US1994457A - Feedback amplifier - Google Patents

Description

March 19, 1935.

VG. w. BARNES- FEEDBACK AMPLIFIER Filed Aug. 26, 1952 3 Sheets-Sheet 1 1mm mas mrms'maz 7 COUPL/N NETWORK cou u/va NETWORK B 7 a 1E A) A' 1 i 256%2795 FIG H/PE 6 f 5 If PHASE CONTROL 4 NETWORK F762 F I63 REJ? HPF A I a f s f 5 no.4 FIG-.5

I HPF T HPF f s a f 5 FIG? HPF 5 J a 6 f s f INVENTOR G. (BARNES ATTO NE) March 19, 1935; G. w. BARNES 1,994,457

FEEDBACK AMPLIFIER Filed Aug. 26, 1932 3 Sheets-Sheet 2 INVENTOR G. W BA RNES BV A TTORNEY March 19, 1935. v 5 w BARNES 1,994,457

FEEDBACK AMPLIFIER Filed Aug. 26, 1932 3 Sheets-Sheet 3 FIG. 9

PHASE-0F mm INTERNAL A flab REGENERATION F FINAL PHASE OF Zlfl WITH A LOW PASS FILTER (E) 200 I80 PHASE 0F =cHA/vaE ab ab l IN PHASE OF Ufl I201 Q0", so; PHASE oF ap WITHOUT INTERNAL REGENERATION -40| 3. I KILOCVCLES- PEI? o a 2 a 4 se7a9uo' 2 a 4 56 00: Q PHASE 01-75 0 40 I PHq E oFA LOW PASS FILTE/i h f} =40o/ cc. 8009: so;

A TTORNEY Patented Mar. 19, 1935 UNITED STATES FEEDBACK AMPLIFIER George W. Barnes, East Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 26,

21 Claims.

This invention relates to wave translating systems, as for example, electric wave amplifiers.

An object of the invention is to control feedback or-retroaction in such systems.

In one specific aspect the invention is a multistage vacuum tube amplifier system of the general type in which waves are so fed back from the output to the input as to reduce the gain of the amplifier, in order to reduce unwanted modulation or non-linear effects and render the gain stability greater than it would be without feedback. That type of amplifier is disclosed for example in the copending application of H. S. Black, Serial No. 606,871, filed April 22, 1932 for wave translation systems, assigned to the assignee of this application, and in British Patent No. 317,005.

In such amplifiers, where tube modulation reduction for modulation components of givenfrcquencies is to be large it is proportional to. the gain (for those modulation components) in a single trip around the closed feedback loop and consequently that gain should be large. The modulation components that it is desired to reduce by feedback are usually waves of frequencies within the utilized frequency range, e. g. within the range of the frequencies of the signal waves to be amplified by the amplifier. In practice, when the loop gain (i. e., the decibel gain for a single trip around the loop) is large for the frequencies of'the utilized frequency range, it is greater than zero for some higher frequency and if the loop phase shift (i. e. the phase shift experienced by waves in passing once around the loop) is zero or a multiple of 360 for any frequency at which the loop gain exceeds zero decibels the amplifier may sing at that frequency. Moreover, passive networks introduced in the loop to contribute a component of loop attenuation increasing with frequency ordinarily introduce a component of loop phase shift that tends to lower the frequency, and raise the gain, at which the loop phase shift reaches a given multiple of 360. To avoid the singing condition, it is desirable to control the loop phase shift and the loop gain carefully with respect to the entire frequency spectrum. (Such control is desirable also for other reasons, as for example to prevent loop V phase shift from causing increase in the gain of the amplifier, which increase may be undesirable because accompanied by a corresponding increase in the modulation products. This increase in gain may become limiting factor determining the permissible loop phase shift at high frequencies where it becomes difficult to obtain suilinot a limiting factor. For example, assuming the amplifier does not sing, the amplifier gain change produced by feedback will be within 1 decibel of the gain around the feedback loop regardless cient feedback. At other frequencies it is usuallyof phase shift if the gain around the loop exceeds 1932, Serial No. 630,553

(C1. ES -44) 20 decibels. .Further, the feedback does not increase the amplifier gain if the loop gain exceeds 6 decibels). In particular, in order to avoid danger of singing it is desirable that the variation of loop phase shift with frequency, over the frequency range for I which the loop gain exceeds zero decibels, be maintained less than 360. If the value of the loop phase shift were maintained at '-180 it would be as remote as possible from the potential singing values of and multiples of 360.

