US2245671A - Inverse feedback amplifier - Google Patents

Description

June 17, 1941. P, F G, HOLST UAL' 2,245,671

INVERSE FEEDBACK AMPLIFIER Filed Aug. 31, 1939 2 Sheets-Sheet 1 June 17, 1941.

P. F. G. HoLsT E-rAl.

INVERSE FEEDBACK AMPLIFIER Filed Aug. 51, 1939 2 Sheets-Sheet 2 Gttorneg Patenied June 17, 1941 INVERSE FEEDBACK AMPLIFIER Paul F. G. Holst and Loren R. Kirkwood, Oaklyn,

N. J., assignors to Radio Corporation of America, a corporation of Delaware Application August 3l, 1939, Serial No. 292,930

11 Claims.

The present invention relates to inverse feedback amplifiers, and has for its primary object to provide an improved inverse feedback amplifier for audio frequency signal amplification in radio signal receiving and phonograph record reproducing apparatus and the like, which may include a phase inverter stage and impedance coupling with a balanced output amplifier stage in the feedback loop without introducing objectionable phase shift in the signal transmitted through the amplifier Within the pass band of the amplifier.

It is also a further object of the invention to provide an amplifier of the type referred to, wherein a tendency to oscillate and overload at frequencies above and below the useful audio frequency range is prevented.

As is well known, inverse feedback to produce degeneration in amplifiers, particularly of the audio frequency type, is accomplished by coupling back from one stage to a preceding or input stage, a portion of the output energy. The phase relationship and relative magnitudes of the feedback and input voltage are the most important considerations. The phase relation of the energy fed back must be such that degeneration v occurs, and, for this condition, the phase relation between the feedback and input voltages must be such that said voltages are approximately 180 degrees out of phase.

if, for any reason, the phase relationship above referred to is caused to shift to such an extent that it approaches 180 degrees therefrom in either direction, a condition of regeneration occurs and may result in oscillation, in which case the amplifier becomes inoperative for transmitting normal signal energy.

Accordingly, it is also an object of the present invention, to provide an improved inverse feedback amplifier including a phase inverter stage having a coupling network including means for preventing high and low frequency oscillation outside of the useful range of signal frequencies, thereby preventing an oscillating condition from being initiated whereby the amplifier may be rendered inoperative.

It is a still further object ofthe present invention to provide an audio frequency amplifier having a driver stage and a phase inverter stage for converting a single-ended input circuit into a push-pull or balancedoutput circuit for driving an output amplifier of the balanced or push-pull type, without the use of a coupling transformer in the coupling network, and without introducing appreciable phase distortion. In a push-pull or balanced -amplifier stage, the signal voltage applied to the grids of the push-pull or balanced tubes must be substantially degrees out of phase, and substantially equal in amplitude. The elimination of a coupling transformer greatly simplifies the application of inverse feedback because `of the elimination of a source of phase shift which may cause a possible 180-degree phase shift over the audio frequency range.

It is well known that audio frequency amplifiers employing pentode output tubes, while highly efcient, are subject to distortion resulting from the pentode tubes operating into a load which varies in impedance with frequency. It is, therefore, a further object of the present invention to provide an inverse feedback amplifier circuit in connection with a balanced pentode amplifier stage for driving a loudspeaker, wherein the effect above referred to has been minimized or eliminated without the use of the usual anode circuit or primary circuit loadings.

It is also an object of the present invention to provide an amplifier which tends to damp out motional impedances of the loudspeaker voice coil refiected over into the output circuit of the amplifier stage.

It is a still further object of the present invention to provide an inverse feedback amplifier having a feedback loop comprising two stages in one branch of the loop and three stages in the other branch thereof in which overall phase distortion is less than the phase distortion through the three-stage circuit.

As is well known, phase distortion results when the phase relation between the input and output signal voltages are changed in passing through the amplifier, this phase shift ordinarily being variable with frequency.

Phase distortion ordinarily is relatively unirnportant in 'audio frequency amplifiers as the phase relation of the various voltages in a complex wave may be shifted over a considerable r-ange without noticeable effect upon the ear. However, in inverse feedback amplifiers, the overall phase distortion is highly important in that a shift in the phase relation established in the circuit by an amount approaching 180 degrees may result in serious regenerative effects and oscillation tending to render such amplifiers inoperative, as hereinbefore mentioned. In fact, the damage may in some cases involve burn-out of the circuit elementsl and of the tubes constituting the output stage.

