US2255757A - Audio frequency amplifier - Google Patents

Description

Sept. 16, 1941.

NON

R. A. BIERWIRTH AUDIO FREQUENCY AMPLIFIER Filed July 29, 1939 T Q Q M Wom'o (Ittorneg Patented Sept. 16, 1941 AUDIO FREQUENCY AMPLIFIER,

Rudolph A. Bicrwirth, naaaon Heights, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 29, 1939, sesame. 287,306

Claims. (C1.

The present invention relates to audio frequency amplifiers, and has for its object to provide an improved volume control system therefor having a single control means for varying the signal attenuation in connection with one amplifier stage and the gain by inverse feedback on a preceding stage, whereby improved volume control of signals is obtained and overloading of said preceding stage in response to appliedv signals is minimized. v

It is also an object of the present invention to provide a multi-stage audio frequency amplifier having means for variably attenuating signals in a plurality of stages by operation of a single control element in a coupling network between two of such stages providing control of the signal voltage amplitude applied to the input circuit of one stage and of an inverse feedback voltage to, the input circuit of the preceding stagein predetermined relation, whereby signal distortion by overloading in said preceding stage is substantially prevented.

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

In the drawing,

Figure 1 is a schematic circuit diagram of a multi-stage audio frequency amplifier provided with volume control means embodying the inven- 1 Referring to Fig. 1, 5 is a first stage amplifier tube having a signal input grid 6 selectively connectable through a switch I to an input circuit 8 comprising the secondary 9 of a microphone input transformer ID on the primary of which are provided suitable input tap connections indicated at H, I! and Ill. The alternate connection'provided by the switch 1 for the input grid 6 includes an input grid resistor l5 having a common cathode return circuit connection 16 with the secondary 9, and having an input circuit l1 coupled thereto through a coupling capacitor 13. A terminal IQ for the circuit l1 provides a connection for a crystal microphone (not shown).

Bias potential for the amplifier is provided by a cathode resistor 20 having a capacitor 2!. The tube may be of any suitable type adapted for audio frequency amplification and in the present example is shown as a screen grid pentode having a screen grid 22, a suppressor grid 23 and an output anode 24. The latter is connected to. a positive anode potential supply lead indicated at 14, through an output or anode coupling resistor'25 and is coupledv through a suitable coupling capacitor 26 to an attenuator network 21 in the input circuit 23 of a second stage amplifier tube 29. The tube 29 in turn is coupled through a suitable capacity couplingcircuit or network 30- with a third stage amplifier tube 3! having an output anode 32 connected to an output coupling device 33. H

' The succeeding amplifier-stages maybe provided with inverse feedback, such as the amplifier tube 3| which is provided with aninverse feedback connection 34 comprising a capacitor 35 and series resistor 36, to the cathode 31 of the second stage amplifier tube 29 and supplies inverse feedback voltage thereto across a cathode resistor 38. A portion of the resistor '38 is provided with a bypass capacitor 39 for controlling the high frequency response by limiting the inverse feedback in the upper portion of the audio frequency range. v

The coupling networkjzl between the input circuit 280i the second stage amplifier 29 and the output circuit of the first stage amplifier tube 5 is preferably in the form of a volume control potentiometeror attenuator device 46, connected at one terminal H to the input circuit 28 and at the opposite terminal 42 with a second feedback circuit 43. The latter circuit isprovided with a branch connection for each of the input circuits 8 and Il, through impedance ele-- moved over the potentiometer element 46 between the terminals 41 and 42, the arrangement being such that as the contact lfi is moved in the direction of the terminal 42 a signal transmitted through the amplifier from the firststage to the second stage is increasingly attenuated in theinput circuit of the second stage by reduction of the applied signal voltage to the input circuit amplitude atthe input terminals. 1 i

28 while, at. the same time, the feedback current is increased through the feedback circuit 53.