In designing an amplifier with negative feedback for distortion reduction, assuming the vacuum tube or tubes of each stage in the loop to introduce a phase shift having a constant componentof 180 (in addition to any component due to interelectrode capacitance, for example), the number of vacuum tube stages used in the loop may be made either odd or even, to facilitate control of singing tendency. The question whether an odd or an even number is more suitable will depend upon whether the loop is made to have phase reversing means other than the tubes, (as for example, effective reversal or crossing over of ,the connections somewhere in the loop in the manner disclosed in the copending application ofE; Peterson, Serial No. 617,557, filed June 16, 1932, (Patent No. 1,955,827, April 24, 1934), for Wave translating systems, assigned to the assignee of this application), and upon what other phase shifts are present in the loop. If, over the frequency range for which the loop gain exceeds zero decibels (herein called the frequency range of loop 'gain); the constant component of the total loop phase shift is any odd multiple of 180, then it is necessary that the total variation of the quency range be maintained withinlimits of.

+180 and -180 in order to make the total loop phase shift differ from zero and every multiple of 360 for every frequency in that frequency range.

The dimculty of insuring that the variation of loop phase shift with frequency is maintained less than 360 or within the required limits over the frequency range of loop gain is in general increased by the fact that, (as brought out in the above'mentioned copending application of H. S. Black), when the distortion reduction and associated amplifier gain reduction produced by feedback action is to be large the gain of the amplifier without feedback must then correspondingly exceed the gain required with feedback; because when the gain without feedback, required to produce the desired amount of distortion reducing feedback and the desired amount of gain with feedback, necessitates use of a plurality of stages and a plurality of interstage coupling circuits the phase shifts around the closed loop may become large. For example, they may become large at frequencies well above the utilized range because of shunt capacitance, for instance, tube and wiring capacities. The singing tendency may become particularly troublesome when the amplifier is called upon to transmit wide frequency bands extending to very high frequencies, for example.

Moreover, as brought out in the above mentioned copending application of H. S. Black, it is often highly desirable to connect an attenuation equalizing network (having its attenuation increasing with frequency over the utilized frequency range) in the feedback path of the ampli fier, for equalizing line attenuation; and the presence of this equalizer may greatly increase the difiiculty of insuring that the variation of loop phase shift with frequency is maintained within the required limits over the frequency range of loop gain.

These difficulties caused by (relations of attenuation to) phase shift in tube and wiring capacities, attenuation equalizing networks in the feedback path, and othercircuit elements arise from the fact that such passive networks have the slopes of their phase-frequency characteristics all of the same sign, conventionally called positive, or the characteristics of the networks that give positive phase shift trend oppositely to those of the networks that give negative phase shift, with the result that the passive networks yield a total loop phase shift that varies widely with frequency, becoming high at high frequencies in the frequency range of loop gain, the variation tending to exceed the maximum permissible 360 or :180 for that frequency range. That is, with both the passive networks that produce positive phase shift and the passage networks that produce negative phase shift having the slope of their phase-frequency characteristics positive (the positive phase shifts increasing with frequency increase and the -negative phase shifts and (2).

1 decreasing with frequency increase), it results that at high frequencies in the frequency range of loop gain the total of the negative phase shifts in the loop is too small to counteract the total of the positive phase shifts sufficiently to prevent the loop phase shift from becoming zero or a multiple of 360 or crossing the zero axis at a high frequency in the frequency range of loop gain, (much less to yield, over the frequency range of loop gain, the 180 loop phase shift whose value is as remote from the potential singing values as it isgpfissible to have the value of the loop phase shift). This difficulty could be overcome if suitable corrective networks were available having 1) attenuation increasing with frequency, without positive slope of phase shift-frequency characteristic, or (2) negative slope of phase shiftfrequency characteristic without attenuation decreasing with frequency increase, or (3) both (1) Unfortunately, (a) passive networks that have attenuation increasing with frequency not only do not have negative slope of phase shift-frequency characteristic, but have positive slope of that characteristic; (1)) passive attenu- V ating networks that give negative slope of phase shift-frequency characteristic do so only over a limited frequency range, and at high frequencies the attenuation decreases with frequency; and (c) (non-dissipative networksnot only do not have negative slope of phase shift-frequency characteristic but have positive slope of that characteristic. However, as explained in the above mentioned copending application of H. S. Black, by inserting in the feedback path of an amplifier with negative or gain reducing feedback and loop gain large compared to unity a passive network whose (negative) phase shift increases with frequency (i. e. whose phase shift-frequency characteristic has a positive slope), the amplifier phase shift can be made to be substantially the negative reciprocal of the phase shift of the passive network, and thus to compensate for or counteract the variation of phase shift with frequency represented by the positive slope of the phase shift-frequency characteristic of a circuit in which the amplifier is connected or of any apparatus associated with the circuit, so that, for example, by making the phase shift-frequency characteristic of the passive network substantially identical with that of the circuit or apparatus, the variation of the phase shift of thecircuit or apparatus with frequency can be completely compensated for or annulled if desired. As further pointed out in that copending application, a feedback amplifier can be used as part of the forwardly transmitting portion of a multiple feedback amplifier to alter the phase shift with a view to making the latter amplifier more readily comply with the requirements for freedom from singing.

