US1904552A - Automatic gain control - Google Patents

Description

April 18, 1933. T I A SMH-H l AUTMATIC GAIN CONTROL Filed oct. 29, 1929 sheets-sheet l INVENTOR THEODDRE A. SMlTH BY 7 illu .Nhk

ATTORNEY AUTOIVIAT IC GA IN CONTROL Filed oct. 29, 1929 3 sheets-sheet 2 Jig. 5

rau mmm a I l lI I l l I I I I I I I I I l l I I I I l I I I l I Il I IAI 1 INVENTOR THEODoRE'A-sM1TH April 18, 1933. T. A- SMH-H 1,904,552

AUTOMATFIC GAIN CONTROL Filed Oct. 29, 1929 3 Sheets-Sheet '5 mvENToR THEODORE A SMITH' ATTOR N EY lzo Patented Apr. 18, 1933 PATEN T OFFICE IHEODORE A. SMITH, OF RIDGEWOOI), NEW JERSEY, ASSIGNOR T0 RADIO CORPORATION 0F AMERICA, A CORPORATION OF DELAWARE I AUTOMATIC GAIN CONTROL Application filed October 29,'

My present invention relates to an' automatic gain or` amplification control, and more particularly to an lmprovement 1n a .gain control circuit of the type disclosed in my co-pending application Serial No. 310,- 497 filed October 5th, 1928.

In the aforesaid co-pending application there has been disclosed an audio frequency amplifier whose gain varies inversely with the input level so that a sudden increase in input is limited to a predetermined consta-nt output. Briefly, the manner in which this is accomplished is by varying the anode load of an amplifier tube by automatically l adjusting the anode impedance of a control tube in a predetermined manner, said anode impedance being in parallel with the amplifier anode circuit, the uni-directional component of a rectified ortion of the alternat#f ing'input current to e amplified being empl'oyed to secure the adjustment. Thus, the change in the anode load of the amplifier alters its amplification.

Now, by further experimentation various improvements have been made upon the invention in the said co-pending application, it being observed that the present arrangement is primarily designed for, and has for its main object, a use where very sudden increases in input voltage, (due for example, to a very sudden increase in volume during a musical program), may cause overloading to occur, and additionally, to augment rather than to supplant manual operation of a gain control, since 'for sudden increase in input voltage the arrangement is faster than manual operation.`

Another important object of the invention is to provide an audio frequency amplifier including controlling equipment broadly comprising an amplifier, a rectifier, a control tube and a time control and filter arrangement for controlling the speed at which the impedance of the control tube varies, the control equipment including a plurality ofaudio amplifier tubes, the plate circuits of the control tube and the first of said plurality of amplifier tubes being inductance coupled to maintain the plate voltages constant regardless of the impedance of 1929. Serial No. 403,281.

the control tube whereby the dynamic characteristic of the said plurality of amplifier tubes remain more constant during operation of the control tube.

Another object of the invention is to provide lan amplifier circuit having a predetermined'ratio between the input energy applied to the amplifier and the output energy derived therefrom which comprises a control equipment including a plurality of amplifier tubes, the input to the last tube being obtained from a drop across a part of a load resistance connected in the output circuit of a control tube, the'latter being supplied with a rectified portion of the altern ating current to be amplified, there being a condenser connected in series with said load resistance and forming the alternating current output circuit of the control tube, the system thereby being less 'likely to cause distortion, since high resistance is always in the circuit regardless of the plate impedance, of the control tube.

Still another object of the invention is to provide an audio amplifier whose output is maintained substantially constant while the input is varied, means for accomplishing the latter result, said means being designed to respond quickly or slowly, as desired.

Other objects of the invention are to improve generally the simplicity and efiiciency of automatic gain control devices for amplifiers, and to especially provide an automatic gain control for an audio amplifier, which is durable and reliable in operation, and economical to manufacture.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention ,itself, however, as both its organization and method of operation will best he understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically one circuit organization whereby my invention may be carried into effect.