Therefore, it is an object of the present invention to provide means for stabilizing an inverse feedback circuit as part of a coupling network between a single-ended input circuit and a push-pull or balanced output circuit, which prevents the transmission of signal frequencies through the inverse feedback loop which tend to cause regeneration and oscillation.

It is also an object of the present invention to provide, in conjunction with an inverse feedback amplifier of the character referred to, a tone control system which is outside of and not included in theY feedback loop or either branch thereof, whereby effective tone control is made possible with an inverse feedback amplifier.

In providing a tone control circuit of this type in a preferred embodiment of the invention, the selectivity of the intermediate frequency amplifier portion of the receiving system is made adjustable or variable independent of the control of the frequency characteristic provided by the inverse feedback amplifier, and with or without additional tone control means in conjunction with volume Control elements in the receiving system, also outside of the inverse feedback loop and either branch thereof.

The invention will be better understood from the following description, when considered in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawings,

Figure 1 is a schematic circuit diagram of a radio receiving system provided with an amplifier and tone control means therefor embodying the invention;

Figure 2 is a schematic representation of a portion of the circuit of Figure 1 rearranged to illustrate an operating characteristic thereof;

Figure 3 is avector diagram of certain signal voltages indicated in Figs. 1 and 2;

Figure 4 is a further schematic representation of a portion of the circuit of Figure 1 illustrating a further operating characteristic of the circuit thereof; and

Figures 5 and 6 are graphs showing curves 1ndicating the response characteristic of the tone control portions of the system.

Referring to Fig. 1, the radio receiving system shown by way of example is of the superheterodyne type comprising a radio frequency amplifier, detector and oscillator portion together with suitable tuning means represented by the rectangle 5 which is coupled through a suitable intermediate frequency amplifier 6 and control system 1 with a second detector and audio frequency amplifier 8,

The two-stage intermediate frequency amplifier ii-Iii illustrated is provided with two tuned intermediate frequency input circuits II and I2 in which are connected additional coupling windings i3 and I4 under control cf selector switches l5 and I5 which operate in steps to connect into the tuned circuits i I and I2 a portion of or the total additional coupling windings I3 and I4 as selectivity adjusting or varying means to increase or decrease the fidelity, as will hereinafter be referred to. The selectivity adjusting means represents any suitable means for this purpose.

The second detector 3 includes a diode rectifier anode I8 and grounded cathode I9, the latter being common with an amplifier portion in the same envelope, comprising a control grid and an output anode 2l. The detector diode is connected with the grounded output resistor 22 for rectified signals which are transferred through a resistor 23 to a volume control network including a volume control potentiometer 24-25 to which the control grid 25J of the amplifier portion is coupled through a coupling capacitor 26 and a grid resistor 21. The volume control network also includes tone control or compensating shunt circuits 28 and 29 connected with the volurne control element 24 for adjusting the tone or fidelity with variations in volume, as is Well known. The signal output from the diode detector IS-IB is amplified in the amplifier portion I9-2il-2I and the amplified signals are applied through an output circuit 35 to the remainder of the audio amplifier system through aY suitable audio frequency filter 3l preferably adjusted to cut off 10 kc. beat notes, the single-ended output from the detector and amplifier being derived across the output resistor 32 in the anode circuit 30 in the amplifier portion of the tube 8.

The audio frequency signals appearing across the output resistor 32 are applied to a driver stage comprising a tube 35 through a suitable coupling means such as a capacitor 36 and appear as the voltage e across the grid resistor 31 and a bypass capacitor 38 to ground in the inputcircuit of the amplifier 35.