The present invention, in its specific aspect mentioned above, is a negative feedback amplifier system in which a portion of the forwardly transmitting part of the closed feedback loop, which portion includes a part only of the tubes of the forwardly transmitting part of the closed feedback loop, is itself a negative feedback amplifier, for altering the phase shift around the outer closed feedback loop to reduce singing tendency. The feedback path of the local or internal feedback loop comprises frequency selective phase shift correcting or phase shift compensating means which includes a high pass filter with cutoff frequency at or above the utilized frequency range. The high pass filter has a positively sloping phase shift-frequency characteristic that causes the internal loop to produce in the external loop a phase shift, that has a negatively sloping phase shift-frequency characteristic, combining with or adding to the phase shift (that has a positively sloping phase shift-frequency characteristic) of the remainder of the outer loop (i. e. the part of the outer loop that is not common to the inner loop) to increase the minimum departure of the loop phase shift (of the outer loop) from 0 and multiples of 360 over the frequency range of loop gain for the outer loop. Or the internal loop produces in a portion of the external loop a phase shift combining with that of the remainder of the loop to make the phase shift of the (total) outer loop differ from zero and every multiple of 360 at every frequency for which there is an (outer) loop gain, or prevent the phase shift of the (total) outer loop from becoming zero or any multiple of 360 at any frequency for which the loop gain around the outer loop exceeds zero.

The frequency selective phase shift correcting or compensating means introduces in the outer loop, principally by the high pass filter, frequencyselective loss (or gain reduction) which tends to lower the outer loop gain at which the outer loop phase shift reaches 0 or a multipleof 360; and the phase shift (including that of the high pass filter) which the frequency selective phase shift corrective means introduces in the outer loop is such that it does not tend to make the outer loop phase shift reach zero or a multiple of 360 at a lowered frequency and attendant higher outer loop gain, i. e., does not tend to lower the frequency. at which the loop phase shift reaches zero or a multiple of 360, but on the other hand is such that it tends to increase the frequency at which the loop phase shift reaches zero or a multiple of 360. (The effect upon the outer loop gain of variation of the loop phase shift for the inner loop is small; because-the effect upon the gain of the inner amplifier is small since, as brought out in the above mentioned copending application of H. S. Black, in a feedback amplifier with loop gain large, only slight change is produced in the gain of the amplifier by varying loop phase shift of the closed feedback loop that includes the feedback path from the output to the input of the amplifier.)

Since the high pass filter has its cut-off frequency at or above the utilized frequency range it. suppresses transmission through the filter of waves of frequencies of the utilized frequency range, so that for waves of those frequencies there is substantially no feedback in the internal loop and the amount of the local negative feedback and the consequent reduction of (distortion reducing) gain around the outer loop is zero or much smaller than for waves of higher frequencies, which ordinarily have greater tendency to produce singing because of the larger phase shifts that they experience in traversing the outer closed feedback loop. That is, the high pass filter causes the reduction that feedback produces in the gain of the inner feedback amplifier to be relatively small for utilized frequencies (where reduction of this gain would decrease the distortion reduction produced by the feedback around the outer loop, for a given over-all gain of the amplifying system), and to be relatively great for higher frequencies (there tending to cut off the gain around the outer loop and so tending to prevent singing around the outer loop at those frequencies regardless of the phase shift around that loop). Thus, with the high pass filter included in the phase compensating means, the internal feedback is such as to not only introduce in the external feedback loop. a phase shift that is helpful in preventing singing, but to also reduce singing tendency by reducing the gain around the outer loop above the utilized frequency range, without materially reducing that gain in the utilized frequency range where it is useful in reducing distortion.

If the phase shift of the phase corrective network (i. e. of the feedback path of the internal loop) were made to be substantially the same as that of the portion of the external loop that is not common to the internal loop, the phase shift introduced in the external loop by the internal loop would be substantially the negative reciprocal of this phase shift, and then the phase shiftfrequency characteristic of the external loop would be completely compensated for in the sense that it would be stabilized at 180", i. e. the phase shift around the external loop would then be 180 and would not vary with frequency. In practice this condition need be approached only to the extent of preventing the phase shift around the outer loop from becoming zero or a multiple of.

360 in the frequency range of loop gain. That is, the extent of the phase compensation or phase correct-ion need be only this, that for every' frequency of the frequency range of loop gain for the outer loop, the sum of the negative reciprocal of the phase shift of the phase compensating or phase correcting network (i. e. of the negative reciprocal of the phase shift of thelocal feedback path of the inner loop) and the phase shift of the portion of the outer loop that is not common to the inner loop, shall not be substantially zero or any multiple of 360.