In the accompanying drawings,

Fig. 1 is a lschematic diagram showing the equipment essential'to an amplifier embody ing the present invention,

in which like characters of reference ind1 'cate the same4 parts in the diii'erent figures,

.A second im edance there is fed into the apparatus, as shown in Figs. 1 and 2, at input a varying alternating input. A portion of this input is s, fed directly into the first amplifier tube 2 of the control equipment. Another portion of the alternating linput is fed into an amplifier tube 3 of the controlling equipment, and thence into a rectifier 4. It is to be understood that, if desired, the amplifier tube 3 may be omitted.

The uni-directional component of the rectiier current is then fed into the time control and filter device 5.' The latter controls the speed at which the bias on the control electrode of control tube 6 Eis varied by the changes in the uni-directional component of the rectified current. v

Control tube 6 has its anode-cathode current supplied throu h the anode impedance 7 The first ampli er`tube2 of the control equipment has connected between its anode and cathode a resistance tapped at a point 16, the latter dividing the resistance into two portions R8, Rg. The impedance 7, in this case a choke coil, is connected in series with the portion R2 R1, and in series with a condenser 10, the resistance R1 having one of Iits/terminals connected to point 16.

The-anode of tube 2 is connected in series with the portion R3 through acondenser 14. 15, in this case also a choke coil, 1s connected in the anode circuit of the tube 2, anode current being supplied therethrough. The input to the last amplifier tube 2 is obtainedfrom the drop across part of the grid leak resistance 13.

which together with a series condenser 12, the latter having one of its terminals connected' to point 16, forms the alternating current output circuit of the amplifier. It will also be observed that the portion of the input which is fed directly -to the control amplifier tube 2 rst passes through the re-Vv sista-nce Rz, R3. i i r` In Fig. 2 I have shownV in'gneater detail the connectionsand apparatus employed in the construction schematically shown in Fig. 1. The amount of the input energy applied to the controlling equipment amplifier 3 is varied by meansronf armanually controlled resistance 8, the output of the amplifier tube 3 being fed, by means of an audio transformer 9, into rectiiier tube 4, the latter having a cathode of the heater type.

byA means of a resistance' The control electrode and anode ofthe rectifier 'tube` 4 are connectedtogether forming a two elementArectiier,` used so as to produce a more nearly perfect rectilinear characteristic. The other portion of the input alternating energy is fed through a condenser 20 to the irst amplier tube 2 of the control equipment. Y

The time control, and filter device 5 shown diagrammatically in Fig. 1 preferably comprises, 31, choke coil 32, variable resistance R and condenser 33. With a suiiciently large input the negative grid bias of control tube- 6 is reduced so that controlling action results. That is, under those circumstances the internal impedance of the tube 6 is reduced, and, as its anode current passes through the anode impedance or choke coil 7 A of the rst control ampliier tube, a change occurs in the effective value of the anode impedance or load of tube 2`which latter consists of the impedance 15, and resistances R5, R3, in parallel with the control tube 6. The latte effect lessens the amplifying eii'ect of tube onpthe input supply to it through condenser The speed of control is varied by Varying resistance R. The reason why variation of resistance R will control the speed of opera- 'lol tion of the apparatus maybe explained as.

follows: When a high input which has been applied is sudderly removed, and assuming that the condensers 31 and .33 remainl charged, the charge on condenser 31 can only discharge through high resistance 30, if the impedance of therectifier tube 4 is high. Condenser '33 can only discharge through the series combination of variable resistance R, choke 32, and resistance 30, since the internal impedance of the control tube is also high. p When the rectier impedancefis low, as

for example, when a large `Voltage is sup,-A plied by transiormer 9 to rectifier 4 thenthe charge on 'condenser 31 can leak ofi' across However, ncondenser 33 willstillhave to discharge through resistance R which .may be varied to control its speed of discharge- Hence, the speed at which the bias on th control electrode of the tube 6 may be-varied, is secured by 'the manipulation of resist-lV ance R. Y'lVhen a stron signal is applied. to the controlling amplifier tube 3, condenser-s 31 and 33 will become highlyv charged and Ahecause of the reducedvr control electrode -or negative grid bias'on wijl have its impedance reduced. this strong signal is suddenly cut oif the im pedance of rectifier 4,Wil1 increase rapidly.

Condensers 31 and- 33 ywill retain their charge until resistors 30 and R discharge them.

