that signals of opposite phase are applied to tubes V1 and V2. These tubes have their cathodes connected in a common cathode circuit including the resistor R1. Tubes V1 and V2 are preferably identical, one with the other, with respect to their electrical characteristics. The average bias Voltage applied to the grids Aof these tubes are maintained at corresponding values with respect to some predetermined voltage level, such as ground, by means of the bias battery C and the resistor R. Due to this resistor, the opposite phase voltages applied to the Igrids of tubes V1 and V2 nuctuate about the predetermined bias lvoltage level in response to the signal E1. The cathodes of tubes V1 and V2 are maintained at some voltage greater than ground potential, depending upon the voltage drop across the resistance R1. Thus the input signals for tubes V1 and V2 comprise the bias voltage due to battery C and the voltage drop in the resistor R1 plus the alternating components of the signel E1, which cause the grid voltages of tubes V1 and V2 to iiuctuate about the bias voltage level.

As described more particularly hereinafter, the two signals of opposite phase applied to the input circuits of tubes V1 and V2 produce an output signal E0, but any signal voltage appearlthe same as the impressed voltage.

ing across the resistor R1, which will be applied in the same phase to tubes V1 and V2, will not produce an output signal. Consequently, the signal E1 will be transferred through the system, but variations in the bias voltage of the tubes V1 and V2, which will appear as input signals of the same phase to both tubes, will not be transferred through the system. Tubes V1 and V2 may have the characteristic that their mutual conductance, which is a measure of gain or amplification, is a function of the input voltage. The amplification of the system may then be varied by varying the voltage drop across the resistance R1 which will vary the mutual conductance of tubes V1 and V2 and, consequently, will vary the degree of amplification of the signal E1. The voltage drop across the resistance R1 may be conveniently controlled by means of an additional tube V4 whose cathode is connected in common with the cathodes of tubes V1 and V2 and whose anode is connected to the plate supply source B. Thus, the current drain of tube V4, which may be used to control the voltage drop across the resistance R1, may be varied readily by varying the input signal E2 applied to the tube. It will be seen, therefore, that the input signal E1 may be amplified in the device by an amount dependent upon a second or auxiliary input signal E2 to obtain an output signal En similar in wave shape to the input signal E1 and containing no components directly proportional to the auxiliary signal E2.

As will be apparent from considering Fig. 1, the resistance R1 is included in both the input and output circuits of tubes V1, V2 and V4, and the voltage thereacross is included in the total voltage in the input circuits of these tubes as well as in the output circuits of tubes V1 and V2, and in addition this voltage may constitute the total output voltage of tube V4.

In the preferred mode of operation of the device, the signal E1 will be small as compared with the signal E2. Further, in general, the combined space current of tubes V1 and V2 Will be of the same order as that of tube V4. The cathode drive of tube V4 which biases tubes V1 and V2 may then offer certain unusual advantages. It will be seen that when a signal is applied to the input circuit of tube V4, the tube will tend to draw enough current so that the voltage across R1 is Under these conditions, the actual grid-to-cathode or tube input voltage would be simply the biasl voltage. The tube will not, in general, however, draw sufficient current to bring about this condition, but will fail by the diierential grid voltage required to cause the change in space current. In other words, the tube is not actuated by the input voltage but by the difference between that voltage and the voltage obtained across the resistor R1. By designing the circuit so that a small change in actual grid voltage of tube V4 will cause a large change in the voltage drop across the resistor R1 due to the change in space current of tube V4, the actual voltage across R1 may be made to follow the input Voltage E2 very closely.

It will be observed that the operating range of input voltages may be considerably larger than the total rated grid swing of the tube V4, since the actual grid-to-cathode voltage is the difference between the voltage E2 and the Voltage drop across the resistor R1, neglecting, of course, the unidirectional component which may be controlled by the average bias on the grid of tube V4.

Thus, for example, the actual grid-to-cathode Voltage on tube V4 may be a small fraction, say 1/10, of the actual input voltage. Since the circuit may be designed so that the tube operates over only part of its grid voltage-plate current characteristic, signal distortion due to non-uniformity of tube characteristic may be minimized to a considerable extent.

