US1985914A - Automatic volume control circuits - Google Patents

Description

Jan. 1, 1935. w. L. CARLSON 1,985,914

AUTOMATIC VOLUME CONTROL CIRC UIT'S Filed Aug. 29, 1930 AMPL/F/GAT/O/V l I I 1 1 l I AMPLIFIER GRID BIAS POTENTIAL Inventor: Wendell L/Carilson,

by MW His Attorney.

Patented Jan. l, 1935 UNITED STATES 1,985,914 "J AUTOMATIC VOLUME EGQNT ROL CIRCUITS Wendell Carlsom'HaddonfiellljhUJ; a'ssi gnor to General Electric Company,; corporation of New York Application August 29, 1930, serial No. mist Claims. (c1.,250 20)' The present invention relates to radio receiving; apparatus embodying hot-cathode, grid-controlled electric discharge devices commonly known as vacuum tubes, arranged to amplify and detect electric signals, such as.:modulated radio waves, and has for its object toprovide an improved circuit arrangement in connection withan amplifier and detector in suchapparatus, whereby it may operate automaticallwto deliver substantially. a constant signal .output in response to i a signal input of varying strength. y. 5

, .In accordance with the invention, advantage is taken of the fact that the'response. or amplification characteristic of an electric discharge amplifier may be varied between wide limits by slight changesin the biasing potential: applied to the control electrode or .grid circuit thereof when operating such :circuit in the positive range of biasing potentials, and of the fact as 'is well known,;that the anode current of, a detector arranged for bias or anode circuit detection, in-

creases with increases in the strength of an ap-' plied signal.

Further. in accordance with the invention, a biasing circuit common to the output or anode circuit of the detectorand the input or grid circuit of the amplifierisso arranged that the apparatus is, highly sensitive. to variations in the strength of the signalsapplied to the detector to provide a more uniform output from the amplifier, the amplifier bias being automatically increased in apositive direction with increased signal strength applied to the detector.

The invention willhoweverbe better understood from the following description when considered inwconnectionwith the accompanying drawing, and its scopewillbe pointedout in the appended claims; a

In thedrawing, Fig. 1 is, a schematic wiring diagram of an electric signal amplifier and detector which may forma part of radio receiving apparatus and the like, provided withan automatic volume control circuit embodying; the invention, and Fig. 2 is a curve diagram illustrating anoperating characteristic of the, apparatus ofFignl. m I

. Referring to Fig. 1 of the-drawing,"5 is an electric discharge device of the hot-cathode, three-element type representing any'suitable device of this character for operation as a bias .detector of radio signals or modulatedwaves. The detector is providedwith an input or grid circuit 6, an output-or anode circuit 7, and a cathode return-lead 8 common to both the grid and anode circuits. suitable 'anode and grid bias potentials for the operation of device 5 as a bias detector are provided by a source of direct currentoperating potentials represented by a voltage-supply resistor '9,Lhaving positive and negative supply terminalsrlO-IOas indicated, to which resistor the grid and anode .circuits fi and'l, and the cathode return. lead 8 are connected at suitable tap points as indicated at 11, .12and 13 respec tively. s

As will-readily be understood, the above described connections are such that with a desired positive potential appliedxtotheanode circuit 7 with'respect to the cathode, the grid circuit 6 is, provided with :au'negative potential with respect to the cathode such-that the anode current: is reduced substantially to zero, with no modulated signal or .carrier.wave onathe grid'or input circuit, this beingv as is'wellv known, a condition for properoperatiom of. device .5as1a; bias detector. Upon the application of signalavoltage to the input :ora gridlcircuit through "a suitable input device suchasiazh'igh' frequencytransfornier 15, the. anodetjcurrent will increase in responseto increases in the; strength of the. carrier: wave. Modulated signals received :by the detector, are transmitted'to outputterminalslfifrom the an! odecircuitthrough an output device such as a low or audio frequency transformer 17. Amplified signal voltage is appliedto the detector .5 thru asuitable. vacuum tube or, electric; charge amplifier havingsign'al input terminals 18,