As a result, um com 1 mbe gris bias win tube 6, vcontrol tube .6 Y

mains substantially constant.

remain nominally reduced and the amplification of control amplifier tube 2 low. A resistance R4, the latter being of a higher order of magnitude than resistance 30, is disposed across the anode-cathode circuit of rectifier tube 4 and decreases the time of discharge in this case, and makes the controlling action more uniform. Of course, the speed of action can still be manually controlled by adjusting the resistance R which determines the discharge rate of condenser A milliameter A is disposed in the output circuit of control tube 6 t`o indicate its operation. The amount of control is indicated by the milliameter which is mounted on the front panelof the receiver. When the latter is reading zero, the control tube is not acting; when it reads 2.5 milliamperes, then the control tube is acting to its full capacity. The meter, in other words, reads the control tube plate current.

As the control device does not operate from the output voltageA (as in the case of certain radio frequency automatic gain controls) it is necessary to carefully adjust the various resistances, so that the output re- The resistance R1 prevents over control which will occur if the control tube plate impedance becomes too low compared to R2. If R1 is too high, insufficient control will be obtained. The resistance R3 is placed in the amplifier plate circuit to prevent distortion which would otherwise occur when the control tube plate impedance becomes too low.

As indicated heretofore, the hi h resistance R4 is employed to allow the4 lter condenser charge to leak by the' rectifier tube when the voltage employed is low and the rectifier impedance is high. When a high voltage input is applied, the rectifier impedance is low and this resistance is not needed. The effect of this phenomenon is to allow the control to act rapidly in cutting ofi the gain when high inputs are applied but to tend to hangover after the high input has decreased.

The resistance R,t has been selected to partially prevent this. It has been noted that when a high input is applied, and then removed, the milliameter reading increases rapidly, decreases rapidly lat first, and becomes slower and slower as the reading decreases. The time action control'allows individual variations, or sudden checks to be controlled by making the action fast; or to control merely the general level by making the action slow.

It may be found necessary to place a grounded shield between the rectifier tube 4 and the control tube 6 to prevent distorted signals from getting into the output amplifier. The amplifier is 'designed to operate -between a 200 ohm input and 500 ohm load,

which may be readily adaptedl to other input and output impedances. v.

The system is, further, provided with a switch, generally denoted by S, that facilitates connecting the input directly to the output without being controlled. This may be desirable if for any'reason the system should cease to function properly. The systempmay then be disconnected and the manual control of the input utilized until the trouble is rectified.

The main plate battery employs a voltage of 135 volts, the voltages indicated in Fig. 2 being those which have been found to be satisfactory in practice. Resistances 20 are used to lobtain correct voltages for the tube filaments. The .positive A connection is preferably grounded and connected to the positive C and negative B.

The control acts by shunting the plate iinpedance of a tube across a portion of a coupling impedance in an impedance capacity coupled amplifier. The plate impedance of the tube is changed in accordancev with the input level by amplifying some of the input, rectifying it, and applying the direct current component as grid bias for the. control tube. It will be noted that only a single stage of amplification is employed before rectification instead of the two stages of amplification as disclosed in my abovementioned co-pending application. Onestage has been found sufiicient to deliver the necessary output to operate the control tube satisfactorily. p

The actual amplifier from which the output is taken is the equivalent of a two stage amplifi-erand has a gain of about 30 decibells, when the control is not acting. Its gain may be varied manually, of course, by the control 13. The control .will begin to ac t when an input of about 15 millivolts is applied, when the control gain is set at maximum. If the'corftrol gain is reduced, higher inputs are required before the control acts. The control action has little effect on the frequency amplification, as is well shown in Fig. 3 which graphically illustrates 'the `characteristics for full and no control.

frequency characteristic in Fig. 3 indicates that there is practically nov change produced The in relative amplification of high and low ages for various inputs and different degrees of control. The input-output curve in Fig.v

v 4, shows that without control, the output increases proportionately with Ainput as far as the curve has been plotted. With control major differences between the present ar" rangement and that described in my copending application involves the factthat the plate circuits of the control and amplifier tubes of the present arrangement are inductance coupled rather than resistance coupled.

This tends to maintain the plate voltages constant regardless of the impedance of the control tube. This'results in the maintenance of the dynamic characteristic of the tubes more constant during operation of the control tube 6.