The input circuit, including the resistor 31, is connected to the input grid 39 of the amplifier and to a source of fixed biasing potential indicated at 4i) through a supply lead 4I and a filter resistor 42 provided with a filter capacitor 43, one side of the potential source 40 being grounded. The bypass capacitor 38 is of relatively low impedance to audio frequency signals so that the lower potential end of the input circuit or resistor 31 is substantially at ground audio frepuency potential. The input signal voltage e is applied to the grid of the tube 35 and the amplified output voltage of this tube is applied through a coupling capacitor 45 to the signal input grid 4B of one tube 41 of a pair of balanced oiitut tubes, the other of which is indicated a The amplified output voltage from the tube 35 is also applied to a signal potentiometer or bleeder circuit comprising' a lead 43, a blocking capacitor 50 and two bleeder or potential divider resistors 5I and 52 connected serially, as indicated, between .the output circuit 53 and the filter resistor 42 for the bias supply source at a terminal indicated at 54.

The capacity of the blocking capacitor 5E) is such that it offers relatively low impedance to audio frequency .signals so that the portion of the signal voltage that exists across the resistor section 52 of the potentiometer is applied to a signal input or control grid 55 of a tube 56 through a tap connection 51 between the resistor sections 5I and 52. The tube 5S provides the phase inverter stage of the amplifier for deriving a balanced signal output for the tubes 41 and 48 from a single-ended input circuit 31, the arrangement being such that the connection 51 for thecontrol grid 55 of the inverter stage on the signal potentiometer or bleeder is at a point which provi-des a signal voltage on the grid 55 of the same value as the signal voltage applied to the control grid 39 of the driver stage 35. For this purpose, the resistance values of the resistor sections 5I and 52 are adjusted to provide substantially unity gain between the plate circuit 53 and the grid 55 of the output tube 48 through the impedance coupling network comprising plate resistors 60 and 6I, -grid resistors 62 and 63 and the coupling capacitor 64 between the inverter anode 56 and the grid 65.

The grid circuits for the output tubes 41 and 43 are provided with a suitable bypass for audio frequencies to ground by a capacitor indicated at B, and bias potential is applied to the control grids of the output tubes through a suitable connection including a filter resistor 61.

The output voltage from the inverter tube 55 is inverted or 180 degrees out of phase with respect to the output voltage from the tube 35. Consequently, the voltages e1 and e2 applied to the grids of the push-pull tubes 41 and 48 are substantially 180 degrees out vof phase and equal as required for push-pull or balanced operation.

The M30-degree phase inversion is present in any normal tube with resistive load and results from the fact that, when the grid of the tube swings positive, the plate current increases and the A. C. component of the plate voltage swings negative. Thus a positive half cycle applied to the grid produces a negative half cycle of plate voltage and vice versa. Advantage is taken of the fact in providing resistance coupling between the driver and 4phase inverter tubes and the power output tubes 41 and 48. Thus the signal voltage applied to the input circuit of the output tube 41 is in phase with the signal voltage applied to the grid of the inverter tube 55 and, as the phase is through the inverter tube, the voltage e2 which is applied to the output tube 48 is 180 degrees out of phase with respect to the voltage 6i.

The output circuit 68-59 of the balanced amplifier' 41-43 is connected in balanced relation between the output anodes 10 and 1| and cornprises a primary 12 of an output transformer 13 having a secondary winding 14 connected with the operating or voice coil winding 15 ofa loudspeaker 16. The winding indicated at 11 is the hum-bucking winding for the loudspeaker. y Anode potential is supplied to the output circuit through a center tap 19 on the primary of the output transformer as normally provided in such circuits. However, the primary circuit is devoid of loading resistors and capacitors as normally provided to stabilize the amplifier and to prevent distortion occasioned by the reflected loudspeaker load in the output circuit providing a load characteristic which varies' with frequency. This is for the reason that the effective output impedance of the tubes 41 and 48 is lowered by the inverse feedback connection which includes a portion di) of the output secondary 14 between one end thereof which is grounded as indicated at Si and a tap 82 included in circuit with the cathode E3 of the input or driver stage 35 through the connection therewith provided by the external inverse feedback circuit 84 and the ground connection 8l.

In this manner. a portion of the output voltage existing across the secondary 14 between the terminal 82 and ground is applied in series between tlie cathode 83 and ground to the driver stage 35 and in opposition to the input signal voltage e between ground and the control grid ,39 across the resistor 31. These voltages are substantially 180 degrees out of phase and the result is a degree of degenerative feedback dependent upon the voltage derived from the winding 14, and may be increased to over 15 db. without danger of oscillation.