, The above mentioned specific aspect of the invention is, then, a multiple negative feedback amplifier, or amplifier with multiple negative feedback connections providing a plurality of distinct closed feedback loops not reducible to merely a single closed feedback loop, one of the closed feedback loops being located in the forwardly transmitting portion of another of the closed feedback loops, and the one loop having in its feedback path frequency selective phase corrective means including a high pass filter with cut off at or above the upper limit of the utilized frequency range, for contributing to the outer'loop a phase shift that has a negative phase shift-frequency characteristic for reducing the singing tendency of the outer loop, while at the same time contributing to the outer loop a gain variation with frequency that reduces the singing tendency of the outer loop without materially decreasing the distortion reduction resulting from feedback around the outer loop at frequencies in the utilized frequency range.

In the case of the internal closed feedback loop, as in the case of the external loop, in order to avoid singing the phase shift around the loop should not be allowed to become zero or any multiple of 360 at any frequency at which there is a gain around the loop; but this requirement ordinarily is less difficult to meet in the case of the internal loop, where fewer amplifier stages are required, and where no attenuation equalizer is required, and where the high pass filter reduces the frequency range of loop gain, and where the required modulation reduction (which, if large in the case of the outer loop, requires large gain around the outer loop) is not large.

However, the phase shift-frequency characteristic of the inner feedback path'should be chosen to meet this requirement for avoiding danger of singing around the inner loop, as well as to meet the similar requirement for avoiding danger of singing around the outer loop; and for this reason it is often desirable to have the inner feedback path include means for producing a phase reversal, or other desired phase shift, over the frequency range of inner loop gain. For example, if the tube or tubesof the inner loop gave-a 180 phase shift and the high pass filter introduced a 180 phase shift at a frequency in its pass band near the lower edge of .the band, and if these were the only phase shifts in the inner loop at that frequency, and if that frequency were a frequency of inner loop gain, then there would be danger of the inner loop singing; and this singing danger could be avoided by the means for producing a phase reversal, or other appropriate phase shift, just mentioned. Often, also, the phase shifting means is desirable to facilitate meeting the requirements as to the phase shift around the outer loop over the frequency range of outer loop gain. .The phase shifting means may be, for example, a transformer, all pass structure or a special network or some combination of any or all.

While the high pass filter should be as effective as possible in the utilized frequency range (and produce as little adverse effect as possible in the quency of the high pass filter, the high pass filter. or the local feedback path does not give suificient negative feedback to prevent the outer loop gain from exceeding zero. vention, a low pass filter having its cut-off below such high frequency but above the cut-01f frequency of the high pass filter may be connected in the outer loop, preferably in the outer feedback path. A further advantage of the low pass filter in the outer feedback path is that it causes the amplifier to suppress frequencies above the cutoff frequency of the low pass filter, since, as

brought out in the above mentioned copending application of H. S. Black, a low pass filter connected in the feedback path from the output to the input of a negative feedback amplifier acts as a high pass filter in the main transmission circuit.

As indicated by the foregoing discussion, loop phase shift and loop gain present important limitations in operation of feedback amplifiers, especially the loop phase shift and the loop gain at high frequencies in operation of wide band negative feedback amplifiers, for reducing distortion by feedback action and more especially where transmission control means producing phase shift, as for example attenuation equalizing means, are included in the feedback means.

Objects of the invention are to control such loop phase shift or loop gain, or both.

It is also an object of the invention to reduce singing tendency and/ or to increase the distortion suppression obtainable in such amplifiers.

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

In the accompanying drawings,

Fig. 1 is a circuit diagram of an amplifier embodying a form of the invention;

Figs. 2 to 7 are fragmentary diagrams showing various modifications of parts of the amplifier circuit of Fig. -1;

Fig. 8 is a circuit diagram of an amplifier embodying another form of the invention; and

Fig. 9 shows curves for facilitating explanation of the operation of the amplifier of Fig. 8.

In Fig. 1 a negative feedback amplifier A amplifies waves received by input transformer T from line or circuit L and transmits the amplified waves through output transformer T to line or circuit L. The amplifier A comprises a forwardly transmitting path P including vacuum tubes 1, 2 and 3, an outer feedback path F including an attenuation equalizer 4 for equalizing attenuation of line L, and an inner feedback path 1 including a filter 5 and a phase shift adjusting network 6. The filter 5 may be a high pass filter or a network having somewhat similar character-- istics. The path F is from the output of amplifier A to the input of the amplifier, around tubes 1, 2 and 3. The path f is around tube 2. An amplifier output bridge network Brenders the feedback path F and the circuit L conjugate, in the manner disclosed in the above mentioned copending application of H. S. Black. The ratio arms of the bridge are resistances or impedances R0, KRO, KR and R. The voltage fed back by path I F is applied across resistance 7 in series with the secondary winding of input transformer T in the input circuit of tube 1. Interstage network 8 couples tubes 1 and 2. Interstage network 9 couples tubes 2' and 3. Only the alternating current circuits of the amplifier A are shown, the direct current energizing circuits or other circuits for energizing the amplifier or conditioning it for operation being omitted for the sake of sim- To accomplish this preplicity as they can readily be supplied by those skilled in the art. The dotted lines in the figure, indicate that stopping condensers, grid bias volt? age supply sources, etc. are to be added in the cir-. cuit as required.

