US2007172A - Wave translation system - Google Patents

Description

y 9, 1935; H. s, BLACK- 2,007,172

WAVE TRANSLATION SYSTEM Filed March 29, 1955 V FIG. GRID OF FIRST 572a Pl/WE 0F LAST sue/=- FIG-2 INVENTOR I H.$. BLACK A TTORME Y Patented July 9,1935

PATENT OFFICE,

WAVE TRANSLATION SYSTEM Harold S. Black, New York, N. Y., assignor to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application March 29, 1933, Serial No. 663,316

8 Claims. (01. 179-171) This'application is a continuation in part of my copending application, Serial No. 606,871, filed April 22, .1932, for Wave translation systems.

This invention relates to wave translation systems, as for example, electric wave amplifying 1 systems.

Objects of theinvention are stabilization, distortion suppression and impedance control in such systems.

In accordance with one feature of the invention, in a push-pull or balanced system in which balance suppresses distortion to a degree dependent upon the degree of balance attained, the degree of balance is increased by retroaction or feedback in the system, for example, as explained hereinafter. e

.In accordance with another feature of the invention, reduction of distortion is achieved by choosing for a push-pull or balanced system a load impedance which'causes objectionable modulation produced in the system to be less than if the load impedance had the value of the output impedance which it would face in the absence of feedback,'and then, as indicated hereinafter, for example, causing feedback to match the output impedance-to the chosen load impedance in order to prevent objectionable wave reflection, which might result in objectionable crosstalk, for instance. (For any given frequency the impedance of a network between any two points is con-- sidered the ratio that a voltage applied across the points from an external network would hear to the internetwork current.)

Also, in accordance with the invention, the above mentioned features may be combined.

In one specific aspect, the invention is an amplifier with negative feedback of fundamental and distortion waves so as to increase stability of operationand reduce distortion production as-explained in the above mentioned copending application, the. amplifier being of the balanced or push-pull type in accordance with the present invention. It has been found that the distortion suppression attainable in such a negative feedback amplifier exceeds the sum of that attainable by the balancing action without feedback and that.

attainable by the feedback action without balance; feedback can improve the balance or make the two sides of the push-pull system more nearly alike as regards their transmission properties. In general, attainment of this improvement in balance. is facilitated by making the two sides of the push-pull amplifier independent as regards feed-' back operation or-in other words by making each because, as explained hereinafter, the,

side an individual feedback amplifier so that neither side of the push-pull system will materially affect the feedback operation of the other. side, and is also facilitated by preventing reaction of the input or output connecting circuit 5 for either side of the push-pull circuit upgn the feedback operation of that side. To indicate in a simple way why this is so, and how the negative feedback action can make amplifiers more nearly alike as regards their transmission properties, certain aspects of operation of feed-back amplifiers will now be pointed out, after the significance with which this specification uses certain terms and symbols relative to operation of vacuum tube amplifiers has been indicated.

Amplification of an amplifier without feedback is what the voltage on the grid of the first tube must be multiplied by to obtain the phase and magnitude of the resulting voltage generated in the plate circuit-of the last tube, or the voltage of an equivalent fictitious generator in series with the internal plate resistance of the last tube. This amplification will be designated as ,u (and is a complex quantity). Amplification ratio is the absolute-value of the amplification. By the term gain as used in this specification is meant voltage'gain, i. e., twenty times the logarithm of the amplification ratio.

The complex quantity #5 will be used herein to designate the ratio by which a voltage of a wave is modified in a single propagation around the closed feedback loop of a feedback amplifier. It follows that p is the complex quantity by which a driving voltage in the space path of the last tube, in series with the internal plate-filament impedance R0 of that tube, must be multipliedto give the voltage that it--the driving voltage alone-acting through the feedback path, will produce on the grid of the first tube.

As shown in the above mentioned copending application, the amplification of a feedback am plifieris and the corresponding change in amplification caused by the feedback action is The quantity 50 back is describedas positive feedback or negative feedback according as the absolute value of 1 1 is greater or less than unity.

As pointed out in that application, when afl 1 the amplification with feedback approaches are made equal, and therefore, the suppression;

by balance, of the even order modulation products generated in the last, stage is greatly facilitated and the usual difiiculties of. obtaining sufficiently close matching of tubes, interstage networks, etc., are greatly reduced. Moreover, this equality of gains and phase shifts is not affected by tube changes, changes in direct current plate and grid voltages, etc., but is constant or stable with time.

