US2205335A - Push-pull amplifier circuit - Google Patents

Description

June 18.1940.' l.. E. BARToN I 2,205,335

PUSH-PULL AMPLIFIER CIRCUIT Filed oct. 1e, 1937 35.. for one of the two output tubes.

Patented .une 1,8, 194() PATENT yorrlci:

PUSH-PULL AMPLIFIER CIRCUIT Loy E. Barton, Collingswood, N. J., assigner, .by mesne assignments, to Pennsylvania Patents,

IHC Nevada Carson City, Nev.,

a corporation of Application October 16, 1937, Serial No. 169,491

13 Claims. (Cl. 179-171) This invention relates to radio receiving systems and more particularly to circuits for use in radio receivers and the like wherein it is desired to form, in response to an input signal, a correspending power output signal having a minimum of distortion. The frequency range of the input signal may be quite wide and may extend over the l entire audio frequency range.

One object of the invention is to provide a -stable push-pull Class A amplifier in which one output tube is self-excited, that is the driving voltage for the said tube is obtained from the out-v put circuit. l

Another object of the invention is to provide a stable push-pull Class A output system requiring no input transformers or phase-inverter tubes, and in which the full available Class A output of each tube may be supplied to the output circuit.

Other objects and features of the` invention will be apparent from a lconsideration of the two figures ofthe drawingr of which I Fig. 1 illustrates the preferred form of Vention; and

Fig. 2 is a modification thereof.

It will be understood, however, that the invention is capable of modification and is not limited to the speciiic form shown.

Referring now to Fig. l, the space discharge tubes V1 and V2 are power output tubes of the the intype commonly employed in conventional radio' receivers. These tubes may be either triodes or pentodes or beam type output tubes, although preferably pentode 'or beam type tubes are employed. The tube V3` serves as the driver tube A signal source E supplies the desired input signal to tube V3. The signal sourcemay be of any suitable charactenfor example, it may be a detector, a microphone, a phonograph pick-up, etc. The output .circuits for tubes V1 and V2 may comprise the two portions of the center-tapped primary P of the transformer T, whose secondary S may be connected to some suitable utilization means, such as the loud speaker LS. It will be understood that any suitable utilization means may be employed.

Signal from the output circuit of the tube Vs may be built up across its plate load impedance R1 and supplied to the control grid of tube V1 by means of the coupling condenser C1 and the grid-leak resistance R2. The input signal for the tube V2 may be obtained by means of the voltage divider circuit R3R4 which is connected to the side of the primary P opposite from that to which the anode of tube V2 is connected.

Disregarding for the present the resistance R5 which will be discussed later, it will' be apparent that the input .signal supplied to the control grid *ofl tube V1 will cause an amplified signal of opposite phase to appear across that portion of the center-tapped primary P which is connected to tube V1.` This amplified signal is supplied through'the voltage divider RaRi to the control grid of V2, causing a signal of opposite phase't appear across that portion of the center-tapped primary P which is connected to the output circuit of tube V2. Thus, there are produced in the respective output circuits of tubes V1 and V2 signalswhich are of opposite phase with respect to each other.

'I'he circuit described thus far is not useful for the amplification of a signal such as an audio signal, ldue `to` `the fact that such circuit will oscillate'at its natural frequency unless the signal supplied to tube V2 is Very much smaller than that supplied to tube V1. Considering for the moment thel circuit including only the tube V2, it will be observed that the output signal from tube V2 will formacross'the voltage dividing network RsRi a signal of equal amplitude but of opposite phase. A' portion of this signal is supplied to the control grid of tube V2, causing the tube to oscillate when a suicient portion of the output signal is fed back. In fact, such a circuit has been employed as an oscillator. On the 'other hand, if the portion 'of the signal fed back to the control grid of tube V2 is insuflicient to cause the circuit to oscillate, the amountl of power supplied by the tube V2 will bev negligible compared to that which it is capable of supplying or that which'is obtained from tube V1. Obviously, in such case the tube V2 Wou1d=serve no useful purpose.