The negative feedback amplifier A of the closed loop comprising the forwardly transmitting path 'P and the feedback path F, includes an inner 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. The amplification of an amplifier, for example, amplifier A, without feedback from the output to the input as for example without the feedback through path Fv (but with any internal feedback amplifier such as b producing internal feedback, as through path f), will be designated as a (and is a complex quantity). The amplification that an internal feedback amplifier, such for example as amplifierb, would have without feedback as for example without feedback through path i, will be designated at (or [La or 0, etc.). Amplification ratio is the absolute value of the amplification. Gain is twenty times the logarithm of the amplification ratio.

The (complex) quantity p will be used herein to designate the ratio by which a voltage of a For example, p will be used to represent this ratio for amplifier A, and the corresponding ratio for an internal feedback amplifier, as for example amplifier b, will be designated bfib, (or Mafia, or ie 3c, etc.)

As shown in the above mentioned copending application of H. S. Black, the amplification of a feedback amplifier is and the corresponding change in amplification caused by the feedback action is With #9., [1b, #0, etc., representing the amplifications or ,us for the individual stages (such as those comprising tubes 1, 2, 3, etc.) of a feedback amplifier when there is no internal feedback (i. e. when there is no internal feedback path such as j), the value of p for the amplifier (neglecting interstage losses between plate generators and grids, for simplicity) is p(I-B fl-b-#c When there is-local feedback for the stage comprising tube 2 for instance, (as for example through path I), then the value of p becomes As explained in the above mentioned copending application .of H. S. Black, the factor can represent either increase or decrease of amplification, and of gainI For example, an increase of gain may be obtained when the angle of p is less than 90 and the absolute value of 1.5 is two or less depending on the angle; and for any angle when IMBI is greater than two and for any value of |m3| when the angle of be is between 90 and 270 the gain is decreased. Since the angle of m3 appears in the denominator, the reciprocal of the angle of l bBb appears in the external m8. When [Lbflb or the internal p49 1, then l bl b is approximately M51. and the phase shift introduced in the external 43 is opposite in sign and slope to the angle of internal me.

This last property is very important since, as

indicated above, it has not been possible to ob-' tain a. suitable phase corrective passive network with a negative phase slope, because all non-dissipative networks give positive slope and dissipative networks give negative slope over the portion of the frequency range of high loss.

As indicated above'with -bfib 1 and the high pass filter 5 attenuating waves in the utilized frequency range and freely transmitting waves of higher frequencies and the phase shift of network 6 adjusted to values suitable for reducing the minimum departure of the angle of external s from zero and multiples of 360 over the frequency range of outer loop gain, the singing margin of the amplifier A can be greatly increased. Figs. 2 to '7 show variousmodifications of parts .of the circuit of Fig. 1 including the internal closed feedback loop. To facilitate comparison ofFig. 1 with Figs. 2 to "l, the tube around which the local feedback path f is shown in Figs. 2 to 7 is designated 2 in those figures, and the preceding tube is designated 1 in Figs. 5 to '1. However, it is to be understood that the amplifier stage shown in Figs. 2 to 4 can be any stage of a multi-stage amplifier with all of its stages included in a feedback loop, such as the amplifier A of Fig. 1, for example; and that, similarly, the

two stages shown in Figs. 5 to 7 can be any two consecutive stages of such an amplifier.. I

Fig. 2 shows the local feedback provided as in Fig. 1, except that in Fig. 2 the filter is shown as a band elimination filter, attenuating wavesof the frequencies'of the utilized frequency range. Fig. 3 shows the local feedback provided as' in Fig. '1, except that the voltage locally fed back is applied across a resistance 10 in series in the grid circuit of tube 2.

Fig. 4 is like Fig. 3 except that the voltage locally fed back is applied across a resistance or' impedance 11 which, in series-with a resistance or impedance 12 is shunted acrossthe grid sir-- cult of tube 2.

Fig. 5 is like Fig. 4 except that-the interstage coupling impedance 8 replaces the impedance 12. Fig. 6 shows the local feedback provided asin Fig. 1 except that a specific form of high pass filter is shown, consisting of a half section of the constant k type and a half section of m derived type, and a specific form of phase adjusting de phase shift substantially independent of fre-' quency, between its input and output voltages.