With this connection, pentodes for high efciency amplication may be utilized at 41 and 48 in the output stage. As the load impedance shifted substantially 180 degrees increases with frequency, the feedback voltage in-V creases because of the lighter load, tending to cut down the amplification with increased frequency and thereby providing a substantiallyilat overall response. This has the same effect as lowering the output impedance of the output stage. This in turn tends to stabilize the operationof the amplifier; The motional impedance of the loudspeaker-operating coil reflected back into the output circuit is damped out by the feedback action without requiring any loading resistors or capacitors across the primary winding for this purpose, and without the attendant losses in signal strength which otherwise must be accepted.

It will be noted that the inverse feedback loop through the amplier exten-ds from the output vwinding 30 from which the feedback potential isderived to the input circuit of the driver stage 35 in series with the input potential provided across the resistor 31 through ground and the bypass capacitor 33 making a series inverse feedback circuit from the double-ended or balanced amplifier to a single-ended input circuit.

This connection is effected by the simple circuit arrangement which includes tapping the output winding of the amplier to include a portion of that winding in the cathode circuit of the driver stage. Obviously, the resistance of the kwinding included in the cathode circuit should be relatively low and may normally be provided by a relatively low potential tap on the winding. With this arrangement, a xed bias for the inverter stage and the driverstage is preferable, as indicated, in order that both cathodes may be grounded, the driver stagei operating above ground potential by the amount of the feedback potential. 1

It will be Vnoted further that from the driver stage signals are applied directly tothe output amplifier tube 41 and indirectly to the amplifier tube 48 through the inverter stage 55. Thus, from the input circuit of `the amplifier, provided with inverse feedback, two stages of amplification are included on one side between the input circuit and the output circuit from which inverse feedback potentials are derived, and three stages are included on the opposite side, signals applied to the input circuit 31-39 being conveyed through the tube 35, the tube 56 and the tube 4S.

It is obvious that there is a greater possibility for phase shift in the three-stage branch of the circuit than Ain the two-stage branch and, since both branches are coupled to the feedback winding 8e, it would seem that the resultant phase shift might seriously affect the operation of the feedback circuit. However, such is not the case, as has been found in actual operation, as the output voltage es is the resultant of the two voltages e1 and e2 amplified `through the output stage.

This may be represented in Fig. 2 in which the output load provided by the loudspeaker or other device is reflected back through the output transformer 13 into the output circuit, as represented by the dotted rectangle 90, and the plate impedance of the tubes 41 and 48 are represented at 9| and S2 respectively. The input voltages e1 and e2 multiplied by the amplification factor of the tubes 41-48 are indicated in the figure and result in a generated output voltage es, of which the voltage es appears across the output circuit, as shown. The voltage ez has a negative sign for the reason that it is 180 degrees out of phase with the other voltage e1.

'I'his isv shown in Fig. 3 by the vector diagram of the input and output voltages and in which it will be noted that, while the phase distortion indicated by the angle 9 is greater for the threestage branch of the circuit than for the twostage branch, the resultant voltage ea has less phase, distortion than the three-stage branch, clearly indicating the feasibility of operation of an inverse feedback amplifier having a differing number of stages in one branch of a divided feedback loop than in another branch.

Conditions are consequently better than for a normal three-stage amplifier, although heretofore it has been considered that the use of feedback with phase inversion represented a special problem because the gain to the grid 4B is the gain of the driver stage 35, while the gain to the grid 65 is the gain of the driver stage 35 multiplied by the gain of the inverter stage 56, where the gain of the latter stage is made equal to unity, as by the bleeder or potential divider connection shown.

The phase shift to the grid 46 is the phase shift in the driver stage, while the phase shift for the grid E is the phase shift in the driver stage plus the phase shift in the phase inverter. The vector diagram analysis, however, as above referred to, shows that the overall phase shift referred to is less than that for a three-stage amplifier, although somewhat greater than for a two-stage amplifier.

It has been found that the input capacity of the input grid of the inverter tube 55 may cause the high frequency response to be lowered and a phase shift which may be corrected by including a portion of the signal voltage bleeder or potentiometer device in a phase-correcting 'network which includes a capacity of the value of that of the inverter stage input grid.