Thus the input circuits 8 and I! receive an increasing feedback potential which tends to reduce the gain in the first stage by reducing the signal amplitude therein thereby preventing overload of the grid 6. The feedback voltage reduces the signal applied to the grid of the first stage by the amount of the feedback and there by prevents the tube from overloading and from causing distortion in the presence of strong input 3 signals from any source which may be coupled with the input terminals I 9 or I l and [2. The distortion produced by the tube. is. also: reduced by the feedback. 7 As the'volume control element 46 is moved in I ,the direction of the terminal 4I-, an increasing 7 end of the bias resistor 20 and the inverse feed back lead 43 is connected through the series resisting limiter 44 to a terminal 55 on the grid signal voltage is applied to the input circuit of the second stage amplifier while at the same-time the feedback current to the first stage is reduced proportionately. Thus in the presence of weak signals applied to the input. terminals [9 or ll and I2, requiring less signal attenuation in the volume control means, the feedback voltage on the first stage is also reduced and may be made substantially zero when a maximum gain is providedinthe second stage, as, for. example, when.

the contact 46 is moved substantially to. the output terminal anddiie'ctly in connection with i the input circuit 28 of. the secondstagel Under.

this condition, the signal on the firststageampli- 1 fier isirelativelylow'so that there is substantially 1 nodistorti'on, andfeedba'ck is not required; 7

' Theiattenuator network .21 is inthe form of avariable. ladder having resistors 50- connected with space d tap points 5| on the main attenuator resistor 40 and connected together through a lead 52 with the cathode return circuit l6. With this arrangement, as. is well known, the load on the output circuit of'the tube 5 is made substantially constant over the full range of control movement ofthe contact 46.

In. the circuit shown, :the contact 46 on'the attenuator .or coupling network 21 provides, a 1 7 single. control means for both the signal attenuationin the input circuit of. the second stage by direct'control of the signal voltage applied thereto'and signal attenuation in the input cir-j end of the coupling resistor 41 between the last named resistors and the low side of the secondary 9 so that the feedback voltage is applied in series with the signal voltage to the input grid 8.

The feedback voltage'i's' derived from a separate control element comprising a resistor element 56 connected in parallel between the cathode return circuit l6 and the output'coupling capacitor across the output plate resistor .25. The resistor 56 is of the variable type providing a potentiom [eter having a movable, contact 5'! connecting with the feedback lead, the arrangement being such that a's the contact is moved from the high potential end 58 -to the low potential end 59, the

negative feedback voltage derived from the re- I sistor 55 is reduced, thereby reducing the feedback current and the potential drop caused by the 'feedback current in the resistor 41.

' Also connected in parallel with the output plateI circuit resistor 25 of the first stagetube. 5 is a second potentiometer device comprising aresistor element 60 and a movable contact therefor 6!, the high potential terminal 62 of the resistor 60 being connected to the termina1158 and the low potential terminal 63 being connected with the terminal 59 and the lead I6.

The contact BI is connected to the, input circuit 'G'S With the second stage tube 29 and operates-U V to control the signafpotential applied to the second stageas a volume control device. 'As the; contact 6| is moved from the'terminal 63 to "the terminal 62 along the resistor 55, thesignal voltage appliedto the second stage amplifier 25. is increased from minim-um to maximum. 1

The two contacts 5'! and El are connected for continued operation as'indicated by the dot-ted connection 64in such a manner that the feedback voltage or current to the first stage and-the.

signalvoltage'appliedto the second stage ampli- -fiers are-varied in an opposite sense, one with respect to the other.

cuit ofthe first stage by inverse feedback thereto,

the inverse feedback being .varied thereby in. an

' opposite sense to the signal voltage variation.

It has been found that with thevolume and feed-f back control element 46 'adjusted'for minimum volume at the terminal the signal attenuation between stages may beof the order of from 30-35 V d. b. andwith the feedback maximum and of the order of 15 d. b., a total signal attenuation of from -50 d. b. may be provided. The attenuationapplied to the inputcircuit ofthe'first stage byinvers'e feedback has been found sufficient to prevent overloading and distortion with. a condition' of operation providing maximum signal j The attenuator'or coupling network between 3 the input transformer T0: as in the preceding contacts 51 and -61 move in unison bycontinued operation from left to right and right to. left, as shown in the drawing, movement to the'right causes the inverse feedback voltage to be reduced 7 and the signal voltage applied to the second stage to be increased in a predetermined relation to. each other and substantially in the same manner" as when thecircuit of Fig. 1, exceptthat the two potential controlling elements are connected in parallel instead of being included in one resistfigure, with the; series resistor 41 in theinput v circuit 8 at the low potentialsidefthereof adia: 1 cent to thecathode connection at the negative ance or impedance element in the coupling net- 7 work between the two stages.