Fig. 7 shows the local feedback provided as 'in Fig. 1 except that the voltage fed back locally is applied across a resistance or impedance 13 in series in the screen grid circuit of tube 1.

The amplifier circuit of Fig.8 is like that of Fig. 1 except that the outer feedback path includes a low pass filter, the local feedback is around tubes 1 and 2, instead of around tube 2, specific networks are shown in bridge B which corresponds to the bridge B of Fig. 1 and in the blocks corresponding to the blocks shown in Fig. 1, and the tube 3 is shown as a coplanar grid tube of the type described in the above mentioned copending application of H. S. Black. The tubes 1, 2 and 3 can be of any suitable type. Tube 3 is shown with a filamentary cathode arranged to be heated by current supplied from winding 20, which may be, for example, the secondary winding of a transformer. The cathodes of tubes 1 and 2 may beheated indirectly or in any suitable way.

Tube 1 is coupled to tube 2 through the inter"- stage coupling network 8 and a stopping condenser C11 and grid leak resistance R9. Tube 2 is coupled to tube 3 through the interstage coupling network 9 and stopping condenser C13 and grid leak resistance R11. Resistance R5 and con-- denser C1 smooth out negative biasing voltage supplied to the control grid of tube 3' from battery 25 through the grid leak resistance R11. Resistance Re and condenser Ca smooth out positive biasing voltage supplied to the space-charge grid of tube 3' from battery 26.

Plate current for tube 1 is supplied from battery-27 through resistance R19 and other elements of the interstage coupling network 8, the condenser Cashown in network 8 being a by pass condenser which, in conjunction with resistance R19, smooths out the unidirectional plate voltage for tube 1. The plate current of tube 1 passes through resistance R1 in the cathode lead.

The resulting voltage drop in the resistance is smoothed out by a shunting or bypass condenser C1 and supplies negative biasing potential for the grid of tube 1 through resistance R16 which corresponds to resistance '1 of Fig. L

Plate current for tube 2 is supplied from battery 2'7 through resistance R20 and other elements of the interstage coupling network 9, the condenser Ce shown in the network being a by-pass condenser which, in conjunction with resistance R20, smooths out the unidirectional plate voltage for tube 2. The plate current of tube 2 passes through resistance R3 in the cathode lead. The resulting voltage drop in' the resistance is smoothed out by a shunting or by-pass condenser 04 and supplies negative biasing potential for the grid of tube 2 through the grid leak resistance R9.

Screen grid potential for tube 1 is supplied from battery 2'? through resistance R2, shunted by condenser C2, this resistance and condenser.

smoothing out the potential and adjusting it to the proper value.

Screen grid potential for tube 2 is similarly supplied from battery 2'7 through res stance R; and shunting condenser C5.

.Plate current for tube 3' is supplied from battery 27 through choke coil L9 and elements of bridge B. The coil L9, in conjunction with a by-pass condenser C15 shunted across coil L9 and battery 2'7, smooths out the unidirectional plate voltage for tube 3'.

Bridge B is an attenuation equalizer, the equalizer 4 supplementing the bridge equalizer so that the two together give the required attenuation equalization for line L as described in the above mentioned copending application of H. S. Black. In the bridge equalizer the elements designated R0, L8 and R12 form the bridge arms corresponding to arms R0, KRD, and KR of the bridge shown in Fig. 1, the reaetance of condenser C15 being negligibly small; and the elements designated L7 and C14 form the bridge arm corresponding to the arm R of the bridge shown in Fig. 1.

The local or internal feedback path is designated f and includes a high pass filter 5 and all pass network 6 corresponding to the high pass filter 5 and phase adjusting network 6 in Fig. 1 or in Fig. 2. I

The outer feedback path is designated F', and includes a low passfilter which has its cut-01f frequency above that of the high pass filter 5 and functions as described above to lower the gain around the outer closed feedback loop of the amplifier at frequencies above those at which the high pass filter transmits effectually, for insuring that the outer loop gain is always less than zero when the outer loop phase shift is zero or a multiple of 360 degrees.

Where the utilized frequency range is, for ex ample, from 8 to 100 kilocycles, the high pass filter 5 may have its cut-off frequency at 400 kilocycles, for instance, and the low pass filter 30 may" have its cut-ofi frequency at 400 kilocycles, for instance, and in the high pass filter 5 the half section of m derived type may give high attentuation at frequencies in the neighborhood of 100 kilocycles and the half section of constant It type give high attenuation at lower frequencies, down to 8 kilocycles, for example.

