US1970325A - Reduction of distortion in vacuum tube circuits - Google Patents

Description

Patented Aug. 14, 1.934

STATES REDUCTION OF DISTORTION IN VACUUM TUBE CIRCUITS John G. Kreer, Jr., Bloomfield,- N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 2.7, 1932, Serial No. 595,502 r 9 Claims.

, plifiers, modulators and the like, and particularly to the control of modulation in such circuits.

It is an object of this invention to suppress effectively modulation products and particularly third order modulation products, in signaling systems employing electric space discharge devices.

'1 It is well known that the electric space discharge tubes employed in signaling systems because of inherent distortion characteristics, produce in their outputs certain modulation products in addition to the waves of'frequencies which are impressed on their inputs. Thus, when a wave of a single frequency is impressed on the input circuit of a space discharge tube, the output circuit will contain a Wave of the fundamental frequency and other waves of frequencies harmonical y related to the fundamental. If a complex Wave, that is, one composed of a plurality of frequencies, is impressed on the input of the device, the output circuit will contain certain waves of frequencies harmonically related to the frequensignaling system for producing certain transformations in waves transmitted thereover, for example, amplification, it is desirable from the standpoint of quality that the effects of all modulation products produced by the device, be minimized. As the effects of the modulation products in amplification systems on quality of transmission of orders higher than the third are usually negligible compared to the effects of the second and third order modulation products, if the sec- 0nd and third order products present are eliminated or reduced to a negligible value by suitable means, the system for practical purposes is made distortionless. With some other tubes or other systems such as modulators some higher order product such as the fourth may predominate.

.Heretofore, it has been found that the amplivalue from the standpoint of other considerations, such as maximum power output, minimum reflection losses, etc.

In accordance with the present invention, it has been found that the amplitude of third or higher order modulation products produced by electric space dischargedevices ofv any type in a system may be reducedto zero or to an unobjectionable amount by means not dependent on a critical value of load impedance. This is accomplished 5; by inserting in the external circuits of the space discharge tubes (not the load circuit) means to provide such modulation voltages, of low-er order, as substantially to suppress unwanted higher order modulation products.

In one embodiment of the invention applicable to a balanced vacuum tube amplifier circuit, third order modulation products are made to vanish or are reduced to an unobjectionable amount by the insertion of a resistance of appropriate value in the common portion of the plate circuits of the two tubes.

The invention will be better understood from the following detaileddescription thereof when read in connection with the accompanying drawing in which: V

Fig. 1 shows diagrammatically a push-pull vacuum tube amplifier circuit in which the suppression of the third order modulation products is obtained by an impedance of suitable value inserted in the common portion of the plate circuit of the two tubes; 7 r

Fig. 2 shows a series of curves used to illustrate the invention; 7

Fig. 3 shows diagrammatically a modification of the invention applicable to a push-pull vacuum tube amplifier circuit; and

Fig. 4 showsdiagrammatically an embodiment of the invention applicable to a single tube .amplifier circuit.

If the outputcurrent of a vacuum tube is expressed as a power series of the variable component of the grid voltage, it canbeshown that the third order coefficient is a function of the derivatives of the static-characteristic at the operating point, the impedance to the fundamental current, the impedance to the second order currents and the impedance to the third order currents. This may be expressed mathematically as follows:'

the fundamental component being J1=C1e, the second order components are ,zzzcze andv the third order components are J3=C'3e where e is the applied input voltage to the tube; bo1,.b10.,.F102-;. bu, 1220 etc. are coefficients in the power series expansion of the tube characteristic.

It can be shown mathematically that ii the tube. characteristic satisfies certain conditions as to continuity, then i 6J1: as." that is, the partial derivative of the plate current with respect to grid. voltage evaluated at the operatingipoint;

a. 12mg bJE Z! that is, the partial derivative of the plate current with respectto the plate voltage evaluated at the operating point; and the higher order I) coefiicients are higher order partial derivatives, e. g., bmn is'the (m+n) th partial derivative of the plate current with respect to plate voltage m times and with respect to grid voltage n times.