The circuit of Fig. 1' has certain advantages in that the controlling-element is simplified, by

being provided with a unitary controlling. device, whileinthe circuit of Fig. 2, the two resistance elements and 65 may be designed to have V differing resistance characteristics. That is, in'

the case of rotary controldevicesthe change in resistan'ce for the same range of movement in each device may be different and may differover various. portions of thecontrol range of movement of the contact elements.

In any case, however, it will be seen from the preceding description thatin a multiestage am- I plifier, ajdesirablefgain control characteristic may be provided by arranging the interstage couplingnetwork between the first and second stages or between anytwostages, if desired, for the simul-.

taneous control o f gain in one stage and the;

control of degeneration or inverse feedback in a Thus, assuming that. the

preceding stage, thereby controlling the gain in said preceding stage and preventing overload in the presence of strong signals in the said preceding stage.

In operation, this provides for a reduction in inverse feedback and an increase in the gain of the preceding stage as the applied signal falls off in amplitude by increasing the gain without overloading the first stage while the gain in the second stage is correspondingly increased in a certain predetermined relation to the increase in gain in the first stage.

Conversely, as the applied signal increases at the first stage to maintain the same output volume or signal amplitude, operation of the volume control element serves to apply an increasing inverse feedback to prevent overloading and to reduce the gain in the first stage simultaneously with a reduction in the gain of the second stage.

This operation is accomplished through the continued control of inverse feedback in the one stage and the control of the applied signal voltage to the second stage without involving a plurality of circuit elements and variable control devices.

I claim as my invention:

1. In an audio frequency amplifier comprising a plurality of cascade connected amplifier stages, the combination of an interstate signal attenuator network including a variable impedance element for controlling signal attenuation, an amplifier stage preceding said network, and an inverse feedback circuit for applying inverse feedback potentials to said amplifier stage, said circuit including said variable impedance element whereby said inverse feedback potentials are conjointly controllable with signal attenuation, and

said variable impedance element comprising a tapped potentiometer resistor having anoutput grid connection at one end thereof and a feedback output connection at the opposite end thereof, and being further provided with a variable input potentiometer contact movable along said potentiometer resistor between said ends, thereby to vary the signal attenuation and the degree of inverse feedback in opposite sense, one with respect to the other.

2. In a multi-stage audio frequency amplifier, the combination of a variable signal attenuator network for controlling the amplitude of signals applied to one stage of said amplifier whereby the signal output of said amplifier may be adjusted to compensate for variations in input signal amplitude, means for controlling the gain of a preceding amplifier stage and preventing overload of said stage in response to said input signal Variations comprising an inverse feedback connection to said preceding stage through said attenuator network whereby the inverse feedback energy and signal attenuation are conjointly controllable through said attenuator network, and means in said network for jointly controlling said inverse feedback and signal attenuation in opposite sense, one with respect to the other.

3. In an audio frequency amplifier, the combination of a first stage amplifier tube having a signal input circuit and a signal output circuit, a second stage amplifier tube having a signal input circuit, an attenuator network including a potentiometer resistor connected serially between said input circuits, and means providing a variable connection for said output circuit with said potentiometer resistor, thereby to provide conjoint control of inverse feedback to said first named input circuit and signal amplitude applied to said second named input circuit in opposite sense, one with respect to the other, Whereby conjoint control of gain is provided in each of said input circuits and overloading of said first named input circuit in response to increased input signal strength is prevented.

4. In an inverse feedback multi-stage audio frequency amplifier, the combination of a first stage amplifier tube having a signal input circuit and a signal output circuit, a second stage amplifier tube having a signal input circuit and a variable attenuator network connected between said output circuit and each of said input circuits providing conjoint control of inverse feedback to said first named input circuit and signal amplitude applied to said second named input circuit in opposite sense, one with respect to the other.

5. In a multi-stage audio frequency amplifier, the combination of a signal attenuator network for one of said stages including a single variable attenuator element, an inverse feedback circuit for a preceding amplifier stage including said variable attenuator element, and means in said inverse feedback circuit for modifying the feedback. action therethrough, and means in said network for controlling signal attenuation and inverse feedback in pretermined mutual relation to each other.

RUDOLPH A. BIERWIRTI-I.

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