In one specific embodiment of the invention disclosed hereinafter, with the amplification in each of the two amplifiers in the two sides of the push-pull circuit substantially equal to the angle of p is made substantially zero, and

- plifier approaches consequently the phase shift through each am- However, as indicated hereinafter, the invention is not limited to having the angle of 5 substantially zero, nor to having pfi 1.

In an amplifier, ordinarily negative feedback action (apart from balance) reduces distortion products of odd and even orders approximately equally, whereas balance by push-pull action (apart from the feedback action) reduces the even order products primarily. Therefore, since the second order products are usually very much greater than the modulation products of higher orders, it is especially fortunate that by employing both negative feedback and balance, the second order products can be reduced not only directly by feedback action, but also by balance and further by improvement of balance produced by the negative feedback.

In accordance with one feature of the invention, each side of the push-pull circuit can be prevented from unduly effecting the feedback operation of the other side, for example, by employing separate output bridge networks for the two sides (i. e., employing output bridges individual to the two sides), and feeding backfrom the output bridge network on each side of the circuit to the input of that side of the circuit, with each output bridge network giving conjugacy between the feedback path for its side of the circuit and the output connecting circuit for that side of the circuit, for giving any desired closeness of approach to such conjugacy. If de sired, in accordance with the invention, an input bridge, also, can be used in each side of the pushpull amplifier, each input bridge giving conjugacy \(or any desiredcloseness of approach to conjugacy) between the feedback path for its side of the push-pull circuit and the input connecting circuit for that side of'the push-pull'circuit,

' so that theinput connecting. circuits can not unduly afiect the feedback operation of either side of the circuit or in other words can not affeet the pin either side of the circuit.

However, the invention is not limited to the' use of the input bridges or the output bridges.

In one specific aspect the invention is a pushpull amplifier with negative feedbackimproving the balance as described above and with a load impedance substantially greater than the 'output'impedance that the load impedance would face if there were nofeedback, and with the feedback raising the output impedance to a value that matches the load impedance. (The amplifier output impedance without feedback is the value that the amplifier output impedance would have if a could in some way be reduced tozero and, at the same time, the remaining properties of the ,ufi-path or closed feedback loop or system not be altered.) Objectionable modulation produced by the amplifier is reduced by thus working the amplifier into a load impedance larger 7 than the value that the amplifier output impedance would'have in the absence of feedback; yet, since the feedback action matches the output impedance to this increased load impedance, objectionable wave reflection is obviated. Such reflection might, for example, result in objectionable cross-talk, for instance, in the case of a string or single line vacuum tube amplifiers A1 and A2, which are alike, connected back to back in push-pull relation. Amplifier A1 comprises a vacuum tube or any number of vacuum tube stages T1 in tandem; and amplifier A1 comprises a tube or stages T2 like the tube or stages of am- The tubes T1 and T2 areconnected to the outgoing circuit 2 throughtwo output bridge net-' works 5 and 5 and output transformer I. The transformer has two primary windings 8 and 8. The winding 8 forms one diagonal of the bridge circuit 5; and thewinding 8', forms one diagonal of the bridge circuit 5'. The four ratio arms of the bridge circuit 5 'comprise four resistances or irnpedances R0, KRo, KR and R, respectively, the

impedance Rn being the plate-cathode impedance in the last of the tubes or stages T1. The four ratio arms of bridge circuit 5' likewise comprise four resistances or impedances R0, KRo, KR and R, of which the impedance R0 is the platecathode impedance in the last of the tubes or stages T2. 4

