US2103468A - Wave translating system - Google Patents

Description

ec. 28, 1937. I .LG. KREER, JR 12,103,468 WAVE TRANSLATING SYSTEM I v Filed April 1, 1956 5/ FIG.

, INVENTOR JGKREER Patented Dec. 28, 1931 2,103,468. I wavn TRANSLATING SYSTEM John G. Kree'r, n; Bloomfield, N. J., assignor to Bell Telephone Laboratories Incorporated, New York, N. Y., a corporation of New York Application April 1, 1936, Serial No. 71,997

' 9 Claims. (01. 119-111) This invention relates to wave translation and especially to retroaction or feedback inwave translating systems as, for example, in systems involving vacuum tubes or electric wave amplilying devices.

An object of the invention is to control trans mission properties as, forexample} modulationor transmission efflciency, in such systems.

It is also an object of the-invention to control 10 feedback in-such systems or to facilitate application of feedback in such systems.

A further object is to reduce singing tendency in such systems, especially amplifiers or systems involving push-pull stages that feed back a portion of the output waves'in gain-reducing phase and in amount sufilcient to reduce distortion below the distortion level without feedback.

In one specific aspect, the invention is embodied ,in such an amplifier inwhich the singing tendency is a tendency to -sing around the feedback loop c'aused by parallel operation of the push-pull vacuum tubes or amplifying elements and, in accordance with a feature of the invention, the tendency is reduced by auxiliary negative feedback without 'materially loweringutheiamplifler gain.

The-auxiliary feedback can be, for'example, negative feedback around one stageor an odd number of stages of the vacuum tubes or amplifying elements for suitably altering the propaga- 1 tion (around the mainfeedback loop) of trans-.

, mission that (in passing around the main feedback'loop) passes through the push-pull tubes in the parallel mode of propagation, to prevent such'transmission from resulting in objectionable tendency toward singing aroundthe main feedbackloop.

The auxiliary feedback voltage can be derived,

for' example, from a mid-branch of a, balanced '40 circuit of anamplifying stage, (so that ft will include only transmission passing through the two sides, of the balanced circuit in parallel), in order to avoid materially reducing the amplifier ,gain. v I j K 4 In itsspecific-aspect mentioned above, the inventionmafbe, fjor'exampl, a two-stage amplifier comprising a-singleetube stage driving apushpull stage. The 'normal mode offpropagation through the push-pull stage is the push-pull 0 mode. Transmission so propagated may be called the normal transmission. For this normal trans mission, one of the push-pull'tubes has its plate potential approximately opposite in phase to the potential of the driver-tube grid. A feedback 55 connection from this one plate to the driver-tube grid may be provided to produce negative feedback of normal transmission. 1. e., of transmission propagated through the push-pull stage in the normal mode. w

The feedback loop for this negative feedback 5 may be referred to as the main feedback loop;

and the feedback around this loop of normal transmission or transmission propagated in the normal mode through the push-pull stage may be called the normal or main feedback. l

Due to unbalance of the push-pull circuit, there may be undesired propagation of transmission through the push-pull stage, this transmission being prop'agatedin the parallel mode, 1. e., with the two tubes of the push-pull stage effectively 15 in parallel. "This transmission'may be called the 7 parallel transmission. For this parallel transmission, the plates of the push-pull stage unfortunately may have their potential in phase with that of the driver-tube grid. This condition may 20 result in strong tendency towardsinging around the main feedback loop, especially since the driver stage 'maybe a high-gain stage. This. high gain may be desired, for example, in order to give 'theloop gain for the normal ormain feedback 25 around the main feedback loop (i. e., the decibel gain for. one trip of the normal transmission around the main feedback loop) a high value,- e. g., several times ten decibels. The large loop gain is desirable, for example, for producing large reduction ofdistortion. The distortion reduction required may be especially large in case'the push-pull stage is operated as a class B amplifier.