To facilitate practice of the invention, anamber of appropriate circuit constants for the amplifier of Fig. 8. when it is to amplify waves of a frequency range from 8 to kilocycles are given below. However, such values are merely illustrative, and the invention is not limited hereby.

Tubes 1, 2 and 3 may have amplification constants of 900, 380 and 5.77, respectively, and internal plate resistances of 828,000 ohms, 320,000 ohms and 3,900 ohms, respectively.

Values of resistances, inductances and capacities may be as indicated in the table below, (wherein M.F. signifies microfarads and H signifies henries) R Ohms C M. F L H.

3 060 3 75 3 0199 4 25500 4 ll 4 1236 5 200 5 01 5 l. 21

8 25000 8 75 8 61. 25X 10- 9 2X10 9 0000955 9 l0. 0

10 51337 10 0000955 10 0124 ll 2X10 ll .07 11 .0036 12 3500 12 0000401 12 00202 13 2410 13 .07 13 .0561 14 2410 14 000019 14 00975 15 1340 15 035 15 .00975 16 3500 16 000288 16 .00255 17 90000 17 001 The curves of Fig. 9 are plots of phase shift versus frequency applying to the amplifier of Fig. 8, over the frequency range from 100-ki1ocycles to 1000 kilocycles. The internal 1,8, i.e. the B for the internal closed feedback loop, is des ignated ltabflab, since the focal feedback is around tubes 1 and 2. The [Labfiab phase shift, i. e. the angle of [Labfiab or the loop phase shift for the inner loop is shown for the frequency range just mentioned by curve A. The improvement or 8 without the low pass filter and without the in-' ternal feedback, and curve D gives the values change in the phase or angle of extern'alj fith at that this same 1/3 phase has when the internal I feedback action is taking place. This curve D shows that more change than necessary was obtained, -and the 1;; phase now crosses through 360. However, the low pass filter 30 placed in the outer feedback path changes the total 13 phase to that shown by curve F. This low pass filter'in addition introduces in B transmission loss sufficient to insure that the outer loop gain is lessthan zero. Singing around the outer loop cannot occur now when the phase does cross back through zero as 13 is now a loss.

As indicated above; since with llnbfiab 1 the gain of the internal feedback amplifier is substantially pub it follows that if over the frequency range of interest the angle of were'made equal and opposite to the angle of the portion of the outer loop that is not common to the internal loop, then the phase shift around the outer loop would be over that frequency range. This loop phase shift of 180 would be obtained, in other words, if A, the phase shifttive feedback of waves of said frequency but giving the amplifier a tendency to sing at a different frequency, and means for reducing the singing tendency comprising a frequency selective negative feedback path for feeding waves back in said amplifier, said path discriminating in favor of transmission therethrough of waves of said different frequency as compared to transmission therethrough of said waves of given frequency.

2. An amplifier, a feedback path for producing in said amplifier negative feedback of waves of 'thefrequency range of the waves to be amplified,

anda frequency selective negative feedback path I for selectively feeding back negatively in said amplifier waves of frequency exclusive of the frequency range of the waves to be amplified' 3. An amplifier, a, feedback path for producing in said amplifier negative feedback of waves of the frequency range of the waves to beamplified, a

to form a closed loop circuit, and means for pro ducing for a portion of said forwardly transmitting path a total resultant phase shift that has its phase shift-frequency characteristic of negative slope and forms a component of the loop phase shift.

5, A wave amplifying system comprising a forwardly transmitting wave amplifying path, a feedback path, a source of waves to be amplified, a load circuit, means connecting said source, said amplifying path, said load circuit and said feedback path to form a closed loop circuit normally effecting feedback of waves originating in said' I source that reduces the gain from source to load below the value it would have with no feedback in the system, and means for producing in said forwardly transmitting path a phase shift in waves transmitted through that path that has its phase shift-frequency characteristic of negative slope and forms a component of the loop phase shift.

6. A waveamplifying system. comprising a forwardly transmitting wave amplifying path, a feedback path for producing feedback in said system, a source of waves to be amplified, a load circuit, means connecting said source, said amplifying path, said load circuit and said feedback path to form a closed loop circuit, and means for producing in said forwardly transmitting path a phase shift in waves transmitted through that path, said phase shift having its phase shiftfrequency characteristic of negative slope, and said last mentioned means comprising an amplifier amplifying waves in said forwardly transmitting path and feeding back in said amplifier waves that reduce the gain of said amplifier below the value it would have with no feedback in the amplifier.

7. A wave amplifying system comprising aforwardly transmitting wave amplifying path, a'

feedback path for producing negative feedback in said system, a source ofwaves to be amplified, a load circuit, means connecting said source, said amplifying path, said load circuit and said feedback path to form a closed loop circuit having loop gain large compared to unity, and means for producing in said forwardly transmitting path a phase shift in waves transmitted through that path, said phase shift having its phase shift-frequency characteristic of negative slope, and said last mentioned means comprising a negative feedback amplifier amplifying waves in said forwardly transmitting path.

