US2485369A - Push-pull amplifier system - Google Patents

Description

Oct. 13, 1949. g E 2,485,369

PUSH-PULL AMPLIFIER SYSTEM Filed Dec. 4, 1947 SOURCE Inventor: v Robert flDome,

His Attorney Patented Oct. 18, 1949 UNITED STATES PATENT OFFICE PUSH-PULL AMPLIFIER SYSTEM Robert B. Dome, Geddes Township, Onondaga County, N. Y., assignor to General Electric Company, a corporation of New York Application December 4, 1947, Serial No. 789,647

may be driven by a single-ended source. It is a primary object of my invention toprovide improved means for obtaining push-pull output from a single-ended source.

As it is well known in the art, it is desirable for high fidelity sound output to employ a pushpull out-put amplifier. In such systems as have been used in the past to obtain push-pull output from a single-ended source, the output capabilities of the push-pull amplifier have not been utilized to full advantage, since a series resistance has customarily been employed in the anode output circuit of one of thepair of output tubes in order to obtain a driving voltage of suitable phase for the control grid of the second output tube. Furthermore, some of the prior systems proposed have an inherent tendency toward selfoscillation due to regeneration. In addition, some of these systems are characterized by a certain amount of undesirable hum which appears at the loud speaker.

It is a particular object of my invention to provide a push-pull amplifier circuit which obtains improved output. I

Another object of my invention is to provide such a system which is free of any tendency toward self-oscillation between the push-pull tubes as is sometimes encountered because of the similarity of the circuits to the Hartley oscillator. I

Another object of my invention is to provide such an amplifier in which undesirable hum is substantially eliminated from the output.

A further object of my invention is to provide a push-pull amplifier, driven by a single-ended source, which employs a transformer in the anode circuit of the first-tube to supply grid excitation for the second tube, thereby effecting an economy of parts. 1

The features of my invention which I believe to.

be novel are set forth with particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. 1 is a schematic circuit diagram of a push-pull amplifier system which suitably embodiesmy invention, and Fig. 2 is'aschematic diagramof another modification. Like numerals indicate like elements in the two figures.

Referring to Fig. 1, an electron discharge device I having cathode, anode, and control electrodes 2, 3, 4, respectively, serves as a singleended audio amplifier. Audio-frequency signal voltage is applied to the control electrode 4 of device I from a suitable source 5, which may comprise the second detector of a superheterodyne receiver. The customary cathode bias resistor 6 and by-pass condenser l are provided between the cathode 2 of device I and the low potential side of the audio-frequency source 5, which is shown connected to ground. Positive direct operating voltage is applied to the anode 3 of device I from a suitable source, here shown as a battery 8, through the customary anode load resistance 9. The amplified audio-frequency output appearing across the load resistance 9 is coupled to the control lectrode H) of device H, which is one of a pair of electron discharge devices I I, [2 by means of the customary coupling capacitance l3 and rid leak resistance l4. Devices ll, I2 are connected in push-pull, the cathodes I5, I6 being directly connected together and thence to ground through a common cathode bias resistance I1 and by-pass condenser 18. The screen grids I9, 20 of devices ll, l2 are directly connected together, positive direct screen operating voltages being supplied from a suitable source, here shown as a battery 2|.

I provide an anode output circuit, comprising the parallel combination of the primary winding 22 of a conventional output transformer 23 and a capacitance 24, connected in series between the anodes25, 26 of devices II, [2 respectively. Also included in the anode circuit of device II is the primary winding 21 of an auxiliary transformer 28, shunted by a resistance 29. Suitable positive unidirectional operating potentials are supplied to the respective anodes 25, 26 from battery 2| through a tap 30 on the primary winding 22 of output transformer 23. The secondary winding 3i of output transformer 23 is connected to a suitable load, here shown as a loudspeaker 32.

In order to obtain true push-pull operation, the

secondary winding 33 of auxiliary transformer 28, being isolated from the primary winding 27 by an electrostatic shield 34, is connected between the control electrode 35 of device 12 andground.

In operation, signals emanating from the audiofrequency source 5 are applied to the control grid 4 of device I, and the amplified output is impressed on the control electrode to of device I l' in a manner well known in the art. The resulting variations in the anode current of device ll set up audio voltages across resistance 29 which are -degrees out of phase with the grid voltage.

A corresponding audio voltage is induced across secondary winding 33 of auxiliary transformer 28 and applied to the control grid 35 of device l2. Since the respective control grid voltages of devices II, I2 are always opposite in phase, the varying anode currents of devices ll, [2 combine in the primary winding 22 of the output transformer 2-3 to produce additive effects on the load current in push-pull fashion. The electrostatic shield 34 between the windings of the auxiliary transformer 28 is necessitated by the high audio-frequency potential diiference; be-. tween the windings.