The singing tendency can be effectively reduced without materially reducing the amplifier gain, 35 for example," by producing local negative feedback around the drivertube of the parallel transmission to the exclusion of the normal transmission. As one example of ways in which this may be done, a voltage can be derived from the mid- 40 branch of the balanced input circuit of the pushpull tubes, and fed back to the driver-tube grid in gain-reducing phase, i. e., in such phase as to reduce the derived voltage.

Other objects and aspects of the invention will be apparent from the following description and i claims. g

Figs. 1, 2 and 8 are circuit diagrams of three forms of the invention.

Fig. 1 showsa two-stage push-pull amplifier for amplifying waves received from circuit I and transmittingthe amplified waves to circuit 2.

The waves may be, for example, speech waves or a broad band of carrier waves transmitting a number of speech messages by multiplex carrier linear distortion, comprising two similar vacuum tubes 3 and 3, shown by way of example, as

pentodes. The second stage may be a power stage.-

It comprises two tubes 4 and 4' which are also alike and which are shown, by way of example, as triodes.

The tubes 4 and 4' are connected to the outgoing circuit 2 through output transformer Lhaving closely coupled primary windings 8 and 8.

The incoming circuit I is connected to the tubes 3 and 3' through input transformer I0 and bridge networks II and II. The transformer has secondary windings I2 and I2. The winding I2 forms one diagonal of bridge II and .the winding I2 forms one diagonal of bridge II. The four ratio arms of bridge II comprise resistances 11, km, Zcr and r, respectively, the grid v and cathode of tube 3 being connected to receive the voltage across arm h. The four ratio arms of bridge II likewise comprise four resistances r1, kn, hr and r, the grid and cathode of tube 3' being connected to receive the voltage across this resistance 11.

The normal mode of propagation of transmission through the two push-pull stages is the pushpull mode, and transmission so propagated may be called the normal transmission.- For this normal transmission, the potentials of the plate of tube 4 and the grid of tube 3 are approximately opposite in phase, and the potentials of the plateof tube 4' and the grid of tube 3' are approximately opposite in phase.

- A feedback path comprising two feedback connections I6 and I1, each including a stopping condenser I9, connects the output circuit of the amplifier to the amplifier input circuit, to produce negative feedback of normal transmission, for example, for reducing distortion. The feedback loop for this negative feedback may be referred to as the main feedback loop; and the feedback around this loop of normal transmission may be called the normal or main feedback;

The gain reduction effected by this feedback may be large as, for example, several times ten decibels. The gain of the amplifier without feedback should then correspondingly exceed the gain with feedback. For instance, the gain without feedback may be of the order of decibels, and the feedback may reduce the gain to, say, a value of the order of 35 decibels.

With the two-stage balanced amplifier, the normal feedback is obtained in the desired phase by 65 reversing the feedback leads I6 and I1, i. e'., crossing them over from one side of the push-pull circuit to the other side, as disclosed in E. Peterson Patent 1,955,827, April 24, 1934. Thus, lead I6 is connected from the plate of tube 4 (which is 60 in the upper side of the push-pull circuit) to the bridge circuit II in the input circuit of tube 3 (which is'in the lower side of the push-pull circult); and lead I! is similarly connected between the plate -of tube 4' (which is in the lower side of 5 the balanced circuit) to the input bridge circuit II of tube 3 (which is in the upper 'side of the balanced circuit). The bridge circuits II and II. render the feedback path comprising connections l6 and I1 conjugate to windings I2 and I2 and circuit I at balance of the bridges, as pointed out in the above-mentioned Peterson patent.

Due to unbalance of the push-pull circuit, there may be undesired propagation of transmission through the push-pull stages, this transmission 76 being propagated in the parallel mode, i. e., with l tube 3';

the two tubes of the push-pull stage effectively in parallel in the case of each stage. This transmission may be called the parallel transmission;

Since this parallel transmission maybe fed back through the feedback path comprising the feedback connections I6 and I1; tendency toward singing around the main feedback loop may result. Such singing will be referred to as parallel singing or over-all parallel singing. Unfortunately, the parallel singing tendency may be strong, since the amplifier in the main feedback loop has an even number of stages and may include tubes suchas 3 and 3', with large amplification constants. Means for reducing the parallel singing tendency will be described hereinafter.

