US2179263A - Audio frequency amplifier - Google Patents

Description

w. R. KOCH 2,179,263

AUDIO FREQUENCY AMPLIFIER Filed Nov 30, 1936 +5 iaazzk g [44 comma/v POM/67E a mom SUPPAY 6864427? fiscal/5e 54 26 $1 30 27 49 f 47 35 at 42 LL 4g i 19.6. 7 ,z

26 5; 46 r 52 Ac ///97EK 6( 40 a F/Zfl/Vf/Vffi 7 FIELD 3v 3 nnentor attorney Patented Nov. 7, 1939 {TY-UNITED STATES PATENT OFFICE AUDIO FREQUENCY AMPLIFIER Winfield R2 Koch, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware 2 Application November 30, 1936, Serial No. 113,473

4 claims. (or. 179-171) v The present invention relates to audio frequency amplifiers, and more particularly to audio frequency-amplifiers in connection with radio receiving apparatus and the like having a common power supply connection with the receiver,

.power amplifier stage of a radio receiver or the like whereby the amplifier stage maybe operated from a single power supply device Without undue interference or flutter in the receiver circuit.

It has been found that the tendency of a radio receiver to flutter may be minimized considerably if the grid filter capacitorin the audio amplifier output stage which creates an interfering surge in responseto strong signals, is returned to the' plate supply positive lead rather than to ground or. chassis.

The grid filter, with the filter capacitor returned to ground, has normally a time constant much slower thanthe time constant of the power supply means, consequently the grid bias voltage tendsto remain constant during a surge. If the grid filter capacitor is connected to a point at which the surge originates, a counter-voltage is impressed. upon the grid minimizing the surge andits effects."

The tubes inthe amplifier may be considered to act as a low impedancealternating current path, while atthe same time offering a high resistance to direct current.

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

In the drawing, -Fig. 1 is a schematic circuit diagramof an amplifier embodying the invention and Fig. 2 is a further schematic circuit diagram of an output amplifier for a radio receiver showing a modification of the invention.

1 Referring toFig. 1, an amplifier tube 5 is provided with an input circuit 6 for receiving audio frequency signals, and an anode output circuit 1 connected through an anode output resistor 8, andacoupling capacitor 9, with an audio frequency output circuit I 0. i

1 Anodecurrent for the amplifier 5, is provided between a positive supply lead H and a negative ground lead |2.

series filter resistor I3 is connected in circuit and is provided with a by-pass filter circuit I4,

to ground.

The filter circuit comprises a second amplifier tube l5 having a control grid l6, an anode l1, and a cathode l8. The latter is connected to the ground lead l2, through a self bias resistor l9, and grid potential is applied therefrom to the control grid l6, through a grid resistor 2|] connected between the grid and ground.

Potential surges in the supply circuit, causing a variation in the potential between leads I l and i2, and surges caused by strong signals applied to the grid circuit 6, tending to make the anode current vary rapidly at audio and sub-audio frequencies are applied to the control grid of the regulating tube through a coupling capacitor 2|, connected between the junction of the output impedance or resistor 8 and the filter resistor l3 at the terminal 22.

The instantaneous surges are conveyed through the coupling capacitor 2| and cause the plate current of the regulating tube I5 to vary, thereby tending to maintain the potential at the point 22 constant and opposing the change which is occurring at that instant.

The reactance of the coupling capacitor 2| between the control grid N5 of the regulating tube and the anode I! should be relatively low, while the grid impedance to ground provided by the resistor should be relatively high.

The variation in potential which may occur at the terminal 22 may be represented by an alternating voltage E which is applied to the plate l1 and to the grid l6 through the capacitor 2|. In the plate circuit of the tube the amplified grid voltage E, is effectively in series with the applied voltage, the alternating component of the plate current will, therefore, be

and the impedance, in effect, will be In the positive supply lead between the output resistor 8 and said lead, a

The tube circuit l4 therefore takes the place of a by-pass capacitor and operates to amplify the effect of the capacitor 2|- At a flutter freing upon the bias on the grid. With the same tube connected as shown in Fig. 1, the plate circuit impedance to alternating current will be only one-fiftieth as much as the resistance to direct current. Because of the low impedance between point 22 and ground, variations in the voltage of the anode of tube will cause the voltage to appear chiefly across resistor 8, and only a small voltage will occur between point 22 and ground. Thus variations inplate current of tube 5 will cause onlysmall variations in current through resistor i3, and the potential of the supply voltage lead I! will be little affected. Likewise, any variations in the voltage supply potential, caused by other amplifier tubes, would made to drop most of the voltage in resistor 53, only small part of such variation in supply voltage would reach the anode i and succeeding parts of the amplifier through condenser Q and lead it.

Referring now to Fig. 2, a pair of balanced or push-pull connected audio frequency amplifier tubes 25 and 25 are connected between a push-pull or balanced input transformer ii? and a push-pull or balanced output transformer 28, to supply audio frequency power to a loudspeaker 29 when audio frequency signals are applied to the input winding 39 of the input transformer 2'5. The cathode heating supply circuit for the amplifier, indicated at 3!, is in common with a radio receiver 32, which may supply signals to the input terminals 30, and is connected to ground, as indicated at 33, within a power supply device indicated at 34, which receives power from alternating current supply leads 35-35.

The power supply. unit 3 includes a rectifier indicated at 3'5 to provide rectified and filtered direct current potential at suitable anode potential between output leads 3'! and 38, the final filtering being provided by a shunt by-pass filter capacitor 39.