The incoming circuit l is connected to the tubes T1 and T2 through input transformer 10' and two input bridge networks II and II. The transformer III has two secondary windings l2 and I2. The winding l2 forms one diagonal of the bridge circuit II; and the winding I2 forms one diagonal of the bridge circuit II. The four ratio arms of the bridge circuit H comprise resistances r1, kn, kr and 1, respectively, the grid and cathode of the first of the tubes T1 being connected across arm n. The four ratio arms of bridge circuit H likewise comprise four resistances 11, km, hr and r, the grid andeathode of the first of the tubes T2 being connected across this resistance r1. Negative feedback in amplifier A1 is provided through a feedback path F comprising conductors l6 and I1, and, as explained in the above mentioned copending application, renders the operation of the amplifier more stable than it would be without feedback Cal and reduces distortion produced by the amplifier. Similarly, negative feedback in amplifier A2 is provided through a feedback path F comprising conductors I6 and I1, and increases the stability of 'operationof the amplifier and reduces its distortion. As indicated above, the feedback action improves the balance of the two amplifiers for second order modulation components, rendering their fundamental driving voltages in the last stage more nearly equal and opposite. Over the utilized frequency range, each of the amplifiers may have ;3 1 so that the amplification for each is approximately and B may be the same constant quantity for each-amplifier. As noted above, with the phase angle of 15* equal to zero, the phase shift through each amplifier (from the first grid to the lastspace path driving voltage) approaches 180.

As described above, the windings 8 and I2 are in diagonals of bridges 5 and II, respectively. The other diagonals of these bridges comprise the feedbackpath F, so. the path F is rendered conjugate to the windings 8 and I2 by the bridges 5 and II, respectively. Similarly, path F' is rendered conjugate to windings 8 and I2 by bridges 5' and II, respectively. Thus, neither of the amplifiers A1 and A2 can affect the operation of the other, (i. e. neither amplifier can act as a transducer, either active or passive, affecting the value of B for the other amplifier), nor can any of the input or output transformers or connecting circuits for either amplifier affect B or the feedback operation of either amplifier. In the circuit, as shown, when the bridges are balanced and their ratio arms are resistances, the angle of p is substantially zero. The output bridges, especially, are advantageous in preventing the feedback in either of the amplifiers A1 and A2 from being affected by the outputfrom the other amplifier. However, if desired either the input bridges or the output bridges, or both, can be omitted; and moreover, with or without the omission of bridges, the feed-back for the pushpull amplifier can be effected through the input or output transformers, or both, so as to reduce the distortion introduced in the transmitted waves by the transformer as pointed out in the abovementioned copending application, or can be effected without feeding back by primaryto-secondary transmission through the transformers.

Only the alternating current circuits of the amplifier A are shown in Fig. 1, the electromotive force sources, choke, coils, stopping condensers,

etc., required for energizing the amplifier or con-.

57,, 65 or 66.of the.above mentioned copending application.

Fig. 2 shows a two-stage push-pull amplifier without input or output bridges suchas those of The amplifier of Fig. 2 may besuitable Fig. 1. for amplifying frequency waves of a wide frequency range, as for example, for use in repeaters of an open wire multiplex carrier'telephone system of the general type of the present commercial type C carrier telephone system describe-d by Messrs. H. A. Afiel, C. S. Demarest and C. W. Green in the Bell System Technical JournalQJuly 1928, pages 564 to 629. In such cases it is important, especially from the point of view of reducing reflection or suppressing crosstalk, that the amplifier output impedance match theload impedance. (The load impedance may here be considered as including the impedance .of the amplifier. output transformer; and the amplifier output impedance may be considered as the imimpedance that is to match (i. e. equal) the amplifier output impedance becomes correspondingly greater and, consequently, objectionable modulation products are reduced.

In the push-pull amplifier of Fig. 2, the first stage comprises two tubes 23 and 23 in push-pull relation. The second stage comprises two tubes 24 in parallel and two tubes 24' in parallel, the tubes 24 being connected in push-pull relation with thetubes 24'. Each of these six tubes may be, for example, Western Electric 101-F type tubes. An interstage transformer 30 connects the two stages. It has two secondary windings 32 and 32'. Winding .32 is connected in the input circuit of tubes 24, in series with a 450-ohm feedback resistance 25 which is common to their input circuit and their output circuit. Similarly, winding 32' is connected in the input circuit of tubes 24', in series with a, 450-ohm resistance 25' which iscommon to their input circuit and their output circuit. Thus, the second stage is a pushpull, negative feedback amplifier A comprising two separate single string or single line negative feedback amplifiers A'i and A2, the amplifier A'i comprising tubes 24 in parallel, and the amplifier A2 comprising tubes 24 in parallel. Batteries 0 are grid biasing batteries for the tubes 24 and separating the alternating current output cir-- cuits of tubes 24 and 24' from the source of unidirectional plate current for those tubes. With respect to the circuit of Fig. 2 and also the circuit of Fig.1, thmdesignation of amplifiers on opposite sides of a push-pull circuit as separate has refer ence to their transmission circuit, and not to the .circuits for energizing the amplifiers or conditioning them for operation.