-A plate current supply source 25 and a grid bias potential source I22 are shown for tubes 4 and 4. The source I22 may bias the tubes for so-called class B operation in' which the space current of the tube is interrupted for the order of half the period of the waves to be amplified. Further, the source I22 may maintain the grids always negative during operation of the amplifier. The grid biasing potentials for the tubes 4 and 4' are supplied through-high resistances 28 and 29, respectively.

The plate of tube '3 is connected to the grid of tube 4 through a stopping condenser 21; and the plate of tube 3 is connected to the grid of tube 4' througha stopping condenser 21'.

A plate current supply source 40 supplies direct space current. for .the tubes 3 and 3 through choke coils 3I and"32, respectively, which are' closely coupled. The source 40 comprises two sources M and 42 serially connected by an impedance 43 shown, by way of example, as an inductance element 44 and a capacity 45 in parallel.

- ( Sources 42 and 25 may be common, if desired).

of parallel transmission around the main feedback loop), without materially reducing the amplifier gain (1. e.,' without reducing the gain of the amplifier between circuits I and 2 for normal transmission). Since the impedance 43 is in the grid mid-branch of the push-pull stage comprising tubes 3 and 3', the normal transmission in the grid circuits of these tubes is in one direction or phase in impedance 43 at any given instant for transmissiom through tube 3 and is in the opposite direction or phase in impedance 43 at the sameinstant for transmission through and so the normal transmission produces. no voltage drop across impedance 43' in these grid circuits. However since it is of course impractical to make the parallel opposing impedance of coils 3| and 32 zero, parallel transmission through the two sides of the push-pull stage may become appreciable. For such'parallel transmission, in impedance 43 the plate current of tube 3 passing through impedance 43 and coil 3| at any given instant has the same direction or phase as the plate current of tube 3 passing through impedance 43 and coil 32; so the of the small,

condition the tubes.

to prevent a series The close coupling of coils 3| and" 32 and likewise the close coupling of coils 8 and 8', tends to short-circuit the parallel singing path and elim--v inate parallel singing. The mid-branch feedback around the driver stage is effective in preventing parallel singing regardless of this close coupling, and is especially useful for this purpose when the close coupling is inadequate for the purpose,

By having the resistance of the inductance element 44 small, the direct currentvoltage drop across the impedance 43 can be made small. Moreover, by having the voltage of source 4| equal and opposite to this direct current voltage drop across 43, the direct current potential of ,the cathodes of tubes 3 and 3 can'be kept at ground potential or the potential .of the terminal of impedance 43 electrically remotefrom the cathodes, negative grid bias for the tubes being supplied by a voltage source 46. The source 46 may maintain the control grids of the tubes always negative during operation of the tubes and condenser serving in connection with the inductance 44 as the impedance across which the auxlliaryfeedback, voltage is produced, may have a capacity of the order of 800 micro-rriicrofarads, for example.

Feedback resistors 50 are shown in leads l6 and I! for con-trolling the amount of feedback. Small capacities 5| are shown in shunt to these resistors. These capacities reduce the phase shift around the main feedback loop at very high frequencies sing,.i. e.) singing due totransmission propagated through the push-pull stages in the, push-pull mode. The value of these capacities is not critical. Each may have a value of the order of 50 micro-microfarads, for example.

