US2076814A - Automatic gain control - Google Patents

Description

April 13, 1937. c. J. FRANKS AUTOMATIC (FAIN CONTROL 2 Sheets-Sheet 1 Filed July 11, 1933 Fay -Z.

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Patented Apr. 13,1937 g l V 1 UNITED STATES PATENT QFFIE Y AUTOMATIC GAIN CONTROL Christopher J. Franks, Boonton, N. 3., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application July 11, 1932,, Serial No. 679,947

' 2 Claims. (01. 250-20) This invention relates to automatic gain con- Fig. 4 is a fragmentary circuit diagramof a trol circuits of the type in which the carrier voltreceiver embodying the invention; and age is rectified to obtain a direct current poten- Fig. 5 is a family of receiver input-detector tial for varying the gain of an amplifier. input curves plotted from data obtained for a 5 The automatic volume control systems cusplurality of adjustments of the resistance which 5 tomarily employed to control the gain of a radio controlled that fraction of the available gain conamplifier place the rectifier of the control system trol voltage that passed to the amplifier stage after the amplifier, and the rectified potential is succeeding the gain control system. fed back to the amplifier. Such systems neces- In the drawings, the reference characters AG sarily result in a substantial variation in the outidentify a collector structure of any desired form 0 put level since the control depends upon a variafor receiving carrier wave signals and transfertion in the magnitude of the amplifier output ing the same to amplifier RF that includes a pluvoltage which is impressed upon the rectifier and rality of cascaded amplifier tubes 1, 2, and 3. The upon th d t t coupling circuits between successive tubes may An object of the present invention is to probe of any desired construction, and the ampli 15 vide an automatic gain control which is capab1e fication maybe at the received carrier frequency of giving a constant output level for all radio ind/0 at a in e med ate requ ncy. put voltages above the critical value which, at According to the invention, the .automatic gain maximum gain, will give the desired output level. t ol voltage obtained b fica ion of the An object is to provide methods of and circuit amplified radio voltage E1 developed in a circuit 20 arrangements for automatic gain control which of the intermediate tube 2 is app t y t are characterized by a control of the gain of a n or more of the preceding tubes l, as is e portion of the radio amplifier which succeeds known pra tic but also to one or more of the that point in a multistage radio amplifier at succeeding tubes 3 0f the mum-Stage p fi which an amplified radio voltage is passed to As shown in Fig. 1, the rad o Voltage E1 at 25 the gain control system. More specifically, an the outputcircuit of the tube 2 is impressed upon object is to provide a multi-stage amplifier in the succeeding amplifier tube and also upon a which the radio voltage at anintermediate point rectifier 4 of the automatic gain control system in the amplifier is rectified to obtain again conwhich is indicated generically at C. The direct trol voltage, andthe gain control voltage is imcurrentlpotential developed across the output 1' 3o pressed upon one or more stages preceding and sistance 5 of the rectifier is passed, through the subsequent to the intermediate or control point. fil er n w rk f rm by r i an es 6, 6' and A further object of the invention is to provide a, condensers I, to the leads 8 that run to the conradio receiver of the automatic gain control type, rol. grids of the amplifier tubes l and in which the gain control unit includes a ra- Any desired portion of the control voltage is 35 dio amplifier for impressing upon the control ele-. a e y adjustable n 9 and d W, to ments a radio voltage of greater magnitude and, the control grid of one or more of the amplifier if desired, more sharply: restricted t th fr tubes 3 which follow the tube 2. The last tube 3- quency of the desired carrier wave signal than y Serve y as an a p it m y be 40 is the radio output of the amplifier stage which both a radio amplifier and detector, for p works into the control unit and into the deteca plate C c t e ector. When the tube 3 func-. t r, y tions only as an amplifier, the first tube ll of These and other objects and advantages of the the audio freque y system AF acts as the deinvention will be apparent from the following ee erspecification when taken with the accompanying The operation and v a s of the inven- 4.5 drawings in which: tion will be apparent from the generalized curves. Fig. 1 is a somewhat diagrammatic i w of a, of Fig. 2. With automatic bias applied to tubes circuit embodying the invention; I, and not to the tube or tubes 3, the relation? Fig. 2 is a curve sheet showing the general rebetween the output level, or voltage E0, and the lationship between receiverinput and audio outsignal voltage E, will be substantially as indicated put level with known systems and with a conby curve 0A. As the carrier wave voltage introl system embodying the invention; creases, the radio voltage Er at tube 2 increases, Fig. 3 is a curve showing the relation between thereby increasing the bias on the control grids the grid bias and the gain of oneof the conof tubes 1 and decreasing the gain of the amplitrolled tubes; a h r, At that normal signal strengthEnwhich, 55