Each of the amplifiers A'1 and A'2 in Fig. 2,

.having a feedback impedance 25 or 25' common to the input and output circuits of the amplifier, is a negative feedback amplifier circuit of the type referred to in the above mentioned copending application asthe common impedance type,

A retard coil cated in the description of that figure, the negative feedback action increases the amplifier outshown in Fig. 34 of that application. As indiput impedance,- (from Z0, its value without feedback), by the factor [1-(pfi) z]. The expression ([Lfi)z designates the value that s would have if the load impedance were zero; and

. grid-cathode impedance, which faces the incom- .ing circuit (winding 32 or 32 in Fig. 2) and the feedback impedance in series; and C is the impedance of the incoming circuit. Thus, in

. Fig. 2 the negative feedback action can be caused to increase the output impedance of amplifier A'i and also that of amplifier A: to a value, for example, approximately twice-R0, i. e. twice the plate-cathode impedance in tube 24 or tube 24?. The transformer 1 then can be given the proper impedance transformation ratio to match the impedances on the two sides of the transformer, so that circuit 2 will be terminated in its own impedance at the amplifier and-reflection, consequently, will be obviated and at the same time the transformer will present to the tubes twice their own impedance.

The 101-F type of tube, when worked into twice its own impedance, has a better modulation characteristic (i; e. has a lower modulation power output fora given fundamental power output) than when worked into a matched impedance; and the circuit of- Fig. 2 combines (1) this improvement in linearity due to working the outputtubes into. twice their impedance with (2) the improvement in linearity that the negative feedback action produces by reducing the ratio of modulation voltage to fundamental voltage generated in the tubes 24 and 24' (3) the improvement in stability that the negative feedback action produces in the operation of each of the amplifiers A1 and A's, (4.) the improvement in linearity that results from merely pushpull operation, and (5) the improvement in linearity that results from the improvement that the negative feedback produces in the balance of the two sides of the push-pull circuit, (as explained above in connection with Fig. 1). Thus, modulation is reduced in four ways simultaneously, the reduction produced directly by push-pull action and that-produced directly by feedback action combining and being augmented by that due to the increased load impedance made feasible by feedback and that due to the improvement in balance effected by feedback.

Since the amount-by which negative feedback action in amplifier A reduces the amplifier gain depends on the amount of the negative feedback,

the gain reduction may be made small by having the amount of feedback vsmall. Moreover, since the load impedance has been increased (by increase in the ratio of transformer 1) to match the amplifier output impedance as increased by the negative feedback, the alternating plate current of the output stage has thereby been decreased for a given power output to the load and consequently the electron emission from the cathodes of the last stage need not be as great as in the case of the lower load impedance. Therefore, in the case of the increased load impedance, tubes may be used in the last stage which require less heating energy than the tubes that would be required in-tlie case of the lower load impedance; and this not only effects important economy in power supply but facilities using tubes of higher amplification factorjto compensate for the effect of the negative feedback in reducing the amplifier gain, so that the amplifier gain may, for example, be approximately the same as it would be if the tubes of lower amplification factor were employed and no feedback were employed. The feedback tends to reduce the amplifier gain; but this tendency can be counteracted by increasing the amplification factor of the tubes, (since.afl 1), so that while the amplifier. output impedance is changed not only by the use of tubes of higher amplification factor or plate .resistance, but moreover, by the feedback action; nevertheless, the amplifier gain remains substantially unaltered. It has been found that in the system of Fig. 2 employing the negative feedback and using the101-F type tubes mentioned above, which have a normal filament current of approximately one half ampere, the gain and the power output capacity of the system may be substantially as great as for the system without the feedback but with tubes (two Western Electric 101-D or L-type tubes in the first stage and four Western Electric 104-D or O-typetubes in the last stage) having a normal filament current .of approximately one ampere, and that the modulation produced by the system is reduced sufiiciently to render unnecessary tube selection for push-pull balance, which was a practical necessity without the feedback- It is noted that the output transformer in Fig. 2 may step the amplifier output impedance down to the impedance of circuit 2, the negative feedback action in the amplifier being used to increase the amplifier output impedance from a value greater'than the impedance of circuit 2 to a value still greater.