Fig. 2 shows an amplifier'circuit similar to that of Fig. 1, but with a single tube'orsingle-sided driver stage connected to circuit I by an input transformer H0 and driving the push-pull stage through a coupling circuit comprising a plate circuit resistor l3l for tube 3, the coupling con- 21 and a balanced retard coil. 'or' autodenser transformer having itsbalanced windings I28 and I29 tightly coupled. Over-all negative feedback is obtained by a feedback path through connection II which gives the desired revePse phase'of feedback voltage by connecting the plate of tube 4' to the input bridge H. v Using a singletube in the first stage not only reduces the required amount of apparatus, but reduces the powerdraln of the complete ampli-'- ,fier. The latter factor is especially important under no-load conditions when the power drain high-gain driver stage is comparable to the drain of the class B power stage. Such a two-stage, three-tube negative feedback amplifier is subject to a kind of over-all singing which may be called parallel singing and which is somewhat similar. to the over-all parallel singing discussed in connection with the four-tube circuit of Fig. l. The mode of oscillation-is such that the output tubes operate inparallel, (that is, with the potentials of the two grids the same ine -steadpf 180 degrees out of phase). though the oscillations are transmitted through the feedback of the interstage 'autotransformer, while thh out-' for -class A operation. The

' normal transmission;

3 put tubes faces the parallel opposing. impedance of the output transformer. transformers had their balancedcoils perfectly coupled and had zerp resistance, such mode of oscillation (i. e., this over-all singingwith parallelfoperation of, the two sides of the' power stage). would be impossible; but such construction is, of course, impracticable.

To reduce the over-all parallel singing tendency, an impedance shown,'for example, as an inductance I43, is "connected in the mid-branch of the balanced input circuit and the mid-branch of the balanced output circuit of the. push-.pull

If either of these stage. This inductance functions similarly to the inductance 43- of Fig. 1. It produces local negative feedback of parallel transmission around the stage in which'it' isconnected, lowering the gain of that stage for parallel transmission and serving to reduce the over-all parallelsinging tendency.

' In tests of a voice frequency, negative feedback, two-stage, three-tube amplifier, such as the amplifier of Fig. 2, two types of parallel singing were feedback was used. Although the frequency of the first type of sing- (20 to kilocycles) was not encountered. Neither type occurred except when.

greatlydiscriminated against by the output transformer, due to the balance in the-parallel path-no appreciable output was obtained in the load circuit at the singing frequency. This singing was stopped by usingthe' impedance I43 in the commomplate-grid mid-branch of the class B stage, the specific impedance employed being an a inductance of 22 milhenrys. v

The second type of singing occurred ,at critical values of load (transmission level) A Braun tube pattern showed the singing as highly damped oscillations occurring over a fraction of the fundamentai cycle'a'nd synchronized in fre- J quency with a high harmonic of the fundamental This singing was stopped by frequency applied.

1 and shunt capacity, 62

inserting a resistance in the balanced stage grid mid-branch ofsuchvalues as to approximately resonate out the parallel opposing reactance of the .interstage coupling retard I28, I29 near the singing frequency.

To avoid increase of third ordei', distortion and consequent limitation of the fundamental output available, it was found desirable to keep the impedance l43- introduced in the plate circuit of the class B stage 'as small as possible. h

The circuit of Fig. 3 avoids the necessityof introducing such impedance in the plate circuit of,

the class B'stage and has the advantage of in-,

'troducing the local feedback in the linear, highgain driver stage; yet it retains the'feature that the auxiliary or local negative feedback is so effected as to avoid reducing thegain for normal transmission or, in other words, to avoid mate- .rlally reducing the amplifier gain.

. This circuit is a two stage, three-tube, negative feedback amplifier generallyv similar to that of Fig. 2. However, the feedback voltage for the auxiliary negative feedback is derived from a feedback impedance in a portlon'of the grid circuit of the class B stage that is traversed by the parallel transmission to the exclusion of Ashe and the voltage so derived is fed back to the grid of the tube in the high-gain linear driver stage.

