US2207905A

US2207905A - Automatic volume control

Info

Publication number: US2207905A
Application number: US207192A
Authority: US
Inventors: Roy A Weagant
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1938-05-11
Filing date: 1938-05-11
Publication date: 1940-07-16
Anticipated expiration: 1957-07-16

Description

July 16, 1940- R. A. WEAGANT AUTOMATIC VOLUME CONTROL Filed' May ll, 1958 Gmbh WSWS. uS MGS@ E ALLA.;

INVENTOR. POV A.' WEAGNT Tw www...

ENSMP@ QN in "I ATTORNEY.

Patented July 16, 1940 UNITED; 'l STATES 2,207,905 n AUroMATio voLoiim coNrrRoLf Roy A. Weagant, Douglast-on, Long Island, N. `Y., 1 assignor toRadio Corporation-of America, a`

corporation of a Delaware Application May 11, 193s, serial No. 207,192 4 claims. Ci.'2'50'20).4

g tector system tube.

Further objects are to provide a control voltage which may reach a large magnitude, and, also, to obtain a rapid variation of control voltage with increasing strength of signals impressed upon the detector system.

Briefly, I obtain the objects of my invention by connecting the gain control gridor grids of the amplifier tubes by way of separate paths to separate bias potential points whose potentials are the limiting values of the range of control potentials desired, and by utilizing the detector output for rendering one of these paths increasingly effective relative `to theother with increasing detector output and vice versa, so that the effective control potential changes continuously from approximatelyl one 4limiting valuefto the other limiting value as the detector 'output changes from one predetermined value to another predetermined value. It will thus be seen that a very rapid increase of control voltage lwith respect to increase of detector input maybe obtained, with a corresponding improvement in control action. f

In the drawing: v

Fig. 1 shows'the pertinent portions of a receiver circuit employing the present invention,

Fig. 2 shows an alternative form of detector that may be substituted for the detector system of Fig. 1,'

.Fig 3 shows an alternative forml of the in'- vention.

Referring to Fig. l, there may be used a source of modulated high frequency currents -such as an antenna; or in the case ofa superheterodyne receiver, the converter tube. Voltage from the signal source is impressed on the input electrodes of amplifier tube A1, and the amplified output of this tube is impressed on' a'second amplifier tube A2 whose output is in turn applied between grid l and cathode 2 of triode tube T through bypass condenser C. A high resistance Rs is connected across condenser C to provide a leakage vpath between grid land cathode 2. It islnot believed necessary to describe in detail the operation of the amplifying stages including tubes A1 andAz, since any of the well known .amplifier arrange-l ments may be usedg-exce'pt'to point out that tubes Ai and A2y are preferably of the supercontrol type; that'isQthey-are so constructed that the amplification provided lby the tubes is determined bytheir grid bias'and may be varied 'over wide limits by varying the grid biasy withoutfintro-l ducing serious distortion. yCoiider'iser C together with resistance R3 determine a time constant suitable for permitting modulation frequency voltages to build up' across R3 andk a desired por' tion of thisfvoltage is led oli by way of leads 3"and 4, inwhichseries blocking condensers maybe connected as shown, to any desired form of utilization device, such as, forexarnple, an audio frequency amplifier feeding into a loudspeaker-.2

The development and the utilization of modulation-frequency voltages is quite conventional in the present state of the-artand need not be described further,v

`In addition to \modulation-frequenc y voltage ltherealso exists a direct voltage component across While cathode 2 is connected through'a resistanceA R1 to the negative terminal, -B, of the sainel source. l"An intermediate point on this source is connected to the'cathodes of amplifier tubes A1,v

and Az, this connection beingindicated in the drawing by a ground symbol. In the drawing the source of direct potential is indicated as a potentiorneterV P whose terminals +B and -B are supplied with direct current from any suitable source, such as the usual alternating current rectier and filter, not shown. Of course, a battery may be employed in place of potentiometer P if desired.

A small bias, such as suitable for permittingy high amplification, is provided for the control grids of tubes A1 and A2 by a conductive connection from their grids through filter resistors Fi and F2 and resistor R to a point on potentiometer P whose potential is a few Volts more negative than ground. `For the purpose of providing a large bias on the grids of tubes A1 and A2 in the presence of large amplitude `signals impressed on detector T, an auxiliary-two electrode valve D has its anode 6 connected tothe junction of resistances R andl F2 While itscathode 1 is connected'directly to cathode 2 of triode T. Thus,l

valve D may be the diode section of the commonly used triode-diode type of tube, while triode T may be the triode section of the same tube, the

cathodes of the two sections of such tubes being connected together within the tube. It is to be understood that the reference characters M, M1, M2 denote the usual coupling transformers each having primary and secondary circuits tuned to the carrier frequency of received signals.

The action of this arrangement is as follows: In the presence of weak signals or no signals at all, considerable space current traverses triode T` and hence resistor R1. The voltage drop across R1 thus makes the potential of cathode 'i more positivethan the potential of theconnection of R to P, so that no current ows through D andA hence the bias on the grids of tubes A1 and A2 is the potential of the junction of R and P. bias remains constant until signals impressed on grid l of tube T are sufficiently strong to cut down the space current of tube T to a value so small that the potential of cathode 1- of valve D drops below the potential of its anode 6 whereupon the resulting current through valve D, and hence also through resistor R, causes a voltage drop through RV that increases the bias on the grids of tubes A1 and A2. lThus after signals impressed on detector T reach a predetermined value, the bias on amplifier tubes A1 and A2 increases rapidly until the signal strength reaches a value which reduces the space current through triode T, and hence through R1to substantially zero, coincidentally with which the bias approaches as a limit the potential ofthe negative end, -B, of the source of direct voltage.