Again, another important difference and improvement involves the fact that the input to the last amplifier tube 2' of the control equipment is obtained from the drop across part of the load resistance 13 which together with series condenser 12, forms the alternating current output circuit of the amplifier. By the use of this improved method the system is less likely tol cause distortion since a resistance of at leastv 48,000/ohms is always in the circuit regardless of the plate impedance of the control tube 6.

To analyze these differences in more detail it will be noted that in an ordinary amplifier circuit, without the volume control rinciple, for the tube 2', would consist o the tube, its plate load impedance 15', a grid leak 13', and a stopping condenser12'. In this arrangement, the total voltage drop across the impedance 15' would be applied to the idy of the succeeding tube, 2. (See Fig. 5. If now a resistance is shorted lacross the plate load 15', the amplification of tube 2' will vary according to the value of the load. If this resistance is made to vary inversely as the amplitude of the applied signal voltage, a more or less uniform output will be obtained from a varying input. y

If such a load is introduced, inthe form of the plate impedance of a vacuum tube, it will be necessary to isolate its plate circuit from the grid circuit of the tube 2. (See Fig. 6.) This is done by the (grid blocking condenser 12'. It will be note that the anodes of tubes 2' and 6 are in parallel'and that if, a larger positive bias is applied to the grid of tube v6. (caused by a strong signal being applied and hence a large rectified component being present), thevo'ltage drop across 15' will increase, which action will reduce the anode voltage on tube 2. This is essentiallythe circuit described in my aforementioned co-pending patent application. Such variation in plate voltage will cause distortion of the output wave form.-

If however, the plate voltage supply' of vcontrolled tube 2', and control .tube 6, can

producing determined be made independent, this distortion will no longer be present. Furthermore, individual tube adjustments can be made without affecting other tubes. The present improved circuit is analytically shown in Fig. v7. Here, 12 and 10' are plate blocking condensers, and the tubes 2 4and 6 are supplied through individual anode loads, .15' and 7' respectively. These loads may be either resistances or inductances, preferably the latter. Load 15' may be considered as the coupling impedance between tubes 2'y and 2.

It has been found in practice that the plate impedance of tube 6 becomes lower more rapidly than it should for maintaining constant output. To avoid this, the plate impedance of tube 6 is shunted across only a portion of the plate load of tube 2'. This is accomplished by means of the potentiometer R2, R3. (See Fig. 8.) Resistance R1 is used to prevent the plate circuit impedance of tube 6 from decreasing to too low a value.

It is to be understood that while the invention is especially adapted for use with audio frequency currents, it may, if desired be utilized in a radio frequency amplifying system. Furthermore, while I have indicated' andY described one arrangement for carrying my inventlon into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, loutu that many modifications may be em- ,.ployed, without departing from the scope of clalms.

What I claim is:

my. inventionl as set forth in the appended 1. In an amplifying system, an electron discharge tube comprisin an anode. a cathode and a control electrode, means to apply alternating current energy variations to sai tube, means for rectifying a portion of the alternating current energy to be amplified v by said tube, means including a control tube to vary the e'ective anode load of said first tube according to the amplitude of the rectifed current, and a common path, including a pair of inductances, for applying a positive potential Vto the anodes of the first tube and the controLtube.

2. In an amplifying system, an'ampliier tube, means to apply alternating current to the control electrode of said tube', means for a direct current whose value is by the amplitude of.- said alternating current, means including 4an electron discharge tube to apply said direct current to said amplifier tube to control'the anode potential thereof, and means for applying a desired positive potential to the anodes of the amplifier tube and said electron discharge tube, said means including a choke coil connected to each of said last mentioned anodes.

3. An alternating current amplifier comprisin a tube having an anode, cathode, and contro electrode, an impedance in the anode circuit of said tube, another tube shunting said impedance, means for applying a portion of the alternatiu current to said first tube, means for rectiying another ortion of said alternating current, means for applying tothe control electrode of the tube shunting said impedance, the direct current component of said rectified alternating current and a capacitative path connecting the anode of said first tube with the high potential side of said impedance.

4. An audio frequency amplifier comprising a tube, a choke coil connected te the anode of said tube, a control tubel shunting said choke coil, means for applying a portion of an alternating current to be ampli-- fied to the control electrode of said first tube,

means for rectifying another portion of said alternating current, means for applying the uni-directional component of the rectified current to the control electrode of said control tube whereby the anode impedance of said first tube is varied in accordance with the strength of the rectified current.