This feedback impedance is shown as a resistance 248 in the mid branch of the balanced grid circuit of the class B stage. The cathodes of both stages are shown grounded, and the ter--.

minal of the resistance :43 electrically remote from ground is shown'connected to the input bridge I I through a conductor 244 and aportion of the grid-biasing battery I22 sufficient to supply the grid bias for tube 3. Y

' Any voltage drop that appears across the impedance 243 is fed back into the grid circuit of the driver stage. Any voltage applied to the two grids of the balanced stage in parallel, i. e., in the singing mode, will appear across this impedance 243 and, therefore, will be fed back, reducing the gain of the driver stage for that mode of transmission and reducing over-all parallel singing tendency. However, any voltage which appears across the two grids of the balanced stage in series will balance out across the impedance 243 and hence this impedance will not affect the gain of the amplifier for normal transmission, or, in other words, will not materially reduce the over-all amplifier gain.

What is claimed is:

1. An amplifier with a feedback loop comprising a push-pull stage having a mid-branch circuit and connections that introduce a phase re verse] in transmission around the loop when the transmission is in the push-pull mode rough the push-pull stage but not when the tra smission is in the parallel mode through the pushpull stage, and means comprising a feedback impedance in said mid-branch circuit producing negative feedback around a portion of the amplifier stages in said'loop for transmission whose passage through said push-pull stage is in the parallel mode but not for transmission whose passage through said push-pull stage is in the push-pull mode.

2. An amplifier with a feedback loop comprising a push-pull stage having a mid-branch circuit and connections that introduce a phase reversal in transmission around the loop for transmission in the push-pull mode through the pushpull stage but not for transmission in the parallel mode through the push-pull stage, and means for reducing parallel singing tendency in the amplifier without materially lowering the amplifier gain, said means comprising a feedback impedance in said mid-branch circuit producing feedback around a portion of the amplifier stages in said loop that reduces the gain around said loop for transmission taking place in the parallel mode through said push-pull stage, said portion of the amplifier stages including a stage having but one amplifying element. g

3. An amplifier with a feedbackloop producing negative feedback around an even number of stages including a push-pull stage, and means producing negative feedback around an odd number of those stages in. a mode that suppresses parallel singing in the amplifier without materially reducing the amplifier gain.

4. An amplifier with a feedback loop producing negative feedback around an even number of mode.

- a,1os,4ee v stages including a push-pull stage and another stage having but one amplifying element; and means producing around an odd number of those stages including said other stage, feedback that reduces the gain around said loop for transmission in the parallel mode through said push-pull stage without reducing the gain around said loop for transmission in the push-pull mode through said push-pull stage.

5. An amplifier with a normal negative feedback channel around a feedback loop including a single-sided stage feeding a push-pull stage, and a circuit feeding back from the common branch of the input circuit of the push-pull stage to the single-sided stage waves that, without reducing the gain around the normal feedback channel, reduce tendency toward singing around a feedback channel including the two sides of the push-pull stage in parallel.

.6. An amplifier with a feedback loop that in-- I cludes a push-pull stage and produces negative feedback in said amplifier for transmission in the push-pull mode through said stage, and means producing feedback from a mid-branch circuit of said push-pull stage around an odd number of amplifier stages in said loop, said stages in-- cluding said push-pull stage.

7. An amplifier comprising a push-pull stage having a mid-branch circuit, a normal negative feedback channel around a feedback loop including said push-pull stage, and connections comprising a feedback impedance in said mid-branch circuit feeding back in the amplifier waves that, without reducing the gain around the normal feedback channel, reduce tendency toward singing around a feedback channel including the two sides of the push-pull stage in parallel.

8. An amplifier with a feedback loop that includes a push-pull stage and produces negative feedback in said amplifier for transmission in the push-pull mode through said stage, and means producing feedback from a mid-branch circuit of said push-pull stage around a feedback loop having an odd number of amplifier stages, one of which is in said loop.

9. The method of operating a signal wave a mplifying system with a push-pull stage which comprises producing negative feedback in the system by transmission of the signal waves in a given mode around a feedback loop that includes said stage, producing positive feedback around said loop of waves transmitted through said Push-pull stage in the parallel mode of transmission, and reducing the gain around the loop for the latter waves by auxiliary negative feedback in a portion of the loop of those waves to the exclusion of waves transmitted'in said given JOHN G. KREER, J ii.

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