with maximum gain would give the desired output voltage, the gain has been substantially reduced by the automatic volume control. A better control, such as shown by the line ONB, may be obtained by the known systems for delaying the operation of the automatic gain control until the carrier voltage rises to the normal value En, but even with such systems, the branch NB of the control curve must rise with increasing signal strength since an increased radio voltage E1 must obtain at the output of the last controlled tube to produce that increase in rectified radio voltage which is required to reduce the gain as the signal strength increases.

By applying the gain control voltage to a tube 3 which follows the intermediate point, tube 2, at which the control system is connected across the amplifier, the variation in the amplified radio voltage E1 may be ofiset by corresponding variations in the gain of the tubes 3, thus giving a branch NN of the control curve which may be substantially flat over an extended range of signal voltages.

To prevent the reduction in gain for signals below the normal value En, the initial bias on the tube or tubes 3 may be such that the gain increases as the automatic gain control first comes into action with increasing signal strength, and then decreases. As shown in 'Fig. 3, the gaincontrol grid bias curve D changes curvature within the operating range of bias voltages and the initial bias aa may be so chosen that, as the gain control component of the bias voltage increases, the gain of the portion of the controlled r amplifier succeeding the control unit C increases to counteract the decreased gain of the tubes I, thus giving a net gain-curve for signals below En that approximates the branch ON, and then decreases to effect the reduction in gain that cannot be produced by applying the control to tubes preceding the control unit C.

It will be apparent, however, that the invention is generally applicable to known types of gain control systems to modify the relation between the output level and the signal input in any desired manner. The compensation for the variation of the voltage El may be either greater or less than that necessary to maintain a constant amplifier output E0. The control of the gain of stages succeeding the control point may also be applied to systems which include some provision for delaying the action of the gain control until the received signals exceed the normal input voltage En- As shown in Fig. 4, the radio amplifier RF, which may include one or more controlled amplifier tubes and, if desired, one or more amplifier tubes on which the gain is not controlled automatically, is coupled in any appropriate manner to the input circuit of the radio amplifier tube 3. The input circuit is shown as an inductance l2 which is tuned by a condenser I3, the blocking condenser 14 being connected between the lower terminal of the inductance and ground to permit the application of a gain control bias through the lead Ill. The tube 3 works into a combined diode detector and audio amplifier tube I5 which may be of the known Type 55, and the interstage coupling may be a transformer [6 having tuned primary and secondary windings. The details of the circuits of tube [5 do not constitute an essential part of the invention and therefore a description of the same is not believed to be necessary, it being sufiicient to note that an audio frequency voltage Ea is developed across the tube output impedance IT for application to a succeeding amplifier stage or reproducer.

The amplified radio voltage E which is impressed upon the amplifier tube 3 is also impressed upon the control system 0 through the lead 18 and condenser l9. The tube 4 of the gain control system is not a simple rectifier but comprises a combined pentode amplifier and diode rectifier. The input radio voltage is impressed between control grid G1 and the cathode K to produce an amplified radio voltage in the plate circuit, and the amplified voltage is impressed on the anode A through the tuned transformer 2B. The cathode circuit of tube 4 preferably includes bias resistors 2|, 22 for establishing a delay bias on the rectifier AK, the control grid being returned through the leak resistance 23 to the junction of the resistor to place an appropriate bias for amplification on the control grid.

The anode output circuit includes a fixed resistor 24 and an adjustable resistance 25 between the lower terminal of the transformer secondary and the terminal of resistance 22 which is remote from the cathode. With this arrangement, the delay bias on the rectifier is equal to the potential drop established across series resistances 2|, 22 by space current flow in tube 4 and, as indicated by the legend +40 volts, the anode A may be so biased that no rectification takes place until the peak input voltage on the rectifier exceeds 40 volts.

The lead 8 which extends to the controlled tubes of the radio amplifier RF is connected to the anode terminal of resistor 24 through a filter comprising resistance 26 and condensers 21, and the total direct current voltage developed by the control unit C is therefore impressed on one or more of the amplifier tubes preceding the tube 3. Only that fraction of the available direct current voltage which is developed across the resistance 25 is applied through lead 10 to the tube 3 as a gain control bias.