It is worthy of note that degeneration of the signal E1, which is usually present when an appreciable impedance is introduced in the cathode circuit of a vacuum tube amplifier having an anode load circuit, is avoided due to the property of this circuit of balancing out signals introduced in both branches. It should be noted further that the eiect of variations in space current of tubes V1 and V2, which variations are small, is further materially reduced by the use of the cathode load for tube V4. Since this tube is actuated by the difference in voltage between the input signal E2 and the voltage drop across the resistor R1, which difference voltage Will be small, it makes little difference Whether the latter voltage is due to space current in tube V4 or to some other cause, and in any event, the tube V4 will operate to minimize the discrepancy in voltage between signal E2 and the resistor voltage. Thus, the circuit of tube V4 not only establishes the bias on tubes V1 and V2 in accordance with the signal E2, but it also functions to minimize any tendency of these tubes to have any otherbias, and thus minimizes any degeneration of the signal E1 due to the cathode resistor R1.

In order to obtain full advantage of the cathode load, it is desirable that tubes V1 and V2 have an appreciable load in their respective anode circuits. The invention provides for loading of the anode circuits to the full extent. Tubes V1 and V2 are supplied with load impedances R3 and R4, respectively, which are of suitable magnitudes to act as proper loads for their respective tubes. The amplied signals obtained across R3 and R4 are supplied through blocking condensers to the grids of tubes V5 and V5. The cathode of tube V5 may be connected to the plate of tube V5, the anode of tube V5 may be connected to the B supply, and the cathode of tube V5 may be connected to ground. If desired, the plate voltages of tubes V5 and V5 may be increased by connecting the cathodes of tube V5 to the negative side of a second B Voltage supply unit whose positive side is connected to ground. The common junction of the tubes V5 and V5 is connected to an output load resistance R5 which may be connected to an intermediate point on the whole B voltage supply. Obviously, this intermediate point might alternatively be provided by a drop wire or other similar device. Tubes V5 and Vs should preferably have identical electrical characteristics, and their respective input grids may be suitably biased to similar operating points by the grid-leak resistors R5 and Rv, as shown.

Tubes V5 and V5 are thus connected so that their output circuits are in series with each other and the battery B. 'Ihe output circuit for the tubes, however, is the resistance R5 which for A. C. components is eiiectively connected across each tube output circuit. Signals across R4 are supplied to the input circuit of tube V5 while signals across R3 are combined With the output voltage across R5 and then supplied to the input circuit of tube V5.

By this interconnection of input and output circuits, a circuit is obtained such that the output signal is proportional to the difference between the input signals, and yet the system is perfectly stable.

This result is due to the fact that as far as results are concerned the output circuit has the properties of a cathode load output circuit, but the circuit interconnections are such that signals introduced in one input circuit are transferred without any phase reversal, while signals in the other circuit are transferred reversed in phase. Thus signals supplied to the input circuit of tube Vs are reversed in phase and transferred to the output circuit from which they are fed back to the input of tube V5 which tube operates to control the amplitude of the output signal so that it will be the same as that due to an input signal supplied to tube V5 which signal, however, would be transferred to the output circuit without reversal of its phase. Thus the output signal will be proportional to the dilierence between the input signals. For example, in the case in which the same signal is applied in each input circuit, the current in each tube would change by the same amount and there would b-e no current change in R5 since the current through this iinpedance is the difference between the current in each tube, Whereas, if equal and opposite signals were applied to each input circuit, the current in one tube would increase while that of the other would decrease, the dilference current through R5 would produce an output signal which would feed back affecting the grid voltage of the one tube and the plate voltage of both in such a way as to cause equal amplification of both input signals. The resultant output signal would then be proportional to twice the input signal.

Consider now the operation of the circuit as a whole. The, group of tubes V1 and V2 may be biased to the same point by means of the common resistance R and the common Voltage across R1. age across R1 and supplied in opposite phase to tubes V1 and V2 by means of the phase inverter V3 as shown. Output signals from tubes V1 and V2 may be derived from resistances R1 and R4 respectively and supplied to the group of tubes V5 independent of tub-es V1 and V2 and determined only by the control signal E2. The grid-to-cathode voltage of tube Vs will correspond to the voltage across Ri while that of tube V5 will be the voltage across R3 combined with that across R5.