5. An audio frequency amplifier comprising a tube, a choke coil connected to the anode of said tube, a control tube shunting said choke coil,'means for applying a portion of an alternating current to be am lified to the control electrode of said `rst tube, means for rectifying another portion of said alternating current, means for applying the uni-directional component of the rectifiedcurrent to the control electrode of said control tube whereby the anode impedance of said first tube is varied in accordance with the strength of the rectified current, and a capacitative path connecting the high potential side lof said choke coil with the anode of said rst tube.

6. In an amplifier system, an amplifier tube, means to apply an alternatin current to be amplified to the input of said tube, means orrectifying a portion of the alternating current to be amplified by said tube, a'

control tube to vary the effective anode lo'ad of said amplifier tube according to the amplitude of the rectified current, means to control the-s eed at Awhich said anode load is varied, andp a capacitative path connecting the anodes of the control tube and amplifier tube.

7. In an amplifier system, an amplifier tube, means to. apply an alternating current to be amplified to the input of said tube, means for rectifying a portion of the alternating current to be amplified by said tube, a control tube to vary the effective anode load of said amplifier tube according tothe amplitude of the rectified current, means to control the speed at which said anode load is varied, a capacitative path connecting the anodes of the control Vtube and amplifier `ternating current, a control tube to ap tube and la resistor connected between a. point on the capacitative path and the cathodes of the control tube and amplifier tube.

8. In an amplifier system, an amplifier tube, means to apply an alternating current to be amplified to the input off' said tube, means for rectifying a portion of the alternating current to be amplified by saidtube, a control tube to vary the effective anode load of said amplifier tube according to the amplitude of the rectified current, means to control the speed at which said anode load is varied, a capacitative path connecting the anodes of the control tube and amplifier tube, a second amplifier tube, and an adjustable path, including a condenser, connected between the id of the second amplifier1 tube and a point on said capacitative pat 1.

9. An audio frequency amplifier comprising a multi-electrode amplifier tube, a choke coil in the anode I'circuit of said amplifier' tube, a multi-electrode control tube, a choke coil connected to the anode of said control tube, means for connecting both said choke coils to a source of positive potential, means for applying a portion of an alternating current to be amplified to the control electrode of said amplifier tube, means' for rectifying another portion of said alternating current, means for applying the uni-direc-4 tional component of the rectified current to the control electrode of said control tube whereby the anode impedance of said amplifier tube is. varied, in accordance with the strength of the rectified current, means for controlling the speed at which said anode impedance varies, said means including a variable resistance and a condenser, a capacitative path connecting the high potential sides of said choke coils, a resistor connected between a oint on the capacitative path and the catho es of the control tube and said amplifier tube, a second amplier tube, and an adjustable path, including a condenser, connected between the grid of the second amplifier tube and point on said` capacitative path.

10. In an amplifying system, an electron discharge tube including a plurality of electrodes, means to apply alternating current to the control electrode of' said tube, means for producing va ldirect current whose value is determined by the amplitude of said all saiddirect current to said `first tube to ciin-7 trol the anode potential thereof, and a network including lumped inductive and condensive impedances connecting the anode and cathode of said first tube with the anode and cathode of said control tube.

11. In an amplifying system, an electron discharge tube including a plurality of electrodes, means to apply alternating current to the control electrode of said tube, .means 'ode of said first tube with `for impressing on t directcurrent to said first tube to control the anode potential thereof, a network in- 4cluding lumped inductive and condensive anode and caththe anode and cathode of said control tube, and means to control the rate at which said anode potential is varied. 1,

` 12. In an amplifying system, an electron discharge tube including a plurality of electrqdes, means to apply alternating current to the control electrode of said tube, means for producing a direct current whose value impedances connecting the -is determined by the amplitude ofsaid alternating current, a control tube to apply sald direct current to saidl rst tube to control the anode potential thereof, a network including lumped inductive and condensive impedances connecting the anode and cathode of said first tube with the anode and cathode of said control tube, a second ampliier tube andan adjustable impedance path, including a condenser connecting the grid of said second ampliier tube to a point on said network.