R1, R2, R3. are the impedances of, the external.

vacuum tube circuit to fundamental, second. or.- der, and third order modulation components, respectively. 1

Examination of expression. (1) shows that the third. order coefiicient ismade up of two portions, one, indicated by the first term, which. is independent of second. order currents, and the other, indicated by the second term, which is dependent onthe' flow of second order currents. It is obvious that if the two terms of expression 1) have the same sign and are of the same magnitude, the third order coefiicient, C3, will vanish and there will. be no third order modulation products.

Now, since the denominators of the two terms involved in C3 are identical, only the numerators need be considered. Then,

New 5 can be put in the form:

6E; R0+R1 6E1, R0 OE obtained directly from. the above definition of the bmns'where R0 equals the internal impedance and a the amplification factor of the vacuum tube. It has been found from inspection of available experimental data on a number of commonly used types of three element vacuum tubes that for most tubes,

by. On D and 0,

are positive with the latter much larger; also that be n 0E1, OE,

arenegative. Under these conditions [3 will usually be positive. No such general conclusions can be drawn concerning a, but such values as have been computed are positive also. Therefore, if R2 is adjusted until on is equal in magnitude to p, then C3 may be made to vanish.

It so happens that if the impedance to the secand harmonic R2, can be varied independently of the impedance to the fundamental, R1, the third order coemcient for most types of tubes can be made zero While the fundamental impedance'remains at some value desired. for other circuit reasons. The balanced vacuum tube amplifier illustrateddiagrammatically in Fig, 1 presents just such a meansv for varying the impedance to theeven order currents independently of. the impedance to the odd order currents.

In the circuit of Fig. 1., the modulated waves to be amplified are impressed upon the input circuits of the space discharge devices 1 and 2 by means of input transformer 3.. The, devices 1 and 2 are connected in opposing. or push-pull relationship in the. usual manner well known in the art. The currents in the output circuits of these devices are impressed by means of output trans,- former 4 upon a suitable load or line which-is represented by the resistance 5..

Assuming a balanced amplifier with an ideal output transformer and identical tubes, as is. well known the second order currents of the tubes balance out in the output transformer and it acts as a low impedance. However, thesecond order currents are in. phase in the common portion 6 of the plate-filament circuits of the. tubes 1v and 2 and the second order voltages of the two tubes. are thus effectively connected-in. parallel sov that the impedance to each is twice the common. branch impedance. If the impedance of the commonbranch 6 is R, as indicated in Fig. 1, then The odd order voltages operate in the output circuits of the tubes 1 and 2 in series aiding, so if the load impedance as reflected through the output transformer 4. is R, then the impedance to the fundamental currents will be and be quite complex. and will not, therefore, be given 1 here.

expected that the third order modulation products could be reduced substantially to zero merely by obtaining the proper value for the impedance R.

As shown mathematically above, it would be I of the common branch of the output circuits of the two push-pull tubes 1 and 2 by making the resistance element '7 in that branch of suitable value, and experiments which have been carried out under direction of the applicant indicate that with standard types of three element tubes commonly used in such amplifier circuits, the third order modulation products may be made negligible by this method.

Fig. 2 shows curves illustrating the variation of third harmonic outputs with different values of resistance used in the mid-branch of a balanced amplifier using Western Electric Company 104-D vacuum tubes. These tubes were operated with a plate potential of 129 volts and a 22.5 volt grid bias, and a specially designed low. leakage output transformer was used. in Fig. 2 each curve represents the result obtained with a different input voltage. The experiments indicated that the best results from the standpoint of third harmonic reduction in thecase of balanced amplifiers using 104-D tubes were obtained at most operating points if the tubes were made to work into an output impedance equal to their internal plate impedance, although the optimum midbranch resistance varied somewhat when the operating point was changed. The operating points tried out varied from a plate potential of to 130 volts and a grid bias of 2G to 40 volts with the lot-D tube. When ordinary output transformers were used in place of the special low leakage output transformers, it was found necessary to insert a reactive network of suitable value in the mid-branch to annulthe leakage reactance to insure suitable reduction of the third order modulation products for impressed waves over a wide frequency band.