The object in correcting for phase shift and frequency loss is to stabilize the inverse feedback circuit, the stability of the circuit depending upon the amount lof attenuation which may be obtained prior to or in advance of the 180- degree phase shift in the inverter stage. In order' to reach a stable condition of operation and thereby to make possible a greater amount of degeneration in an inverse feedback amplifier of the type shown providing a phase inverter, the high frequency cut-off has been separated as much as possible in the various stages.

In the input circuit of the inverter, the phase shift is compensated or controlled by a cornpensating capacitor 94 connected in parallel with the high potential section 5I of the inverter bleeder resistor and having Va predetermined relation to the input capacity of the inverter tube 55.

Referring to Fig. 4 and Fig. l, with the capacitor at 94 in shunt with the high potential section 5| of the bleeder and with the tube input capacity indicated at 95 across the second resistor section 52, the high frequency response in the inverter stage may be extended and the phase shift minimized by making the product R1C1=R2C2, where R1 and Rz equal the resistances of the resistors 5| and 52, respectively, and C2 and C1 equal the input capacitance 95 and the compensating capacitance 94, respectively.

If the capacitor S4 is omitted, the effective input -capacitance C2 of the inverter stage, together with the voltage regulation of resistances 5I and 52, will cause a high frequency cut-olf together with a negative phase shift in the signal potential transmitted through the inverter stage. However, with the capacitor 94 alone in the circuit, neglecting the effect of the input capacitance of the tube 55, a high frequency gain or increase in signal response will be obtained, together with a positive phase -shift in the signals transmitted through the inverter stage.

A combination of the two opposing effects may thus be obtained, involving values of resistance and capacitance in accordance with the formula above referred to, which may cause substantially zero signal attenuation and phase shift Within the high frequency end of the audio frequency signal range of the amplifier. If the value of the capacitor 94 is increased above an optimum value for substantially zero high frequency signal attenuation and phase shift, an increase in the high frequency response may be obtained Ywith a slight positive phase shift.

In the amplifier shown, for a coupling and phase inverter network between two RCA 6J5 tubesA at 35 and 55 and two RCA GF6 pentode tubes at 4l' and 48, the grid coupling resistors 62 and 53may each have a value of substantially 270,000 ohms with a bypass capacitor B6 of .25 mfd. The anode coupling resistors 60 and 61 may have a value of 100,000 ohms with a coupling capacitor 45 of .l mfd. and a coupling capacitor at 64 of .025 mfd. The input resistor has a resistance of 1 megohm, with a bypass capacitor at 38 of .5 mfd.

In the inverter bleeder circuit, the anode potential blocking capacitor 50 may have a value of .01 mfd., the compensating capacitor 94 a value of 4.7 mmfd., and the resistor sections 5l-'52 may have values, respectively, of 560,000 ohms and 47,000 ohms.

In order to prevent undesired high and low frequency oscillations in the amplifier, resulting from a phase shift in signal voltages transmitted through the amplifier in a direction normally providing inverse feedback, for example, a phase shift of substantially degrees and regenerative feedback, the coupling network in the two branches of the feedback loop is modified to effect a high frequency cut-olf in one branch and a low frequency cut-off in the other branch. This is accomplished in the circuit by connecting in shunt with the signal circuit in the twostage branch a capacitor 96 from the output circuit 53 to ground, the connection preferably being made to the anode circuit 53 of the driver stage 35, as shown, in connection with the lead 49. In the three-stage branch between the inverter stage 56 and the output tube 4B, the coupling capacitor 64 is reduced in value from .1 mfd., as in the two-stage branch at 45, to a low value sufficient to appreciably attenuate low signal frequencies, particularly adjacent to and below the useful audiofrequency range and may have a value of .025 mfd., as hereinbefore referred to, for this purpose. The capacitor 96 mtr have a value in the present example of .005 m With this arrangement in the branched inverse feedback circuit, continuous oscillation at relatively high frequencies of the order of 20,000 cycles and above, and at relatively low frequencies substantially below the useful range of audibility, resulting in excessive distortion and overload of the amplifier, is effectively prevented.

By providing a high frequency cut-off in the plate circuit of the driver stage at as low a fre- -the tendency of said amplier to'` oscillate above quency as is consistent with a satisfactory overall frequency response, and by extending the high frequency response range in all of the other signal conveying circuits of the amplifier, the oscillations referred to are prevented, while at the same time the inverse rfeedback characteristic of the amplifier serves to correct for the high frequency signal attenuation injected into the one branch of the feedback loop. Like-wise, this inverse feedback action serves to correct for the low frequency attenuation in the other three-stage branch of the inverse feedback loop.