By the-present invention, the maximum available AClass'Aoutput is obtained from tube V2 and at the same time, the circuit is prevented from oscillating 'and is caused to reproduce faithfully the input 'signal supplied to it. In laccordance withl the invention, a degenerative connection is provided between the output circuit and the input circuit of'tube V1; vA degenerative connection is one in which a portion of the output signal is fed back to the input circuit in such phase relation that a current iscaused to flow in the output circuit in such direction as to opposethe voltage causing it and to lower" the effective output impedance ofthe degenerated amplifier.

Such connection may take different forms but is-preferably provided-by connecting the resistance R5 ibetween/ the anode of tube V1 and the grid circuit of tube V1.- r4Slt-will be noted that byithis ,necessarily occur to any great extent.

connection two direct current paths are provided between the plate of V3 and the power supply, which has the advantage of minimizing the D. C. voltage drop in the plate circuit external of the tube. When this degenerative connection is employed, the amplifier will not oscillate because the tube V1 will oppose any tendency of tube V2 to oscillate. The necessary condition for sustained oscillation is that the signal energy fed to f the grid of tube V2 be suflicient to overcome the losses in the whole circuit. In the absence of the degenerative connection, the tube V1 appears as a relatively high impedance and does not provide sufficient load in the circuit to prevent oscillations. However, when the degenerative circuit is employed, the effective impedance of the `tube V1 becomes quite low and prevents any oscillations in the circuit, even though the input signal supplied to tube V2 be as large or larger than that v supplied to tube V1.

This lowering of the equivalent impedance of tube V1 may be understood by noting that if the voltage between the cathode and anode of tube V1 were to change by some small amount, a portion of this voltage in the same phase would be supplied to the input circuit of tube V1, which' would cause a differential current to flow in the tube in such direction as to prevent the variation in output voltage. Thus the tube V1 operates as a device to prevent any variation in its cathodeanode voltage, except variations corresponding to the input signal supplied toit, or in other Words as a low impedance load for undesired voltage variations. Hence, tube V1 will prevent tube V2 from self-oscillating, since in order for self-oscillations to occur, the voltage across the output circuit of tube V1 must change.

The amount of degeneration may be controlled by varying theratio of the magnitude of resistance R to the equivalent vshunt impedance of 'Rn R5 and the plate-to-cathode impedance'of The condenser C1 serves as a blocking tube V3. condenser to keepthe D. C. -plate voltage outl of the' grid circuit. Thecondenser C1 may have a negligible impedance at audio frequencies compared to the impedance of R2. In general, the useof degeneration will bring about a reduction in the gain 'of the degenerated stage. In this particular case, however, this effect does not Considering` the voltage relations in the input circuit of ductance of tube V3 and the total output load imtube V1 and including the degenerative path, it will be observed that the effect of the feedback path is to reduce the input impedance ofthe input circuit inducing R2,`R5 and the tube v1, that 'is the effective impedance which acts as a load for tube V2. However, the signal supplied to the grid of tube V1 will be equal to the signal across `R1 and themagnitude of this signal will be given by 'a function of the product of the mutual conpedance. By proper design, the equivalent input impedanceof tube V1 may be made sufliciently high so that it will, in shunt with R1, provide an optimum load impedance for the preceding tube.

"Hence it will not materially affect the total gain Hcuit of tube V2.

C2`mjay` be negligible as compared rwith that of resistance R3 at the lowest frequency which it is desired to amplify. 'Ihe condenser C3 is in shunt with R3 and serves to prevent variations with frequency in the proportion of signal fed back. There will be some inter-electrode capacity between the control grid of tube V2 and its cathode and screen grid, as well as a capacity between the control grid and anode. This capacity is effectively in shunt with resistance R4. By providing the condenser Ca in shunt with the resistance Ra, and by arranging the ratio of the impedance of the condenser to that of the equivalent input capacity of the tube so that it is proportional to the ratio of resistance R3 to resistance R4, the proportion of signal transferred to the control grid of tube V2 compared to the total output signal may be made independent of frequency.