Experiments were also made with balanced amplifiers using Western Electric Company 101-D vacuum tubes. In the case of these tubes, it was found necessary to increase the output impedance to twice the internal plate impedance before a maximum reduction of the third harmonic could be obtained with a resistance in the-common branch. In the case of the 101-1) tube, it was found that the external resistance in the mid-branch giving maximum suppression of third order modulation products, was much lower than that required in the case where Western Electric Company 104-13 tubes were used. As in the case of the 104-D tubes it was found that the optimum mid-branch resistance varied for different sets of tubes and different operating points.

The experimental results indicated that by properselection of the operating point, both in the case of the balanced amplifiers employing Western Electric Company 101-D or 104-D vacuum tubes, on the average a ten to twenty decibels reduction of the third harmonic output could easily be obtained by properly adjusting the resistance in the common portion of the plate circuits of the balanced tubes. The exact resistance required is not critical within 10 per cent for any particular pair of tubes, but does vary with the operating point of the tubes between zero and one thousand ohms depending also upon the output at which the maximum suppression is desired. Placing greater suppression at higher inputs usually will not give as great suppression at the lower inputs.

In some cases it may be desirable to reduce the third order products generated in the grid circuits of balanced amplifiers when grid current "is permitted to flow, that is, when the amplifier is overloaded. For that case, it would'be necessary in order to obtain maximum reduction of the third order modulation products, to insert a resistance of proper value in both the common portion of the input circuit and the common portion of they output circuit of the balanced tubes.

Furthermore, it is not necessary inxeither the grid or plate circuit to obtain the necessary second order voltages by means of IR dropsas shown in Fig. 1. They may be obtained in any way desirable in the particular circuit-under considera-' tion as long as the proper phase and amplitude relations are maintained. For example, as illustrated in Fig. 3, a second order modulator 8 with itsinput-paralleled with the amplifier input and its output in series with the mid-branch ,of' the amplifier output might be used 'to provide. the proper second order voltages in the plate circuits of the tubes.

Fig. 4. shows a modification-of the invention embodied in an amplifier comprising a single space discharge tube. The waves to be amplified are impressed upon the input circuit of the space discharge tube 9 by means of input transformer 10 and the amplified waves in the output circuit of the tube 9 are impressed by means of output transformer 11 upon any type of load circuit or line represented in the figure by the resistance 12. Space current for the tube 9 is supplied by plate battery 13 and a suitable biasing potential is supplied to the grid of the tube 9 by means of battery 14. A suitable selective network 15 such as a resonant circuit tuned to reject the second order products while transmitting the fundamental components of the waves impressed on the input of the tube 9, is connected in the output circuit of the tube 9 between the plate and filament thereof. Connected directly across the plate and filament of the tube 9 is a circuit 16 comprising selective network 17, such as a resonant circuit, and a variable resistance 18 inseries. The selective network 17 should be of such a nature as to have a free transmission range including the second order modulation products generated in tube 9, and also as to present a high impedance to other modulation products and'the fundamental.

By suitable adjustment of the resistance 16, the

impedance of the circuit 14 to the second har-- monic or second order modulation products may be made of the critical value at which the third harmonic or the'third order modulation products generated by the tube '7 are suppressed or reduced to low amplitudes, so that substantially, only'the fundamental waves'will be impressed by the output transformer 9 upon the load circuit 10.

Although this invention has been illustrated and described as applied to a particular system, it is not to be limited thereto, but only in accordance with the scope of the following claims.