The higher the resistance R in comparisonwith R1 and the resistance of valve D the more nearly will the potential of anode 5 of valve. D approach that of the -B terminal in the absence of space current through detector T. It is thus seen that for signals above a predetermined strength the automatic volume control voltage, in other words thebias on the grids of tubes A1r and A2, is determined chiefly by the potentials of the points of connection to source P, and is relatively little dependent upon signal .strength or the cathode emission of triode T. Ori the other hand; for very weak signals, the bias is also independent of tube characteristics, being determined only by the point of connection of R to P so that the only effect of variations in the charac'- teristics of individual tubes in the detector position is a slight change in the amount of delay that occurs as signal strength increases before the automatic volume control voltage begins to increase.

Fig. 2 sho-ws how a diode detector` 8 may be used in conjunction with a triode 9 in place of triode T of Fig. l. Since the cathodes of triode 9 and diodes 8 and D are all connected together, a single tube of the'double diode-triode type may be used in place of three separate tubes if desired. A double diode-pentode tube could also be used, it being understood that the screen and suppressor electrodes of such a tube would be suitably connected to proper voltage supply sources as well known to those skilled in the art.

Fig. 3 shows a modified form of the invention wherein a conductive connection is provided from the grid-cathode circuit of amplifier A2 to a source of high bias such as required for reducing the gain of amplier tube Az to an extremely small value, While a unilaterally conductive space chargeV device is employed in a corresponding connection to a source of 10W bias suitable for permitting maximum gain. Signals amplified by tube Az are impressed upon, and rectified in, the grid-cathode circuit of triode T, and audio output is taken from resistor R3 all as described in connection with Fig. 1. Cathode 2 of triode T is connected through resistor R11 to a point on direct voltage source P1 whose potential is considerably more negative than that of the cathode of amplifier tube A2. Thus, if sufficiently strong signal's are impressed on thegridlL of tube T to reduce the space current of tube T to nearly zero, the potential o-f cathode 2 will be considerably negative with respect to the cathode of Az. Since the grid return of tube-A2 is made by way of filter resistor F4 to the cathode of tube T, a sufficient bias is thus impressed on tube A2 to reduce its amplification to a small value. As the impressed signals diminish, the increasing space current of tube T causes its cathode 2 to become less and less negative. If tube T is of unusually low resistance, cathode 2 might evenin `the absence of signals become somewhat positive', thus makingthe .bias on tube A positive. Since'posi.-` tive bias is generally undesirable on amplifier tubes, however, diode D1 is provided having a low` resistance in one direction, with its plate 6' connected to `cathode 2 and its cathode T connected. to. a point ll whose potential is somewhat more negative than that ofthe cathode of tube A2. Thus, if the potentialof cathode 2 tends to rise above the potential at point It, current flows. through the diode 'and thusby diverting the major portion of any further increase of space. current of tube T from flowing through .resistor R1 prevents the Zero signal bias oftube A2 from becoming positive, or less than a suitable value for optimum amplifying action.

While I have indicated and described several l systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to. the particular organizations shown and described,butl that many modications may be made without departing from the scope of my invention, as set forth in the appendedclaims. n

What I claim` is:

l. In a radio receiver, a signal transmissiony tube, a signal rectifier tube of the type includ- 1 ing atleast a cathode, signal grid and outputv electrode, a signal input circuit for said rectier tube coupled to said transmission tube, a source of direct currentconnected to the space current path of the rectifier, a resistive impedance connected in circuit with said source-between therectifier cathode and output electrode, a direct current voltage connection from again control electrode of the transmission tube to ay point on said source, and a second direct current voltage con-v nection from said electrode to a second point on said source, said second connection including a device of uni-directional conductivity and said device being connected between potential points of said resistive impedance suchv that the device is non-conductive for signals of a predetermined amplitude.

2. In a radio receiver, a signal transmission tube, a signal rectifier tube of the type including at least a cathode, signal grid and output electrode, a signal input circuit forsaid rectier tube coupled to said transmission tube, a source of direct current connected to the space current path of the rectier, a resistive-impedance connected in circuit with said source between the rectifier cathode and output electrode, a direct current voltage connection. from a gain control electrode of the transmission tube to a point onk said source, a second direct current voltage connection fro-1n said electrode to a second point on said source, said second connection including av diode device of uni-directional conductivity, and said diode device being connected in shunt across said impedance between potential points thereof such that the diode isnon-conductive for signals of a predetermined intensity.

3. In a radio receiver, a signal transmissionr tube, a rectier tube having a signal input circuit coupled to'said transmission tube, a source of direct current connected to the space current path of the rectifier, a direct `current voltage connection from a gain control electrode of the transmission tube toa point on said source, and a second direct current voltage connection from said electrode to a second point on said source, said second connection including a device of `unidirectional conductivity, said device consisting of a diode having its anode connected to said first point,v and said second point being negative with respect to the rst point.

4. In a radio receiver, a signal transmission tube, a rectifier tube having a signal input cirv cuit coupled to said transmission tube, a source `oi direct current connected to thespaee current path of the rectiiier, a direct currentvoltage connection from a gain control electrode of the transmission tube to a point on said source, a second direct current voltage connection from said electrode to a second point on said source,

said second connection including a device of uni-r directional conductivity, said device being a diode which has its anode connected to the rectifier cathode, and the vsaid second point being positive relative to the rst point.

ROY A. WEAGANT.