US1998479A

US1998479A - Push-pull amplifier

Info

Publication number: US1998479A
Application number: US730387A
Authority: US
Inventors: Raymond O Wise
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1934-06-13
Filing date: 1934-06-13
Publication date: 1935-04-23
Anticipated expiration: 1952-04-23

Description

Patented Apr. 23, 1935 I y PUSH-PULL AmLIFIER Raymond 0. Wise, Orange, N.y J. assignor tov Bell l Telephone Laboratories, Incorporated,

New

York, N. Y., a corporation of New York Application June 13,1934, Serial No. 730,387

. 6 Claims.Vv (Cl. 179-1171).

This invention relatesV to push-pull vacuum tube circuits and more particularly to means' 'for automatically balancing said circuits. Themain object cf the invention is to maintain an automatic balance between the direct currents in the anode circuits of a pair Yof vacuum tubes.

In accordance with the invention, theequalization of the space currents of a pair of vacuum tubes connected in push-pull is carried out under the control ofa differential voltage that is set up whenever the space currents are unbalanced. The diiferential voltageis introduced in the input circuit of the tubes in serial relation to the two grids, causing one grid to be made more positive .and the other more negative with respect to the cathodes. It is so arranged that the application of these grid voltages causes the larger space current to decrease and the Asmaller to increase until an equilibrium is set up in which the space currents are substantially equal again, although both may have departed from their initial value. 1

The invention is applicable to amplifiers Ywhose filament current supply is either direct current or alternating current provided the following conditions are observed: (-1) If direct current is used, separate batteries should be employed .for the two laments; (2) if directly heated filament type cathode tubes are to be operated'on alternating current through a power transformer, separate filament current windings should be provided for the two tubes; (3) if indirectly heated cathodex tubes are employed both heaters may be connect` The objects and advantages ofthe invention will be better understood from the following def;

tailed description thereof when read in connection with the accompanying drawing in which,

Fig. 1 shows a push-pull electronic vacuum tube amplifier employing directly heated lament type cathodes operated on separate batteries and equipped with automatic plate current balance circuits in accordance with the invention.

Fig. 2 shows the circuit of Fig. 1 modified to employ separate windings on an alternating current power transformer to heat the vacuum tube cathodes, and,

Fig. 3 shows the circuit of Fig. l modied to employ vacuum tubes having indirectly heated cathodes operated either from a common battery or from the same winding of a power transformer.

The circuit of Fig. 1 comprises a source of alternating current I, an input transformer 2 having a center tapped secondary winding for applying energy symmetrically to the grids ofthe vacuum tube amplifiers 3 and 4, and an output transformer 5 for coupling the plate circuits of the vacuum tubes'to a load resistance Ii. vPlate voltage is supplied to the vacuum tubes 3 and 4 by a battery.. 'I through the center tappedprimary winding of the output transformer 5, and a fixed portion-ofthe grid bias is'furnished by a battery 8 through the center tapped secondary winding ofthe input transformer 2. The automatically'variable'portion of the grid bias on the vacuum tubes 3 and 4 is supplied by the plate current flowing through the resistances 9, III, II and I2. Condensers I3 and`l4whose impedances at signal frequencies areV small compared with those of the `resistances 9, I0, II and I2, serve to prevent degenerative action on waves passing through the amplifier. Separate batteries, "I5 and I6, are employed to heat the laments of the vacuum tubes 3 and4.' l

'I'he operation of the circuitis as follows: It

s PATENroFFIc-E- f will first be assumed that thebias on the grids of y the Vacuum tubes 3 and 4` due to the battery Y8 is such 'as to permit. appreciable plate current from thebattery 'I to ow through the'split primary Winding of the output transformer. 5, the plate to cathode impedances. of the tubes, and the resistances 9, IIL: II and I2. It will be further assumed that .the grid .bias versus plate current characteristics `of the vacuum tubes 3 and 4 are somewhat idiiierent so that when identical bias voltages are applied to the grids, the plate cur' rents flowing in the two tubesy arenot the same.

The plate current of the vacuum tube 3 from the-positive terminal of the battery 1 divides at the junction of the resistances 9 and II, one portionnowingfdownthrough the resistances IIY and I2 and up through the resistance I0, while thel other portion ilows down through the resistance 9. The two portions of the plate current of the vacuum tube 3 reunite at the junction of the resistances 9 and I0 to ilow to the negative terminal of the battery 1. The plate current of the vacuum tube 4 from the positive terminal of the battery 'l divides at the junction of the resistances I0 and I2, one portion `ilowing up through the resistances I2 and II and down through the resistance 9, While the other portion ows up through the resistance I0. The two portions of the plate current of the vacuum tube 4 reunite at thejunction lof the resistances I0 and 9 to flow to the negative terminal of the battery 1.