It is this characteristics of inverse feedback amplifiers tending to maintain an overall uniform frequency response which also operates adversely with regard to anytone-control action which may be desired in the circuit. Accordingly, it will be noted that 'tone-control, means applied to the volume-control circuit 2f225 is included outside of the inverse feedback loop of the amplifier, and. for manual adjustment may be provided in the intermediate frequency amplifier, as hereinbefore referred to, so that the signal fidelity prior to amplification in the inverse feedback amplifier may be controlled.

For example, by controlling the coupling between the intermediate frequency amplifier tuned circuits and the input circuits therefor, by adjustment of the coupling windings I3 and i 1i shown in Fig. l, the signal attenuation about the intermediate frequency of 455 kes. may be controlled to provide variation in the selectivity response characteristic, as indicated at 97 in Fig. 5. This may be made to result in attenuation of the high frequency signals in the audio frequency output of the iirst `detector and amplifier 8 in accordarice with the response curves indicated at 98 in Fig. 6, the lower audio frequency cut-off corresponding to the narrow frequency attenuation curves in Fig. 5, as will be understood from a comparison of Figs. and 6. By this means, the signal potential e, applied to the input circuit of the driver stage or iirst stage audio frequency ampliiier within the feedback loop may be controlled independent of the inverse feedback frequency corrective action, thereby resulting inan effective tone control for the receiving systei We claim as our invention:

1. An inverse feedback amplifier comprising, in combination, a driver stage, a phase inverter stage, a balanced output amplifier stage impedance coupled to said first and second-named stages and having an inverse feedback connection with said driver stage, means including a coupling network between said driver stage and said inverter stage for applying a predetermined value of signal voltage to said inverter stage, and means in saidlast-named network for compensating the input capacitance of said inverter stage thereby to prevent phase shift in the feedback loop of the amplifier through said inverter stage.

2. In an inverse feedback amplifier having an input circuit and an output circuit, the combination of means' providing an inverse feedback circuit between said output circuit and said input circuit, a plurality of amplier stages interposed between said input circuit and said output circuit including coupling means for transmitting signals through said amplifier, said signal transmitting means including an interstage coupling network for reducing the overall phase shift of signals transmitted through said amplifier, and said amplifier further including an interstage coupling network having coupling elements for reducing and below a predetermined frequency range.

3. An inversev feedback amplifier comprising, in combination, a driver stage, a phase inverter stage, a balanced output amplifier stage impedance coupled to said first and second-named stages and having an inverse feedback connection with said driver stage, means including a coupling network between said driver stage and'said inverter stage for applying a predetermined value f of signal voltage to said inverter stage, means in said last-named network for compensatingy the input capacitance of said inverter stage thereby to prevent phase shift in the feedbackl loop of the ampliiier through said inverter stage, means included in the coupling between said balanced output amplifier stage and said phase inverter stage for preventing oscillations in the amplifier below a predetermined low frequency, and means in the coupling between said balanced output amplifier stage and the driver stage for preventing oscillations in the amplifier above a predetermined high frequency.

4. An inverse feedback amplier comprising, in combination, a balanced ampliiier stage having an output circuit, a driver stage impedance coupled to one half of said output stage, an inverter stage impedance coupled to the other half of said output stage, said impedance coupling including means for preventing oscillations in said vampliiier above and below the operating frequency range of the amplifier, means providing an inverse feedback circuit between said output circuit and the driver stage, means providing a coupling network between said driver stage and said inverter stage including a pair of series-connected voltage bleeder resistors to the junction of which said inverter stage is connected for applying thereto a predetermined signal potential, and means in said network for compensating the input capacitance of said inverter stage.