and an'output circuit 19 connected with the detector input or coupling device 15. The amplifier may'includejany desired number'of stages; In they present example, for the sake of simplicity and clearnessbut one stage is shown and includesan electric discharge amplifier device 20 of the type employed as a 'detectorat 5, and a couplingor in-- put device21 interposed between the input terminalsxl8g-and an inputor controlv electrode circuit 22 for:,device20. The anode orv output .circuitof device 20:,isprovided bythe.outputcircuit19' of the amplifier; to which suitable operating potential is applied through connections with the poten-. tial supply resistor 9 at a tap point14 and through a cathode"return"lead 23 for-the amplifier :con-,

hectedzwithithe resistor 9. at] a variable tap point24L p J Bias potential for the amplifier grid or control electrode 1 is supplied thereto through a lead" 25 connected with: animpedance suchas 1 a resistor 26in the;outputicircuit of the detector fi, to derive a variable :bias potential: therefrom responsive to; changes in the signal-output." Thewalue of the potentialxapplieditolthei .control electrodeor. bias circuit of the amplifier through lead 25 is provided by an adjustable tap 2'7 on the impedance, which in the present example is arranged as indicated, to vary the voltage by varying the impedance or resistance. The bias resistor or impedance 26 is preferably located as shown in the cathode return lead 8 for the detector 5 whereby it operates to apply an additional bias to the detector in response to increased signal input.

By returning the'control grid or input circuit of the amplifier 20 to the positive end of the bias resistor 26 a variable positive bias or control" potential, having a certainfixed initial value, is applied to the control electrode circuit bythe detector device 5, in accordance with the signal strength. It will be seen that if the variable tap point 24 is brought to a point opposite the tap point 12, the total applied bias potential to'the control grid of the amplifier device 20 will be that provided by the drop in resistor 26 and will be a positive bias,'instead of a negative biasas is common with amplifier devices heretofore used for signal amplification and the like.

The object of the above described arrangement is to provide a means for reducing the amplification of the amplifier by causing the bias potential applied to the control electrode circuit, thereof to become increasingly more'positive'with increased applied signal strength, thereby correspondingly reducing the amplification, to maintain the average value of amplification and the detector output substantially constant.

With thetap point 24 for the cathode return lead of device 20 located substantially opposite the point 12, the detector through the'medium of the resistor 26 would supply all of the bias voltage for the'amplifier device 20. Should this initial bias, with no signal applied to the detector, be too high or too low, the tap point 24 may be moved to 'a'positionsuch as that shown in the drawing, along the potential supply resistor 9.to provide the desired value of initial bias on amplifier device 20 for normal operation. In the present example, tap point 24 is located at a point more positive than tap point 12 whereby the positive bias potential provided therebyis'applied in opposition to the positive bias potential supplied by the bias resistor 26. v

= The bias potential applied to the detector 5 is such that normally with anelectric discharge device'of the typeshown, the anode current isv reduced substantially to zero or to a small fraction of a milliampere as hereinbefore described. The bias potential applied to the amplifier 20 between its control grid and its cathode is then adjusted by applying a suitable potential to the cathode by moving the tap'point' 24 or the tap 27. In the present example and in the usual case with electric" discharge devices of the type shown, the tap point 24-is moved to a point more positive to balance or reduce the positive bias applied through the bias resistor 26. The bias resistor 26 thus simultaneously applies a negative bias to the detector and a positive bias to the amplifier.

It will be seen that with the above-described ar rangement the volume setting of the amplifier depends upon the bias voltage applied thereto and may be adjusted by adjusting the resistor 26Tor the tap point 24.

It will be seen also that the circuit arrangement issuch that a positive bias potential'is applied .to the amplifier from resistor 26, while an additional negative bias potential is applied to the detector overlthat derived from the supply source 9 between tap points 11 and 12.. This arrangement, in

accordance with the invention, provides for the proper operation of the negative bias detector and for the operation of the amplifier with a positive bias in the steep portion of its grid-bias amplificavalue, the amplification falls off gradually, while as the amplifier and grid-bias voltage is increased from zero to a more positive value, the amplification falls off-very rapidly. It is the purpose of the above-described control arrangement to utilize this latter portion of the characteristic in the most effective positive bias control range between points whereby a rapid and effective con trol of the amplification is obtained.