What is claimed is:

1. In combination in a wave transmission system, an electric space discharge device having.

second .order modulation products of waves from tem,a pair of space discharge devices having input and output circuits and connected in pushpull relation" with respectto an incoming and outgoing wave transmission circuit, and means common to the output circuits of said devices and independent of the impedance of the outgoing circuit, reducing in said outgoing circuit third or higher order modulation of the waves receivedover said incoming circuit.

a 3. In combination-in a wave transmission system; a pair of space discharge devices connected between an .incoming and an outgoing circuit, and having a portion of their input and their output circuits in common, and means for reducing insaid outgoing circuit the amplitudes of third order modulation products of waves impressed on :the input circuits of saidspace discharge de vices from said incoming circuit, said means comprising means proportioning the impedance of the common portion of the output circuits of said devices to second order modulation products oftheimpressed waves to the critical value at which. the amount of third order modulationcurrenttransmitted to'said outgoing circuit is a minimum. 4. In a wave transmission system, an incoming circuit and an outgoing circuit, a pair of space discharge devices having their input and output circuits connected in push-pull relation with saidincoming circuit and said outgoing circuit, respectively, said devices being balanced so as to suppress effectively insaid outgoing circuit second order modulationproducts of the waves from the incoming circuit and means common to the output circuits ofsaid devices and independent of the impedance of said outgoing circuit substantially preventing transmission to said outgoing circuit of third order modulation products of the waves from said incoming circuit.

5. The system of claim 4 and in which said means substantially preventing transmission of third order modulation products to said outgoing circuit comprises means making the impedance of the common portion of the output circuits of said devices to second order modulation products'of the critical value at which third order modulation currents are substantially suppressed from said outgoing circuit. 7 V

6. An amplifier comprising a pair of space discharge devices having portions of their input and output circuits in common 'so that said devices areconnected in push-pull relation with respect to an incoming and outgoing wave transmission circuit, said devices being balanced so as .to sup-- press effectively from the outgoing circuit the the-incoming circuit, and resistance means in the common portion of the output circuits of said devices proportioning the impedance of said com- .mon portion to the second order modulation products therein to the critical value at which the third order modulation products of said wavesin the respective output circuits of said devices substantially neutralizeeach other.

'7. In combination ina wave transmission system, a pair of three-electrode space discharge devices, said devices having a portion of their cathode-grid circuits and a portion of their anode-cathode circuits in common, so as to be connected in push-pull relation with respect to a source of waves to be repeated and an outgoing circuit, said devices having substantially similar characteristics with respect to second order modulation products appearing in the in-'- dividual portions of their anode-cathode circuits whereby transmission of said. second'order products to said outgoing circuit is substantially prevented, and a resistance element in the common portion of the output circuits of said devices, and of such value as to make the impedance of said common portion to second order modulation products appearing therein of the critical Value which will reduce the. amplitudes of the third order modulation products transmitted to said outgoing circuit to a minimum.

8. In combination in a wave transmission system, a three-electrode electric space discharge device having a space path, a cathode-control electrode circuit and a cathode-anode circuit, means for impressing waves to be repeated on said cathode-control electrode circuit, a'load circuit coupled to said cathode-anode circuit so as to receive the repeated waves, a circuit connected effectively in shunt with the space path of said device and comprising a resonant circuit tuned to second order modulation products of said waves, and means for reducing the effects of third order modulation products of said waves in said load circuit, said means comprising means for proportioning the impedance of said shunt circuit with respect to the second order modulation products of said waves to the critical value at which the amplitudes of the third order modulation products transmitted to said load circuit are a minimum.

9. In combination in a wave transmission system, a, three-electrode electric space discharge device comprising a space path, input and output circuits, means for impressing on said input circuit of said device waves to be repeated, a loadcircuit, means for impressing the repeated tion products of said waves in said load circuit,

and means for reducing the effects of modula-- tion productsof said waves in said load ciruit,

comprising a selective network in said output cir-' cuit ofsuch nature as to suppress second order modulation components of said waves, a circuit in parallel with said output circuit and directly in shunt with said space path, a selective net-' Work in said parallel circuit of such a nature as to pass the second order modulation components of said waves, and means for proper CERTIFICATE OF CORRECTION.

Patent No. 1,970,325. August 14, 1934.

JOHN G. KREER, IRE

it is hereby eertiiicd that error appears in the printed specification of the above numbered patent requiring correction as ioiiows: Page 4, claim 9, strike out line 122 and insert instead the Words and comma waves in said output circuit on said load circuit,; and that the said Letters Patent should he read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 17th day of December, A. i). 1935.

Leslie Frazer (Seal) Acting Commissioner oi: Pateots,

{M W M

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