It will be seen that the; use or am auxiliary transformer to supply grid excitation; voltage for the second tube eliminates the necessity of the customary coupling condensen and. grid leak reasistor. A further economy of parts is effected by using an output transformer 23 havinga singie tapped primary winding shunted by a single con- I denser,- instead of employing a circuit which usestwoprimary windingseach. shunted by a condenser. Furthermore, theuseof a separate phase inverter: tube is: avoided;

A furtheradvantage of the circuit of Fig. l is that any-hum: voltage appearing across the source of" the direct. operating potential: isisolated from the control grid: of the second: tube by. theauxiliany transformer; A still further advantage over a. system utilizing a resistance in. series. with. one half. of the primary winding; ofthe outputtransformer results from. the low direct voltage: drop in the: primary.- winding Ofi the auxiliary transformen. This; provides; an; increased output from a given. anode voltage: supply; Furthermore, the system; is inherently stable, ther e being very little tendency toward self-oscillation.

Referring to Fig. 2-, a. further. modification is shown in which the auxiliary transformer isv cons nected. in the low potential sideof; the: anodecathqde; circuit. In. this. application,v the parallel combination, comprising; resistance 29, and, the, primary winding, 21 of. the: auxiliary transformer 28: is connected. between the cathode; L of. device; M, and. the cathode. bias. resistance. I1. and by-passcondenser 1.8,. In all. other. respects, the circuitsof. Fig, L and. Fig.2; are identical. By, so, locating the auxiliary transformer, theinecessity of. an. electrostatic, shield between thewindings thereof; is.eliminated, sincelthe primary 2.1. is no, longer, at a high audiorfrequency potential with. respect. to! the secondary. 31.. This, arrangement also. permits the. use, of. a. StSRr-HIJ. transformer; thus, by proper, desigmthe grid, excitation. vol'te age for device 1'2, may be made substantially the. samein magnitude as thatfor device. andatf the same time the impedance of" the primary 2T may bemade low so asto avoidserious degeneration indevice l l. The use of step-up transformer. also... permits, the value, of resistance 29 to be decreased, thus providing a still further increase. in. the net audio-frequency output a1: pearing at the, loudspeaker 32 for agiven excitation voltage at. source 5. The magnitude. of the gridexcitation voltage applied to device l2 may be controlled by suitably adjusting the value. ofiresi'stlance 29'.

While Iihave shown and describedicertainpresent preferred" embodiments of" my invention, it.

will" beund'erstocdi that numerous variations and;

modifications may be: made, and I contemplate inthe appended claims to cover any such variations and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination with a single-ended source of signal voltage, an electronic amplifier system comprising a pair of electron discharge devices connected; in push-pull, said devices, having respective anodes and controlelectrodes, said source being connected to the control electrode of one of said devices, means comprising a first transformer. for coupling the output of said system to a useful load, a second transformer having a: primary winding connected in the anodecathode circuit of said one device and having a resistanceirr parallel therewith, and the secondany of' said second transformer being connected between the control electrode of the other of said devices and ground, and means for applying substantially equal positive unidirecti'onab operating potentiads; to) said. anodes;

22 In combinatiom with; a sing-lesended. source of: signal; voltage;v am electronic: amplifier: system comprising; 3.1. pain. oft elect/mm discharge.- devices connected irr nushz-rmlli, said devices having. re,- spective anodes and: control; electrodes, said source: bein connected to. the-contnolelectrode of one; of saidi devices... a nusiepull. output; circuit for said: devices.- comprising; a transformer having, a primar winding and a second y" Winding. id primary. winding; being connected in; series with the; anode; of said. one: dlwice; said. secondary winding, being conncctedsbetween. the-controlrelectrode; of. the other: of saiddevlices. andithe. low potential. side.- of. said; source, aresistance. connected in Harallehwith.saidlprimary. winding and means for applying, substantially eqnaL positive direct operating petentials, to. said anodes 3.11:1, combination. with a single-ended. source of. signal.-. voltage; an electronic amplifier system comprising, a pain of electron. discharge. devices connected. in. push-pull; said. devices having respective cathodes, anodes, and" contr01..ele.ct'rod'es, said. source. being connected. to the. control; electrode of one. of said devices, an. output, circuit connectedbetween said" anodes, means comprising, a transformerhaving primary and secondary windings; for'applying signalvoltage to the control' electrode. of the: other" of said devices in proper phase: for push-pull operation; said primary" winding being-i connected in series. between said cathodes and in' parallel" with a resistance; said secondary winding being connected between the control electrode of said second device and ground; and means for applying substantially equal positive unidirectional" operating potentials tosaid anodes:

RQBEBFT. B. EIGJME;

REEEBENQESS GLDED The following references" are of record iir the file of this patent:

UNITED STAKES, PATENTS,

Number: Name: Date;

2 061 602 Walsh: Nov.241.1935; 236L282 Gilbema Get. 24 1944i FQR'EIGN P A ILElN'BS Number. Country Date 114,744; Australiamflnhnn. Feb. 20,1942. 5043960 GreatlBritain. May 3,, 19.39;

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