For the above purpose, when there is no signal applied to the amplifier 20 and detector 5, the amplifier bias potential is so adjusted that the amplification is substantially at a point on the curve indicated at 29, the bias potential on the control grid of amplifier. device 20 being then slightly negative. When a signal or modulated wave is applied to the input terminals 18, and through the amplifier 20, the bias potential provided by impedance 26 in the detector output or anode circuit will increase in a positive direction which will cause the amplifier 20 to' operate on the steep portion of the curve such as at 30 for example. The amplifier then operates on a portion of the curve which is more sensitive to bias voltage change than the negative slope and permits a rapid reduction of signal strength when the latter tends to increase.

While but one amplifier device has been shown and described in the present example, it will readily be appreciated thatthe same type of control may be applied to other amplifier devices connected in cascade with the single stage shown, substantially in the same manner as the connection between the amplifier device 20 and the detector-5. In any circuit arrangement, however, the amplifier bias control circuit is returned through the impedance 26 in the detector anode or output circuit whereby the impedance 26 is arranged to apply a negative bias potential to the detector and a controlling positive bias to the amplifier increasingly with increasing signal strength, the amplifier being operated at such a point on the characteristic curve that an increase in the bias potential causes it to operate upon that portion of its characteristic curve substantially between zero and a more positive bias voltage position, that is, on the steep slope of the positive bias portion of its grid-bias amplification characteristic curve as indicated in Fig. 2.

A further advantageous feature of this volume control circuit is found in the fact that the selective input device in connection withthe am- I for use in radio receiving apparatus and the like to which strong local signals are applied and 0perates to improve the fidelity under such conditions while for other signals it operates to increase the selectivity. Stated in another way,

as the bias potential on the control grid circuit upon the input device and thereby decreases the selectivity and increases the fidelity.

The control arrangement of the present example is particularly adapted for use in connection with the second detector of a superheterodyne receiver, the latter being supplied with a signal voltage through the medium of an intermediate frequency amplifier. For example, amplifier device 20 may be consideredto be the last amplifier device of an intermediate frequency amplifier, and device 5 may be considered to be the second deector in a superheterodyne receiving apparatus.

From the foregoing description, it will be seen that an amplifier and detector circuit embodying the invention provides a rapid and effective control of the output volume from the detector through the medium of a bias impedance controlling the applied grid potential to the preceding amplifier, the grid potential of the amplifier being controlled in a positive range of potentials having an amplification characteristic whichis sharply sloped in a short range of positive potentials, substantially as indicated in Fig; 2, and that this arrangement permits the use of a highly plifier without sacrificing the quality of the output in response to strong or local signals.

What I claim as new and desire to secure by Letters Patent of the United States, is V 1. In combination, an amplifier having an anode, a cathode, and a grid, a detector having a I grid-coupled to said anode of said amplifier, a cathode, and an anode, a common source of operating potential for said anodes, a resistance connected between the cathode of said detector and a point on said source negative with respect to both of said anodes, and connections from said resistance to the grid of said amplifier and from said source to the cathode of said amplifier, the

potential of said resistance between said connections thereby being applied in a positive sense to the grid of said amplifier.

2. In combination, an amplifier having anode,

cathode, and grid electrodes, a detector having a grid electrode coupled to said anode electrode, a cathode and an anode electrode, a common source of operating potentials for said amplifier and detector, connections from all of said electrodes to said common source of operating potentials, said connectionsfrom the grid of said amplifier'and cathode of said detector to said source including a common resistance whereby the potential on said resistance produced by anode current insaid detector is supplied in a positive sense to the gridof said amplifier and in a negative grid of said detector.

WENDELL L. CARLSON.

sense to the

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