13. In combination, in an automatic gain control circuit for an ampliier, an amplifier tube having its input electrodes connected to a source of energy to be amplified, a gain control tube, a common path for connecting the anodes of said amplifier and control tubes to a source of positive potential, means for impressing on the input of-said control tube control energy derived from said source whereby the internal impedance of said control tube is adjusted in a predetermined P manner with respect to variationsl of said source energy, and a capacltative path, independent of said common path, connecting .the anodes of said control tube and ampliiier tube. e

y 14. In combination, in an automatic gain control circuit for an amplifier, an amplifier tube' having its input electrodes connected to a source of energy to be amplified, a gain control tube, a common path for connecting the anodes of said amp tubes to a source of positlve potential, means e input of said control tube control energy derived from said source whereby the internal impedance of said control tu e is adjusted in a predetermined manner with respect to variations of said source energy, a capacitative path, independent of said common path, connecting the anodes of said contro tube and amplifier tube, .and a resistor connected between a inton said capacitative path and the I cathodes of said amplifier and control tubes.

15. In combination, 1n an automatic gain control circuit for an amplifier, an amplifier tube having its input electrodes con- `and amplier tube,

lier and controlA nected to a source of energy to be amplified, a gain control tube, a common path for connecting the anodes of said amplifier and control tubes to a source of positive ypotential, means `for impressing on'the input of said-control tube control energy derived from said source whereby the vinternal imp'edance of'said control tube is adjusted in a predetermined manner with respect to, variations of said source energy,'a capacitati've path, independent of said common path, connecting the anodes of said control tube said capacitative path including a resistor and a second resistor connected between an intermediate point of said last mentioned resistor and the cathodes of said amplifier andcontrol tubes.

16. In combination, in anautomatic gain `control circuit for an amplifier, an amplifier tube having its input electrodes coGnnected.

to a source of energy to be. amplified, a gain control tube, a common path for connecting the anodes of said amplifier and control tubes to a source of positive potential, means for impressing on the input of said control tube control energy derived from said source whereby the internal impedance of said control tube is adjusted in a predetermined manner with respect to variations of said source energy, a capacitative path, independent of said common path, connecting the anodes of said control tube and amplilier tube, a second amplifier tube, a resistor connecting the cathodes of said first ampliier tube and control tube to a. point on said capacitative path, and an adjustable impedance path connecting the grid of said second iamplifier tube and said last mentioned om 17. In an amplifying system, an electron discharge tube comprising an anode, a cathode and a control electrode, means to apply alternating current energy variations to said tube, means for rectifying a portion of the alternatin current energy to be ampliied by said tu means including a control tube to vary the efective anode -load of said first tube according to the amplitude of lthe rectified current, a common path, for apto said tube, means for rectifying a portion of the alternating current energy to be ampliiied by said tube, trol tube to vary the eiective anode load of said first tube according to the amplitude of the rectified current, a common path, including a pair of inductances, for applymeans including a con- Y ing a positive potential to the anodes of the first tube and the control tube and a switch mechanism for disconnecting o r connecting at Will the said rectifying means and control tube and the said lirst tube.

19. In an amplifying system, an electron discharge tube comprising an anode, a cathode and a control electrode, means -to apply alternating current ener variations to said tube, means for rectifylnga portion of the alternating current-energy to be amplified by said tube, means including a control tube to vary the effective anode load of said first tube according to the amplitud' of the rectified current, a common path for,

applying a positive potential to the anodes of the rst tube and the control tube, a load circuit coupled to the anode of said rst tube, manually operable means for varying the transfer of energy between the anode circuit of said first tube and said load circuit, and a switch mechanism for disconnecting or connecting at Will the Said rectifying means and control tube and the saidl first tube.

20. In an amplifying' system, an electron discharge tube comprising an anode, a cathode and-a control electrode, means to apply alternating current energy variations to said tube, means for rectiying a portion of the alternating current energy to be ampliied by said tube, means including a-control tube to vary the effective anode load of said first tube according to the amplitude of the rectified current, a common path, including a pair of inductances, for applying a positive potential to the anodes of the first tube and the control tube, and a visual vcurrent indicator connected in the anode circuit of said control tube. l y

THEODORE A. SMITH.

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