By preventing the circuit from regenerating, in accordance with the invention, each tube may be driven so as to obtain the maximum Class A power output. Since tubes V1 and V2 are preferablysimilar, this condition obtains when each tube is supplying the same amount of power and the input signals supplied to tubes V1 and V2 are the same. All of the power output obtained from the two tubes may be supplied to the output circuit, except for that portion fed back to the input circuits of the two tubes by means of the voltage divider RaR4 and the feed-back path including R1, R2 and R5. However, the impedance of these circuits, particularly that of Ra and R5, may be made so large compared to that of the load plate-to-cathode impedance that the amount of power absorbed by the feedback paths is negligibly small. For example, the plate-tocathode load impedance may be of the order of magnitude of 2500 ohms, whereas that of R3 and R5 is of' the order of magnitude of 500,000 ohms. Hence, the amount of power taken by the feedback paths would be one-half of one per cent of the total power available, which is negligibly small.

In Fig. 2 is shown one modification of the invention. In this figure the inputsignal for V2 is obtained from a tapped connection on the primary of the transformer, insteadof by means of the voltage divider of Fig. 1 which at the present time is slightly more economical. .The degenerative feedback connection also differs from that of Fig. 1 in that use is made of a secondary winding of the transformer, which may conveniently be the output winding or an extension thereof, to obtain a feedback voltage in the input circuit. The phase of the feedback voltage must be opposite that of the input signal, and will decrease the overall gain of the amplifier, and hence is less desirable than the circuit of Fig. 1.

It will be apparent that various other modifications of the invention are possible without departing from the spirit and scope of the invention, and therefore the invention is not confined to the specific forms herein described.

I claim:

l. In an amplifier circuit, a load circuit including a winding, a plurality of space discharge devices each having input and output circuits and characterized in that the output voltage of each of said devices is of substantially opposite phase with respect to the input voltage thereof, connections for including a part of the winding in the output circuit of one space discharge device, connections between another part of the winding and the input circuit of the said one discharge device for supplying to the said input 'circuit a non-degenerative signal-voltage of 'opposite 'phaseI of another space discharge device, and means for degeneratively coupling the input andoutput circuits of said other discharge device.-

2. In an amplifier circuit, a load circuit including a winding, a pluralityofspace discharge devices each having input andoutput circuits,`

connections for including a part of the winding in the output' circuit of one space -discharge device, connections between anotherpart of `the winding and the input circuit of the said one discharge device, a voltage divider included-.iin said last-named connections for applying a nondegenerative signal to the input circuit of said one device, connections for including part-of said winding in the output circuit of another space discharge device, and a degenerative connection for coupling the input and output circuits-oi said other discharge device. 1

3. In an amplier circuit, a load circuit including a winding, a plurality of space discharge devices-each having input and output circuits, connections `for including a part of the winding in the output circuit of one space discharge device, connections between another part of the Winding and the input circuit of the said one discharge device, a voltage divider included in said lastnamed connections for applying a desired signal to the input circuit of said one device, a condenser connected across a portion of the voltage divider, connections for including part of said Winding in the output circuit of another space discharge device, and a degenerative connection for coupling the input and output circuits of said other discharge device.

4. In an amplier circuit, a load circuit including a winding, a plurality of space discharge devices each having a cathode, a control grid, a

screen grid, and an anode, and characterized in that the output voltage of each of said devices is of substantially opposite phase with respect to the input voltage thereof, connections for including a part of the winding in circuit with the anode and cathode of one space discharge device, connections for including another/part of the winding in circuit with the cathode and control grid of said one device so as to apply to said control grid a non-degenerative voltage in phase opposition with respect to the voltage on the.

anode of said one device, connections for including part of said winding in circuit with the anode and cathode of another space discharge device to form an output circuit, means for supplying electrical energy to the anodes and screen grids of each of said discharge devices, an input circuit including a source of signal energy connected to the cathode and control grid of said other device, and a degenerative connection for coupling the input and output circuits of said other discharge device.