The negative potential supply lead 38 from the rectifier is connected to ground or chassis 39 through the loudspeaker field indicated at ill, which provides a filter choke coil in the negative lead and a source of grid bias potential for the tubes 25 and 26. A connection lead between the negative terminal of the potential source 40 and a center tap $2 on the input transformer is provided with a series filter resistor 43 for the biasing potential for the tubes 25 and 25. A plate potential supply lead it is connected between a center tap 55 on the primary of the output transformer and the positive supply anode lead 31. A filter capacitor 8% is also provided between the terminal 45 and the ground.

The radio receiver is also connected in common with the amplifier, to the anode potential supply leads 3'? and 38, as indicated at M and a suitable filter capacitor is provided therefor as indicated at 48.

Variations in. anode potential caused by heavy load placed upon the output amplifier and audio orsub audio potential variations resulting therefrom may be transmitted to the radio receiver and may adversely affect the operation of the tube circuits therein, as is well known. To prevent this effect, the grid filter, including the series resistor 43, is completed by a filter capacitor 49 connected between the grid terminal 42 on the transformer 21 and the positive supply lead 3'? through a lead indicated at 50. With this connection, a signal surge tending to cause an increase in anode current through the tubes 25 and 26, and a corresponding reduction in voltage on the positive lead 31, is conveyed through the filter or controlling capacitor 49 directly to the grids of the amplifier tubes, causing said grids to become more negative and this, in turn, tends to counteract the tendency of the anode current to increase, destroys the effect of the surge on the supply circuit, and prevents the same from being transmitted, to the radio receiver circuits.

It will be appreciated that if the usual grid filter is provided for the purpose of reducing the effect of ripple resulting from alternating current rectification, the time constant of the circuit so formed is relatively slow and tends to maintain the grid bias potential constant during the surge, thereby permitting the anode current to rise.

However, with the connections shown, the amplifier tubes are utilized to absorb the alternating current or surge voltage. The circuit feeds back only the undesired effects of the signal or surge and not the surge itself, since the degenerative feedback connection is provided through a reactance device of low impedance to alternating current, while preventing the flow of direct current because of the high direct current resistance.

In an amplifier as shown, using RCA 2A3 tubes and a potential of 300 volts between the leads 3? and 38, a surge conducting capacitor 49 of .25 microfarad is provided in connection with a filter resistor 43 of substantially 56,000 ohms. As in the preceding example, the capacitor 49 should have a relatively low reactance, while the grid filter resistor 43 should have a relatively high resistance. For example, in the circuit of Fig. 1 the capacitor 2| may have a value of 1.0 microfarad and the resistor 2|) may have a resistance of substantially 1 megohm, the requirements depending upon circuit and the tubes used.

It has been found preferable, in commercial radio receivers for high fidelity output, to separate the balanced grid circuit from the filter network by introducing a resistor 43a between the mid-terminal of the input transformer and the mid-tap on the filter, between the resistor and the capacitor. This may have a resistance of 100,000 ohms with 2A3 output tubes as shown and is preferably higher in resistance than the resistor 43.

I claim as my invention:

1. In an audio frequency amplifier, a filter circuit comprising a filter resistor and an electric discharge amplifier tube, means providing a relatively high impedance path between the input electrode thereof and the cathode, and a capacitor providing a relatively low reactance path between the anode and said input electrode and providing the sole impedance element in circuit between said electrodes, said anode being connec'tedto a point in said amplifier at which surges tend to develop.

2. In an audio frequency amplifier of the pushpull type having a balanced input circuit provided with a connection for grid bias potential supply, means providing a balanced output anode circuit ior said amplifier having a centertap connection, means for applying audio frequency and sub-audio frequency voltages to the 5 control electrodes of the push-pull amplifier simultaneously in parallel, said means comprising a capacitor of low reactance at said frequencies connected as the sole impedance element between the anode circuit center-tap con- 10 nection and said bias potential supply connection, and means for supplying a biasing potential to said first-named connection comprising a filter resistor having a relatively high resistance with respect to the reactance of said capacitor.

5 3. In an audio frequency amplifier of the balanced type including a pair of electric discharge amplifier tubes in push-pull relation to each other, the combination of anode potential supply means for said tubes comprising a positive supply lead 20 and a negative supply lead, means providing a balanced input circuit for said amplifier having a bias potential supply connection, means providing a balanced output anode circuit for said amplifier having a center-tap connection with 25 said positive supply lead, a capacitor having a relatively low reactance to audio frequency currents and sub-audio frequency currents connected as the only impedance element in circuit between the said center-tap connection and said bias potential supply connection, a filter resistor connected between said capacitor and a point of negative biasing potential supply in said negative supply lead, and means for isolating said center-tap connection from said last-named point of connection between said resistor and capacitor comprising a seriesresistor having a resistance at least as great as said first-named resistor.

4. In an audio frequency amplifier of the balanced type, comprising a pair of electric discharge amplifier devices connected in balanced relation to each other, the combination of an input transformer for said amplifier having a secondary winding provided with a bias potential supply connection, an output transformer having a primary winding provided with a centertap, a capacitor providing the only impedance in circuit between said bias potential supply connection and said center-tap and providing a path of low reactance for audio frequency currents therebetween, means providing a source of biasing potential for said amplifier devices, and a resistor connected between the center-tap of the input transformer secondary and said source, said resistor having a relatively high resistance with respect to the reactance of said capacitor.

WINFIELD R. KOCH.

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