5. In an audio frequency amplifier, the combination of a driver stage, a phase inverter stage including an amplifier tube having a signal input grid and a predetermined input grid capacitance, means providing a coupling network between said driver stage and inverter stage including a pair -of series-connected voltage' divider resistors for said inverter stage, means providing a connection for said signal input grid with the junction of said resistors, a compensating capacitor connected in parallel with the initial one of said series resistors and having a predetermined capacity relation to the input capacitance of said inverter stage, whereby distortion in said network at high frequencies is minimized, and means providing an inverse feedback connection to said driver stage. Y

6. In an audio frequency amplifier having a signal input circuit, a signal output circuit and a loudspeaker device in said output circuit, the combination of means providing a balanced output stage including a pair of pentode amplifier tubes coupled to said output circuit, a driver stage including an amplifier tube coupled to said input circuit and to one of the tubes in the output stage, a phase inverter stage including an amplifier tube providing coupling means between said driver stage and the other of said output stage amplifier tubes, means providing a series inverse feedback circuit between said output circuit and said driver stage providing correction for motional impedance reflected back from said loudspeaker device to said output circuit, and means providing a signal bleeder coupling network between said driver stage and the inverter stage including a resistance and capacitance element for preventing phase shift and high frequency signal attenuation in said bleeder network.

7. In an inverse feedback amplifier having an input circuit and an output circuit, the combination of a feedback circuit between said input and output circuits, a driver stage, a phase inverter stage, and a balanced output stage and interstage coupling means therefor providing a degenerative loop over two stages on one side of said amplifier and a degenerative loop over three stages on the other side of said amplifier, the overall phase shift through which is less than for a three-stage amplifier, means providing a signal bleeder circuit for said phase inverter stage including circuit elements for preventing phase shift in the inverter stage by reason of the input capacitance thereof, and means in the coupling network for the first-named degenerative loop for preventing v oscillations above a predetermined high frequency, and means in the other degenerative loop of the amplier for preventing oscillations below a predetermined low frequency.

8. In an audio frequency signal amplifier, the Y combination of a driver stage, a phase inver-ter stage, a balanced pentode amplifier output stage resistance coupled to said first and secondnamed stages, a signal output circuit providing a frequency variable load on said output stage, means providing inverse feedback between said output circuit and said driver stage to lower the effective output impedance of said pentode amplier stage, and thereby to eliminate compensatory loading of said output stage, and means for preventing phase distortion between the driver stage and the inverter stage by reason of the input capacitance of the inverter stage.

9. The combination with an amplifier comprising a driver stage, a phase inverter stage and ,v

a balanced amplifier output stage resistance coupled to said rst and second-named stages, a

signal output circuit for said balanced output stage, said first-named stage including a driver tube having a cathode connected to said amplifier output circuit through an inverse feedback circuit, whereby the inverse feedback loop through the amplifier comprises two branch circuits, means for preventing phase distortion between the driver stage and the inver-ter stage by Vreason of the input capacitance of the inverter stage, means in one of said branch circuits for preventing oscillations in said amplifier above a predetermined high frequency, and means in the other branch circuit for preventing oscillations in a range below a predetermined low frequency.

10. In an amplifier, the combination of a driver stage, a phase inverter stage, a balanced output amplifier stage impedance coupled to said rst and second-named stages and having an inverse feedback connection with said driver Stage, means including -a coupling network between said driver stage. and said inverter stage for applying a predetermined value of signal voltage to said inverter stage, means in said lastnamed network for compensating the input capacitance of said inverter stage thereby to fix the frequency characteristic of the amplifier and to prevent phase shift in the feedback loop of the amplifier through said inverter stage, and means outside of the feedback loop of said amplifier for modifying the frequency characteristic of signals transmitted through said amplifier to further enhance the tone quality of said signals.

l1. In an amplifier, the combination of a driver stage, -a phase inverter stage, a balanced output amplifier stage impedance coupled to said vfirst and second-named stages and having an inverse feed-back connection with said driver stage, means including acoupling network between said driver stage and said inverter stage for applying a predetermined value of signal voltage to said inverter stage, means in said lastnamed network Y for compensating the input capacitance of said inverter stage thereby to prevent phase shift in the feedback loop of the amplifier through said inverter stage, means within the feedback loop for attenuating signals in frequency ranges above and below the operating range of the amplier, whereby the frequency characteristic of the amplifier is xed, and control means outside of the feedback loop of v said amplifier for modifying the frequency characteristic of signals transmitted through said amplifier to further enhance the tone quality of said signals.

PAUL F; G. HOLST.

LOREN R. KIRKWOOD.

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