Preferably R3 and R4 should have the same value. and V5 and Ve should be similar tubes. Under these conditions, the output signal across R5 will be proportional to the difference signals supplied to the group of tubes V1 and V2, and the proportionality factor will be dependent upon the control signal E2 which will not otherwise appear in the output circuit. The input circuits of tubes V5 and Vs constitute high impedance sources and consequently do not tend to restrict the use of proper load impedances R3 and R1 in the output circuits of tubes V1 and V2. Hence, the optimum operating conditions for the circuit as a whole may be obtained.

Thus, an input signal is supplied to one of two amplifying branches each of which includes an amplifier tube whose mutual conductance is a function of its grid voltage. A similar signal of opposite phase is supplied to the input circuit of the other branch. A second input signal is supplied to the input circuits of both branches in A signal E1 may be combined with the voltif such a way that the mutual conductance of an amplifier tube in each branch is modified, producing in each branch an identical undesired signal and determining the degree of amplification of the signal E1. The output of each of these branches is then combined in a circuit which forms in its output circuit a signal proportional to the difference between the two input signals, that is, a signal corresponding to E1 but amplified by an amount determined by E2, and which permits the use of load impedances in the anode circuits of the input tubes, thus enabling better control of the gain of these tubes by the control tube.

While it is preferred to apply opposite phase input signals to tubes V1 and V2, as in Fig. 1, it will be apparent that an input signal applied to one of these tubes only will be transferred through the system while a common control signal will be balanced out. It is contemplated, therefore, to apply an input signal to only one of the tubes V1 and V2, if desired.

The amplifier shown in Fig. 1, and described above, is adaptable for use over a wide frequency range due to the fact that, in general, resistancecapacitance coupling is used and, as well known, such coupling devices are substantially uniform over a frequency range extending from some low Value to reasonably high values. It will be noted that there are no transformers employed in the device of Fig. 1 which would tend to restrict the frequency range. The use of the phase inverter Vi makes it possible to obtain the opposite phase input signals for tubes V1 and V2 without employing a transformer.

It is likewise important that the frequency response of each branch be the same at the low frequency as well as the high frequency end. and hence in Fig. l the values of the condensers and resistors connected to R3 and R4 should be so designed that the transfer circuits will have similar frequency characteristics. As R3 and Ri should preferably be equal, this may easily be accomplished by using condensers and grid leal: resistors of equal values.

It is worthy of note that in the circuit of Fig. l., no unusual restrictions are placed upon the source of B voltage for the circuits, and the same B supply as that used for associated apparatus may be employed. In addition, one side of each of the several input and output circuits may be grounded for A. C. components which facilitates the use of the circuit.

The variable gain amplifier provided by the invention finds several uses in television systems where wide frequency ranges are encountered. For example, due to diiference between the center line of the optical system of a television carnera tube and the axis of the electron beam generator which supplies the scanning beam, an effect known as the keystone effect occurs and this effect may be overcome by causing the amplitude of the horizontal deflection of the beam to vary in accordance with the amount of vertical deiiection. This may be accomplished by amplifying the signal for producing horizontal deflection in a variable gain anuoliiier` the gain of which is determined by a second signal derived from the vertical deiiecting system. In this instance it is desirable to have the amplitude of the signal E1 substantially directly proportional to a constant plus the signal E2. This may be accomplished by using for V1 and V2, tubes having the mutual conductance characteristic MZ shown in Fig. 2. If, for example, the signal E2 varies the operating points of the tubes V1 and V2 from the -grid voltage value GV1 in Fig. 2 to the value GVz, the amplincation or gain of the amplified signal E1 will vary from pmi to gmc since the stage gain is substantially proportional to the mutual conductance of the tube.

In another instance it may be desirable to have the gain constant at one value part of the time, and zero or some low value other times. This may be done by using for V1 and V2, tubes having a mutual conductance characteristic such as N in Fig. 2. In this case the control. signal E2 may vary the operating point from GV4 to GVg for which points the gain will be zero and proportional to gms, respectively. It will be noted that if the gain curve N is flat topped from to y, the voltage of the control signal plus the input signal Et may be anywhere in the interval a.' to y, thus permitting the use of larger input signals Ell. Where a push-pull input circuit such as that of Fig. 1 is used, distortion of the signal El due to the slope of the gm curve is largely balanced out, and larger input signals maybe employed. Tubes having characteristics as shown in Fig. 2 are now available commercially. For example, tubes of the type known as 42 have a characteristic similar to M while tubes of the type known as '77 have a characteristic similar to N. Other tubes having similar or different characteristics may, of course, be used, depending upon the particular application.