3. A wave amplifying system comprising a forwardly transmitting wave amplifying path, a feedback path for producing feedback in said system, a source of waves to be amplified, a load circuit, means connecting said source, said amplifying path, said load circuit and said feedback path to form a closed loop circuit, and means for producing in said forwardly transmitting path a phase shift in waves of selected frequency transfrequency characteristic .of negative slope and forms a component of the loop phase shift.

9. A wave amplifying system comprising a forwardly transmitting wave amplifying path, a feedback path for producing negative feedback in said system, a source of waves to be amplified, a load circuit, means connecting said source, said amplifying path, said load circuit and said feedback path to form a closed loop circuit, and means for producing in said forwardly transmitting path a phase shift in waves of selected frequency transmitted through that path that has its phase shift-frequency characteristic of negative slope, said last mentioned means comprising a negative feedback amplifier amplifying waves in saidforwardly transmitting path and a'high pass filter in said forwardly transmitting path, having its outoff frequency above the frequency range of the waves to be amplified by said system.

10. A wave amplifying system comprising. an amplifier, a negative feedback path from the output to the input of said amplifier, a second feedback path from the output to the input of said amplifier, a high pass filter in the first mentioned feedback path, and an. active transducer in said other feedback path.

- transducer in said other feedback path, said high pass filter having its cut-off frequency ofthe order of magnitude of the upper limiting frequency of the utilized frequency range of waves to be amplified by said amplifier, and said low pass filter having a higher cut-off frequency than said high pass filter. I

12. A wave amplifying system comprising an amplifier, a negative feedback path therefor, and

a low pass filter in said feedback path, having its cut-off frequency above the utilized frequency range of waves to be amplified by said amplifier, for preventing feedback of waves of frequency above the utilized frequency range that tends to produce singing.

13. A wave amplifying system comprising an,

amplifier, a negative feedback path therefor, an attenuation equalizer in a portion of said path, and means comprising a high pass filter connected around said portion for reducing the singing tendency of the system, said filter having its cut-off frequency at least as high as a frequency approximately that of the upper limit of the utilized frequencies of the frequency range to be amplified by said amplifier.

14. A wave amplifying system comprising an amplifier, a negative feedback path therefor, an attenuation equalizer and a. low pass filter in a portion of said path and a high pass filter connected around said portion, said high pass filter having its cut-off frequency atleast' as high as a frequency approximately that of the upper limit of the utilized frequencies of the frequency range to be ,amplified by said amplifier, and said low pass filter having a higher cut-off frequency than said high pass filter.

15. A multi-stage amplifier, a feedback path for all of the stages, and means producing frequency selective negative feedback for certain of the- 16. A multi-stage amplifier, means feeding back waves from the output to the input of said amplifier, and means comprising a high pass filter for producing negative feedback in less than the whole number of the stages of waves whose frequency is above the frequency range of the waves to be amplified in said amplifier.

17. A multiple feedback amplifier comprising three stages of-vacuum tubes connected in tandem, means for producing negative feedback of waves from the output of the third stage to the input of the first stage, a local feedback path around less than the whole number of said stages,

and a high pass filter in said local feedback path.

18. A multi-stage amplifier, means feeding back waves from the output to the input of said amplifier, and means comprising a high pass filter and a phase shifting network forproducing negative feedback in less than the whole number of the stages of waves whose frequency is above the frequency range of the waves to be amplified in said amplifier.

19. A multiple feedback amplifier comprising three stages of vacuum tubes connected in tandem, means for producing negative feedback of waves from the third stage to the first stage, a local feedback path around less than the whole number of said stages, and a high pass filter and phase reversing means in tandem in said local feedback path.

20. A wave amplifying systemcomprising a forwardly transmitting wave amplifying path, a feedback path for producing negative feedback in said system, a source of waves to be amplified, a load circuit, means connecting said source, said amplifying path, said load circuit and said feedback path to form a closed loop circuit having loop gain large compared to unity, and means for preventing singing around said loop, said last mentioned means comprising means for producing in said forwardly transmitting path a phase shift in waves transmittedthrough that path that has its phase shift-frequency characteristic of negative slope and increases the minimum departure of the loop phase shift from zero and multiples of 360 for the frequency range of loop gain.

21. The method of operating a negative feedback amplifier which comprises introducing frequency selective gain reduction in the closed feedback loop and at the same time introducing, in the feedback loop, phase shift with a phase shift-frequency characteristic of negative slope that increases the minimum departure of the loop phase shift from zero and multiples of 360 for the frequency range of loop gain.

- GEORGE W. BARNES.

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