The close approximation to the flat gain control curve NN of Fig. 2 which may be obtained by an appropriate choice of the values of resistances 24 and 25 is shown by the curve ONN of Fig. 5. The data from which this curve was plotted was obtained when resistor 24 was 500,000 ohms and resistance 25 had a value of 130,000 ohms. The control bias applied to tube 3' was therefore approximately one-fifth of the available voltage that was impressed on the controlled tubes of amplifier RF. Curve ON'B' was plotted from readings made when resistance 25 was zero, 1. e., when the Fig. 4 circuit was so adjusted that the automatic gain control functions in the same manner as the known gain control systems. On the arbitrary scale of values of carrier voltage on the detector, the desired detector input level had a value of approximately 200 and it will be noted that the curve ONB rose to approximately 600 for a receiver input of /2 volt. As this 3 to 1 rise of detector input amounts to a 9 to 1 rise in the power output at the reproducer, it is apparent that the expected variation in receiver input produces substantial changes in the output level when the invention is not employed. Even greater variations in the output level may obtain when the known control systems are not carefully designed to obtain the best possible gain characteristic.

Curve O'N'N represents the observed relation between receiver input and. detector input iii when the resistance 25 was set at a value of 260,000 ohms, i. e., about one-third of the available voltage was impressed on the control grid of tube 3'. This curve shows the possibility of obtaining a falling output level with increase in receiver input but, in general, the substantially fiat characteristic ONN' is to be preferred. The particular fraction of the control voltage which should be applied toa succeeding tube or tubes 3 will depend upon the type and number of controlled tubes in the amplifier and, for any given receiver design, it is not necessary that the resistance 25 be adjustable since'its value is not to be varied as an operating adjustment of the receiver.

The problem of receiver design, and particularly in the case of high fidelity receivers, is materially simplified by'the invention as the audio system is not subjected to relatively wide variations in input voltage when resistances 24, 25 are so chosen as to produce a substantially fiat gain characteristic. The audio amplification may be set at a definite value to obtain the desired output level with the assurance that there will be no danger of overloading the audio system, and thereby introducing distortion, since the top NN' of the control curve closely approximates a horizontal line for an extended range of input voltages.

The advantage of introducing an independent amplification of the radio voltage E1 for application to the rectifier is that a sharp control of the gain characteristic may be obtained when the voltage E1 is comparatively low, as is the case when the audio system is designed for high gain. When this amplification system includes a circuit tuned to the frequency of the desired carrier wave, such as the tuned transformer 20, there is the further advantage that the gain control system has a sharper characteristic of developed voltage versus frequency 011 tune than does the cascaded amplifier system of the receiver.

The detailed construction and design of the several elements of the radio amplifier and of the control system may be varied at will without departure from the spirit of my invention as set forth in the following claims.

This application is a continuation-in-part of my copending application Ser. No. 600,750, filed March 23, 1932, for improvements in Automatic gain control.

I claim:

1. In a transmission system, the combination with a plurality of tubes and a circuit network connecting said tubes in cascade for amplification of radio frequencies, means initially biasing said tubes to establish the gain thereof in the absence of received signals, and a rectifier having an input circuit effectively connected across points of said network subjected to a radio voltage developed intermediate the input and output terminals of said amplifier, of means for impressing upon a tube preceding and a tube succeeding said intermediate points of said network direct current bias potentials proportional to the direct current potential developed by said rectifier, wherein the initial bias on the tube succeeding said network points is less than the value corresponding to maximum amplification, whereby the gain of said tube increases and thereafter decreases as the magnitude of the direct current bias potential impressed on said tube increases towards its maximum value.

2. In an electrical wave transmission system, the combination with a plurality of tubes and a circuit network connecting said tubes in cascade for transmission of said waves, means initially biasing said tubes to establish the gain thereof in the absence of waves, a rectifier having an input circuit effectively connected across points of said network subjected to a .wave voltage developed intermediate the input and output terminals of said system, means for impressing upon a tube preceding and a tube succeeding said intermediate points of said network direct current bias potentials proportionally to the direct current potential developed by said rectifier, the initial bias on said succeeding tube being less than a value corresponding to maximum gain whereby the gain of said succeeding tube increases and thereafter decreases as the magnitude of the direct current bias potential impressed thereon increases towards its maximum value.

CHRISTOPHER J. FRANKS.

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