In Fig. 3, there is shown in block diagram a schematic outline of a circuit which may be used to avoid the keystone effect above mentioned. In the figure, horizontal and vertical deflecting signals En and Ev respectively are supplied to blocking tube oscillators l and 2 which in turn energize generators 3 and i which form the proper scanning signals. In the vertical signal circuit, the blocking tube oscillator t also energizes a sav/tooth signal generator 5. The output oi the generator 5 is supplied to a variable gain amplilier 6 of the type shown in Fig. 1 which amplifier is included in the horizontal signal circuit and serves to control the amplitude of the horizontal scanning signal in accordance with the vertical scanning signal, by which the keystone eiect is eliminated. Both scanning signals are then arnplied by power ampliiiers 'l and 8 and used to energize appropriate deflecting units 9 and lil, such as those commonly associated with cathode ray tubes.

From the illustrated embodiments and the above description, it will be seen that the invention provides novel means for the accomplishment of the objects and functions herein set forth. It will be understood, of course, that the present disclosure is merely illustrative and imposes no limitation upon the invention, the scope of which is dened in the appended claims;

I claim:

1. In an electrical system, a plurality oi groups of space discharge devices, each of said discharge devices having an input circuit and an output circuit, each of said discharge devices having an electrode common to its input and output circuits, each of said discharge devices of one of said groups having a mutual conductance dependent upon the voltage in its input circuit, another space discharge device having an input circuit and an output circuit, an impedance having a voltage thereacross in the input circuits and output circuits of said one group of discharge devices and said other discharge device, for establishing said voltage in each of said lastnamed circuits, means for introducing an input acer/,81o

signal in the input circuit of at least one of the space discharge devices of said one group, means for introducing an input signal in the input circuit of said other space discharge device, means including an impedance in the output circuit of one of the discharge devices of said one group for deriving a signal therefrom, means including an impedance in the output circuit of another discharge device of said one group for deriving a second signal therefrom, means for connecting the output circuits of the discharge devices of another group in series relation, means including an impedance effectively -connected for signal currents to each of the common electrodes of said last-named discharge devices for forming an output signal, means for applying said rst derived signal to the input circuit of one of said last-named ischarge devices, and means for combining said second derived signal and said output signal and for applying said combined signal to the input circuit of another of said lastnarned discharge devices.

2. In an electrical system, a plurality of groups of space discharge devices, each of said discharge devices having an input circuit and an output circuit, each of said discharge devices having an electrode common to its input and output circuits, each of said discharge devices of one of said groups having a mutual conductance dependent upon the voltage in its input circuit, another space discharge device having an input circuit and output circuit, an impedance having a voltage thereacross in the input circuits and output circuits of said one group of discharge devices and in the output circuit oi said other discharge device, for establishing said voltage in each of said last-named circuits, means for introducing an input signal in the input circuit of at least one of the space discharge devices of said one group, means for introducing an input signal in the input cir-cuit of said other space discharge device, means including an impedance in the output circuit of one of the discharge devices of said one group for deriving a signal therefrom, means including an impedance in the output circuit of another discharge device of said one group for deriving a second signal therefrom, means for connecting the output circuits of the discharge devices of another group in series relation, means including an impedance effectively connected for signal currents to each of the common electrodes of said last-named discharge devices for forming an output signal, means for applying said iirst derived signal to the input circuit of one of said last-named discharge devices, and means for combining said second derived signal and said output signal and for applying said combined signal to the input circuit of another oi said last-named discharge devices.