The portion of the bias due to the battery 8 is,

of course, the same on both grids of the vacuum tubes 3 and 4, so that on the basis of the original assumptions, the plate current of tube 3, say, will initially be greater than that of tube 4. While this condition holds, the plate current from tube 3 flowing down through the resistances Il and I2 will exceed the oppositely directed plate current of tube 4 lflowing through thesame resistances. Thev cathode of tube 3 thus becomes effectively more'positive with respect toA its grid while at the same time the cathode of tube 4 becomes less positive. As a result of these changes in bias, the plate current ofv tube 3 is reduced and that of tube 4 is increased so that they are more nearly equal than they would be if xed bias voltages of equal magnitude were applied to both grids.

The circuit of Fig. 2 differs from that` of Fig. 1 only in that separate windings I1 and I8 on a power transformer I9 are used to supply alter-v nating current to the directly heated Vfilament type cathodes of the vacuum tubes 3'and 4., The operation ofthe vcircuit of Fig. 2.is otherwise identical with that of Fig. 1. v The circuit of Fig. l3 differs from that of Fig. 1 in that indirectly heated cathodes 2U and 2ly are used instead of directly heated filament type cathodes. v The heaters may have direct current supplied from a common batteryl 22y or alternatl, ing current from a common winding 23 of a power transformer 24.,The operation of the circuit of Fig. 3' is likewise identical with that of Fig. 1.

By a similar method of procedure, the plate current of the two tubes can be shown to ap proach equality automatically when that of tube 4 initially exceeds that of tube 3.

The .inventionis not limited to systems employing. three electrode tubes` but may be used with tubes having anydesired-.number of electrcdes. In multi-electrode tubes the space current may include componentsfrom `the several gridsas well as from the anode.

What is claimed is: j 1 1. In an electrical circuit, .vacuum tubes arranged in push-pull relation, each tube including.

a grid, a cathode, and .an anode.. means tol set up a voltage determined in magnitude. and po-r larity by the difference between the space currents of the tubes,. and means to impress por.. tions of said voltage in opposite polarity upon thev grids of the tubes to .substantially equalize the space currents.

2. In an electric circuit, vacuum .tubes arranged in` push-pull relation, each tube vincluding agrid, a cathode, and an anode, means to insure the same space current flow in each tube, comprising .electrical means for controlling the grid potential of each tube, said means being actuated by a difference in the space current iiow in the tubes.

3. In an electric circuit, vacuum tubes arranged in push-pull relation, each tube including a grid, a cathode, and an anode, means to insure equal anode direct current flow in the tubes, comprising a separate resistance element in each anode' circuit, and means controlled by the difference in the IR drops through the resistances due to the anode currents, said means controlling the grid potentials of the respective tubes.

4. In combination with a pair of vacuum tubes in push-pull connection, impedances individual to the space current paths of the tubes and having a terminal in common, a divided impedance bridged between the non-common terminals of the individual impedances, a conductive connection from the grid of each tube to the division point of the divided impedance, and a conductive connection from the cathode of each tube to the non-common terminal of the impedance individual to such tube, whereby thepotential of each grid with respect to its corresponding cathode is made to depend upon the diiference in the space currents in the respective tubes.

5. In combination with a pair of vacuum tubes in push-pull connection, a pair of substantially equal resistances each individual to the space current path of one tube and having a terminal in common, a resistance bridged between the non-common terminals of the individual resistances, a conductive connection from the grid of each tube to the mid-point of the bridging resistance, and a conductive connection from each cathode to they non-common terminal of the associated individual resistance, whereby the space currents of the respective tubes are automatically maintained substantially equal.

6. In coxnbnation'with a pair of vacuum tubes in push-pull relation, a pairA of `substantially equal resistances having a terminal in common and each resistance being individual to the space current path of one'tube, a resistance bridged between the non-common terminals of the individual resistances, a conductive connection from the grid of each tube to the mid-point of the bridging resistance, and a conductive connection from each'cathode tothe non-common terminal of the associated individual resistance, whereby an' inequality of space currents in the tubes causes` a positive increment of potential to be impressed upon the grid of the tube having the smaller space current and a negative increment upon the grid of the tube having the larger space current.

RAYMOND O. WISE.

CII