As indicated above, the invention is not limited to having ,ufi 1. It is within the scope of the invention to have p l in the circuit of Fig. 2 as well as in the circuit of Fig. 1. doing so in the circuit shown in Fig. 2 would tend to increase the degree in which the feedback raises the output impedance).

Moreover, control of impedance by feedback, as contemplated for the invention, is not limited to raising impedance. For instance, in a negative feedback amplifier circuit in which the voltage to be fed back is derived from a voltage across the load, the negative feedback can lower the amplifier output impedance, as pointed out, for instance, in connection with Fig. 31 of the above mentioned copending application showing such a circuit, and in accordance with the present invention such negative feedback, for example, can be used to lower the output impedance of a pushpull amplifier and at the same time to match the output impedance and the load impedance (for example, the impedance of the high impedance winding of a step-down transformer facing the amplifier output impedance and forming part of the load). This may be desired, for instance, in case the tubes of the last stage of the amplifier are pentodes, it being customary (without feedback) to work such tubes into approximately one fifth. of their impedance rather than into a matching impedance, in order to improve their operation, as for example, in order to reduce objectionable modulation that they produce.

While in accordance with the present inven- (Ordinarily,

tion, impedance is controlled by feedback inspeciflc types of circuits or systems, the above mentioned copending appplication and my copendin'g application Serial No. 663,317, filed of even date herewith, for Wave translation systems,'present .dominating claims not limited to those specific Q the two sides of the push-pull circuit, respectivetypes of circuits or systems. ,What is claimed is:

1. A negative feedback, push-pull amplifier comprising two feedback paths for the two sides ,of the push-pull circuit, respectively, two outgoing circuits for the two sides of the push-pull circuit, respectively, and two output bridges for ly, each bridge connecting in conjugate relation one of said feed-back paths and one of said out- 7 going circuits.

2. A negative feedback, push-pull amplifier comprising two feedback paths for the two sides 'of the push-pull circuit,'respectively,. two incoming circuits for the two sides of the push-pull circuit, respectively, and two input bridges for the two sides of the circuit, respectively, each bridge,

plifier having a load impedance of value that causes objectionable modulation produced in the amplifier to be less than if the load impedance had the value that the amplifier output impedance would have in the absence of feedback in the amplifier, and means for producing in the amplifier negative feedback that improves the pushpull balance of the amplifier and matches the amplifier output impedance to said first mentioned load impedance. I

' 5. A wave translating system comprising an amplifying valve and a load therefor having an impedance that-causes objectionable modulation.-

produced in the system to be less than if the load impedance had the value that the output impedance of the valve would have in the absence of feedback, and means for producing in said systern negative feedback that matches the output impedance of the valve to said first mentioned structure'giving it transmission characteristics, with said feedback, approximately the same as those of a corresponding valve of the same power capacity without feedback.

' 6. A wave translating system comprising a vacuum tube amplifying'device, means producing negative feedback in said device, and a load for said device, said device having its feedback, its load impedance and its amplification factor such that the amplification of said device is substantially the same and its ratio of output fundamental to output of modulation is at least substantially as great as if the feedback were absent and said device, were replaced by one of the same type, but with lower amplification factor and with power capacity in the absence of feedback equal to the power capacity of the first men tioned device with its feedback.

7. A wave'translating system comprising two negative feedback amplifiers, each having #5 of greater order of magnitude than unity, an incomingcircuit, an outgoing circuit, and means connecting said amplifiers in push-pull relation between said incoming and outgoing circuits, said means comprising a pair of input circuits and a pair of output circuits for said amplifiers and means opp itelyassociating said input circuits with said incoming circuit and oppositely associating said output circuits with said outgoing circuit. v i

8. In combination, a negative feedback amplifier comprising a feedback path individual thereto' rendering feedback therein negative, a, second negative feedback amplifiercomprising a feedback path individual thereto rendering feedcuits for said amplifiers, said amplifiers being symmetrically and oppositely disposed with respect to each of said circuits.

HAROLD S. BLACK.

, load impedance, said valve having electrode.

'back therein negative, and input and output cir-

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