3. In an electrical system, a plurality of groups of space discharge devices, the space discharge devices in each group having similar electrical characteristics, each of said discharge devices having an input circuit and an output circuit, each of said discharge devices having an electrode common to its input and output circuits, each of said discharge devices of one of said groups having a mutual conductance dependent upon the voltage in its input circuit, another space discharge device having an input circuit and an output circuit, an impedance having a voltage thereacross in the input circuits and output circuits of said one group of discharge devices and said other discharge device, for establishing said voltage in each of said last-named circuits, means for introducing an input signal in the input circuit of at least one of the space discharge devices of said one group, means for introducing an input signal in the input circuit of said other space discharge device, means including an impedance in the output circuit of one of the discharge devices oi said one group for deriving a signal therefrom, means including an impedance in the output circuit of another discharge device of said one group for deriving a .second signal therefrom, means for connecting the output circuits of the discharge devices ci another group in series relation, means including an impedance effectively connected for signal curents to each of the common electrodes of said last-named discharge devices for forming an output signal, means for applying said first derived signal to the input circuit of one of said last-named discharge devices, and means for combining said second derived signal and said output signal and for applying said combined signal to the input circuit of another of said last-named discharge devices.

i. In an electrical system, a plurality of groups of space discharge devices, the space discharge devices in each grouphaving similar electrical characteristics, each of said discharge devices having an input circuit and an output circuit, each of said discharge devices having an electrode common to its input and output circuits, each of said discharge devices of one of said groups having a mutual conductance dependent upon the voltage in its input circuit, another space discharge device having an input circuit and an output circuit, an impedance having a voltage thereacross in the input circuits and output circuits of said one group of discharge devices and said other discharge device, for establishing said voltage in each of said last-named circuits, means for introducing an input signal cluding an impedance in the output circuit of one or the discharge devices of said one group for deriving a signal therefrom, means including an impedance in the output circuit of another discharge device of said one group for deriving a second signal therefrom, said last-named impedances being substantially equivalent one to the other, means for connecting the output circuits of the discharge devices of another group in series relation, means including an impedance effectively connected for signal currents to each of the common electrodes of said last-named discharge devices for forming an output signal, means for applying said iirst derived signal to the input circuit of one of said last-named discharge devices, and means for combining said second derived signal and said output signal and for applying said combined signal to the input circuit of another of said last-named discharge devices.

5. in an electrical system, a plurality of groups of space discharge devices, each of said discharge devices having an input circuit and an output circuit, each of said discharge devices having an electrode common to its input and output circuits, each of said discharge devices of one of said groups having a mutual conductance dependent upon the voltage in its input circuit, another space discharge device having an input circuit and an output circuit, an impedance having a voltage thereacross in the input circuits and output circuits of said one group of discharge devices and said other discharge device, ior establishing said voltage in each of said last-named circuits, means for introducing an input signal in the input circuit of one of the space discharge devices of said one group and for introducing an input signal of opposite phase relation in the input circuit of another of the space discharge devices of said one group, means for introducing an input signal in the input circuit of said other space discharge device, means including an impedance in the output circuit of one of the discharge devices of said one group for deriving a signal therefrom, means including an impedance in the output circuit of another discharge device of said one group for deriving a second signal therefrom, means for connecting the output circuits of the discharge devices of another group in series relation, means including an impedance eiiectively connected for signal currents to each of the common electrodes of said last-named discharge devices for forming an output signal, means for applying said rst derived signal to the input circuit of one oi said last-named discharge devices, and means for combining said second derived signal and said output signal and for applying said combined signal to the input circuit ci another of said last-named discharge devices.

6. lin an electrical system, a plurality of groups of space discharge devices, the space discharge devices in each group having similar electrical characteristics, each of said discharge devices having an input circuit and an output circuit, each of said discharge devices having an electrode common to its input and output circuits, each of said discharge devices of one of said groups having a mutual conductance dependent upon the voltage in its input circuit, another space discharge device having an input circuit and an output circuit, an impedance having a voltage thereacross in the input circuits and output circuits of said one group of discharge devices and said other discharge device, for establishing said voltage in each of said last-named circuits, means for introducing an input signal in the input circuit of one of the space discharge devices of said one group and for introducing an input signal of opposite phase relation in the input circuit or another of the space discharge devices of said one group, means for introducing an input signal in the input circuit of said other space discharge device, means including an impedance in the output circuit of one of the discharge devices of said one group for deriving a signal therefrom, means including an impedance in the output circuit of another discharge device oi said one group for deriving a second signal therefrom, said last-named impedances being substantially equivalent one to the other, means for connecting the output circuits of the discharge devices of another group in series relation, means including an impedance eectively connected for signal currents to each of the common electrodes of said last-named discharge devices for forming an output signal, means for applying said rst derived signal to the input circuit of one of said last-named discharge devices, and means for combining said second derived signal and said output signal and for applying said combined signal to the input circuit of another of said last-named discharge devices.