5. In an amplifier circuit, a load circuit including a winding, a plurality of space discharge devices each having input and output circuits, connections for including a part of the winding in the output circuit of one space discharge device, connections between another part of the winding and the input circuit of the said one discharge device, connections for including part of said winding inthe output circuit of another space discharge device,l an additional space discharge device having an output circuit connected to the input circuit of said other device, and a conductive impedance connected between the output circuit of said other device and the output circuit of said additional device, for degeneratively coupling the input and output circuits of said other discharge device.l

. 6. In an amplifier circuit, a load circuit including a winding, a plurality of space discharge .devices each having input and output circuits, connections for including a part of thelwinding in the output circuit of one space discharge device, connections between another part of the winding and the input circuit of the said one discharge device, connections for including part of said winding in the output circuit -of another space discharge device, an additional `space discharge device having an output circuit connected to the input circuit of said other device, and a resistance connected between the output circuit of said other device and the output circuit of said additional device, for degeneratively coupling the input and output circuits of said other discharge device.

7. In an amplier circuit, a load circuit including a`winding, a pair of space discharge devices each having a cathode, a control grid, a screen grid and an anode, connections between the ends of said winding and said anodes, a connection including a source of electrical energy between the mid-point of said Winding and both of said cathodes,` a circuit for .supplying electrical energy connected to said cathodes and said screen grids, a voltage divider connected to the anode-cathode circuit of one of said devices, a circuit for deriving an input signal from said voltage divider and supplying it to the control grid of said other device, an additional space discharge device having an output circuit, a circuit for supplying a signal fromsaid output circuit to the control grid of said one device, and a resistance connected in circuit with the anode of said one device and the output circuit of said additional device for degeneratively coupling the anode-to-cathode and control grid-to-cathode circuits of said one device.k

8. In an electrical system, a pair of space discharge devices, each having a cathode, an anode and a control element, an output circuit including a winding and means for connecting one end of said winding to one anode and the other end of said winding to the other anode, means for supplying electrical energy to said devices, an input circuit for one of said devices connected between the cathode and the control element thereof, means for deriving a signal from the output circuit of said one device and for applying the derived signal between the cathode and control element of the other device, and a degenerative connection between the output and input circuits of said one device.

9. In an amplier circuit, a source of signals comprising unbalanced or single-sided circuits, a balanced or double-sided circuit comprising a pair of space discharge devices each having input and output circuits, coupling means for applying signals from said source to the input circuit of one of said space discharge devices, means for deriving signals from the output circuit of said one space discharge device, coupling means for applying said derived signals to the input circuit of the other of said spaced discharge devices, a load for said space discharge devices, and a connection for degeneratively coupling the output and input circuits of said one space discharge device.

10. In an amplier circuit, a source of signals comprising unbalanced or single-sided circuits, a

balanced or double-sided circuit comprising a pair of space discharge devices in push-pull relation, said devices including control grid and plate electrodes, coupling means for applying signals from said source to the control grid of one oi said space discharge devices, means for deriving signals of reversed phase from the plate circuit of said one space discharge 1 device, means for applying said reversed phase signals to the control grid o1" the other of said space discharge devices, a balanced or double-sided load for said space discharge devices, and means connected between the plate and grid circuits of said one Space discharge device for degeneratively coupling the output and input circuits thereof.

11. In an ampliiier circuit, a load impedance, a plurality of space discharge devices each having input and output circuits, connections for including a part of the load impedance in the output circuit of one space discharge device, connections between another part of the load impedance and the input circuit or" the said one discharge device, a Voltage divider included in said last-named connections for applying a non-degenerative signal to the input circuit of said one device, connec-y tions for including part of said load impedance in the output circuit of another space discharge device, and a degenerative connection for coupling the input and output circuits of said other discharge device.

12. In an electrical system, a pair of space discharge devices,veach having a cathode, an anode and a control element, an output impedance, and means for connecting one end of said impedance to one anode and the other end of said impedance to the other anode, means for supplying electrical energy to said devices, an input circuit for one of said devices connected between the cathode and the control element thereof, means for deriving a signal from the output circuit of said one device and for applying the dcrived signal between the cathode and control element of the other device, and a degenerative connection betweenfthe output andinput circuits discharge device for supplying to the said input` circuit a non-degenerative signal voltage of opposite phase with respect to the voltage in the output circuit of saidone discharge device, connections for including part of said load impedance in the output circuit of another space discharge device, and means for degeneratively coupling the input and output circuits of said other discharge device.

LOY E. BARTON.

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