'7. In an electrical system, a plurality oi groups of space discharge devices, the space discharge devices in each group having similar electrical characteristics, each of said discharge devices having an input circuit and an output circuit, each of said discharge devices having an electrode common to its input and output circuits, each of said discharge devices of one of said groups having a mutual conductance dependent upon the voltage in its input circuit, another space discharge device having an input circuit and an output circuit, an impedance having a voltage thereacross in the input circuits and output circuits of said one group of discharge `devices and said other discharge device, for establishing said voltage in each of said last-named circuits, means for introducing an input signal in the input circuit of at least one of the space discharge devices oi said one group, means for introducing an input signal in the input circuit of said other space discharge device, means including an impedance in the output circuit of one of the discharge devices of said one group for 'deriving a signal therefrom, means including an impedance in the output circuit of another discharge device of said one group for deriving a second signal therefrom, said last-named impedances being substantially equivalent one to the other, means for connecting the output circuits of the discharge devices of another group in series relation, means including an impedance effectively connected for signal currents to each of the common electrodes of saidV last-named discharge devices for forming an output signal, means including a transfer circuit having a certain frequency response for applying said rst derived signal to the input circuit of one of said last-named discharge devices, and means including another transfer circuit having a substantially similar frequency response for combining said second derived signal and said output signal and for applying said combined signal to the input circuit of another of said last-named discharge devices.

8. In an ampliiier circuit, a balanced or double-sided circuit, an unbalanced or singlesided load impedance for said amplifier, a pair of vacuum tubes, input circuits for said tubes coupled to said balanced circuit, a direct currentV supply source for said tubes, means for connecting the direct current plate circuits of said tubes in series with respect to one another and said direct current source, and means for connecting the signal output circuits of said tubes in parallel with respect to said load impedance, said load impedance being included in the direct current plate circuits of both of said tubes and in the input circuit of one of said tubes.

9.'In an amplifier circuit, a pair of space discharge devices each having a cathode, anode, and control grid, signal input circuits for said devices, a source'of direct current, means connecting said source and the anode-cathode paths of said devices in a series loop circuit with respect to the direct current supplied by said source, and an alternating-'current output impedance for said devices, said impedance being connected by a degenerative cathode load for one of said devices and as an anode load for the other of said devices.

10. In an amplifier circuit, a pair of space discharge devices each having a cathode, anode, and control grid, a source of direct current, means connecting said source and the anodecathode paths of said devices in a series loop circuit with respect to the direct current supplied by said source, an alternating-current output impedance for said devices, said impedance being connected as a cathode load for one of said devices and as an anode load for the other of said devices, means for applyingan input signal to the grid-cathode circuit of one of said devices, and means for combining another input signal with the output signal across said outp-ut impedance and for applying said combined signal to the grid-cathode circuit of the other of said devices.

l1. In an amplifier circuit, a pair of space discharge devices each having a cathode, anode, and control grid, a balanced input circuit connected to said control grids forr driving said grids in opposite phase relation, a source of direct current, means connecting said source and the anode-cathode paths of said devices in a series loop circuit with respect to the direct current supplied by said source, an alternating-current output impedance for said devices, and connections for shunting the anode-cathode paths of said devices across said output impedance in mutually reversed relation, said output impedance being included in the direct current anodecathode circuits of both of said devices and in the grid-cathode circuit of one of said devices.

l2. In a signal transfer circuit, a pair of space discharge devices each having a grid-cathode input circuit and an anode-cathode output circuit, a source of direct current, means connecting said source and the anode-cathode paths of said devices ina series loop circuit, and a common load impedance for said serially connected devices, said load impedance being disposed in both the anode-cathode and grid-cathode circuits of one of said devices, but only in the anodecathode circuit of the other of said devices.

13. In a signal combining circuit, a pair of space discharge devices each having a gridcathode input circuit and an anode-cathode outputh circuit, a source of direct current, means connecting said source and the anode-cathode paths of said devices in a series loop circuit, a common load impedance for sai-d devices for forming an output signal thereacross, means for applyingV an input signal to one of said input circuits,V and means for combining an input sig-` nal with said output signal and for applying said combined signal to the other of said input circuits.

DAVID B. SMITH.

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