US2237409A

US2237409A - Automatic volume control circuit

Info

Publication number: US2237409A
Application number: US268465A
Authority: US
Inventors: Don G Burnside
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1939-04-18
Filing date: 1939-04-18
Publication date: 1941-04-08
Anticipated expiration: 1958-04-08

Description

D. G. BURNSIDE 2 Sheet-Sheet 1 AUTOMATIC VOLUME CONTROL CIRCUIT 2 70 FOLLOW/N6 7'0 m/ok TUBES- a j e To GR/DS OF PRIOR TUBE-5' Filed April 18, 1959 April 8, 1941.

70 PRIOR TUBES 70 69/05 0F PR/OP. TUBE IN VENTOR. 0 6. BURNS/DE p 1941- i D. G. BuRNsmE 2,237.409

AUTOMATIC VOLUME CONTROL CIRCUIT Filed April 18, 1939 2 Sheets-Sheet 2 70 FOLLOWING EMISSION au/mEA/r 1'; (MICRO-AMPS) 40 45 .50 F/LAMENT CURRENT- I f (MIL Ll-AMPS) INVEN TOR. DON (I. BURNS/DE ATTORNEY.

Patented Apr. 8, 1941 AUTOMATIC VOLUME CONTROL CIRCUIT Don G. Burnside, East Orange, N. J., asslgnor to Radio Corporation of America, a corporation of Delaware Application April 18, 1939, Serial No. 268,465

8 Claims.

My present invention relates generally to automatic amplification control, and more particularly to automatic volume control (AVC) circuits adapted to produce control voltage without rectification ofsignal waves.

In the past there have been devised many types of circuits capable of controlling signal amplification automatically. Most of these circuits have been designed to provide the gain control voltage by rectification of the signal voltage. While there have been proposed circuits for gain control wherein rectification of carrier voltage is not depended upon, yet such methods have not been as simple as is desirable.

It may be stated that it is one of the main objects of my present invention to provide an automatic amplification control circuit wherein the control voltage is derived from variations in emission current of an electron discharge device; variations in emission being dependent upon the amplitude of a radio frequency voltage which is being amplified.

Another important object of this invention is to provide an automatic gain control circuit in a signal tranmission system, the control voltage being developed across an impedance located in the emission current path of an electron discharge device, and the emission current being varied by the application of signal voltage to the emission element of the electron discharge device.

Still another object of the invention may be stated to include the utilization of a radio frequency, or carrier, voltage to heat the filament of a diode, or even a triode or other multi-element tube, and a steady voltage being employed to cause the diode current to fiow through a load resistor thereby to provide the required gain control voltage for the signal grids of the amplifier tubes under control.

Yet other objects of this invention are to improve generally automatic volume control circuits of the type employing a diode as the gain control device, and more especially to provide a control circuit wherein the signal voltage is employed to vary the diode filament heating, the control circuit being efficient and reliable in operation, and economically manufactured and assembled in radio receiving systems.

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 to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have er supply transformer of a radio receiver.

indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:

Fi 1 illustrates one gain control circuit embodying the invention,

Fig. 2 shows a modification of the invention,

Fig. 3 shows still another modification,

Fig. 4 graphically shows the relation between filament current and emission in the gain control device.

, Referring to the accompanying drawings, wherein like reference characters designate similar circuit elements, Fig. 1 shows a signal amplifier tube I whose cathode is at ground potential,

while the signal grid 2 thereof is connected to the high potential side of the resonant signal input circuit 3. The latter can be tunable, as in the case of the tunable radio frequency amplifiers prior to the first detector of a superheterodyne receiver. However, circuit 3 can be the fixedly tuned input network of the intermediate frequency amplifier of such a receiver. Again, the circuit 3 may be coupled to one or more prior tuned amplifiers of the same type, and the receiver can be of the broadcast sound reproduction type, or it may be a television receiver. Assuming that the receiver is of the superheterodyne type, and that amplifier I is in an intermediate frequency amplifier, the plate circuit 4 will be fixedly tuned to the operating intermediate frequency. The amplified energy may be utilized in further amplifiers, demodulated, the modulation voltage further amplified and finally reproduced.

In order to maintain a substantially constant carrier amplitude at the demodulator input circuit, it is, of course, necessary to vary the gain of the pro-demodulator stages in such a manner that substantially constancy at the demodulator input circuit is attained. The diode device 5 is provided to act as the gain control element, The signal energy, at radio or intermediate frequency, is impressed upon the diode by connecting the plate of tube I to the diode filament B by a condenser I. order of mmf. (micromicrofarads), or less. The radio frequency path is then through the diode filament 6 and capacitor 8 to ground. The

transformer T, whose primary winding is connected to an alternating current source, has its secondary in series in the diode filament circuit. Hence, T acts as the filament heating means, and may comprise, if desired, a tap on the usual pow- A sep- The latter has a capacitance of the arate winding on the supply transformer may also be used. Of course, a battery may be employed to heat the filament, where this expedient is desirable.

The radio frequency choke coil 9 is inserted in series with condenser 1 and the transformer primary to prevent radio frequency current from passing to ground through the winding. If desired, the condenser 1 can be additionally employed to resonate the coil of output circuit 4 to the operating frequency, and in such case the usual shunt condenser can be omitted. The diode plate It! is connected to filament 6 through a path including load resistor R in series with current source II. The positive terminal of source II is grounded. The positive terminal of the latter is connected to the resistor so that a steady direct current voltage of positive polarity is applied to the diode plate. The signal grid 2 of amplifier tube I is connected by lead I2, which includes an alternating voltage filter l3, to the plate end of load resistor R. The lead [2 applies the direct current voltage developed across resistor R to the various signal grids of the controlled amplifier tubes. The lead is the automatic volume control (designated on the drawing as AVC) connection.

In operation, the filament 6 is left at such a temperature by the voltage from source T that the steady voltage from source ll causes a current to flow of about the magnitude indicated by point a on the Filament current-emission cur rent characteristic shown in Fig. 4. This normal, or initial, emission current flowing through load resistor R provides a normal, or maximum amplification, bias for the signal grids connected to lead l2. This eliminates the necessity of providing an auxiliary source of grid bias.

Upon the impression of radio frequency voltage upon diode 5, a current flows through the filament 6 which adds, in a more or less complex manner, to the already present current from T. This augmented current flow causes an increase in the temperature of filament 6; an increase in the electron emission from the filament results. The curve in Fig. 4 shows the manner in which emission current increases as the filament current rises. As the emission current flow through resistor R increases, the control voltage applied through lead l2 increases and in a negative, or gain-reducing, sense. By way of illustration, and in no way restrictive, it is pointed out that a diode can be employed at 5 which is designed for a filament voltage of about 0.5 and a filament current of about 40 ma. (milliamperes). An increase of the current through the filament of about 2 or 3 ma. causes a large increase in the emission. Fig. 4 is a curve of the emission characteristic of a diode of this type.

In Fig. 2 there is shown a method of avoiding the use of the choke coil 9 of Fig. l. The circuit 4 is coupled to the diode 5, in this modification, by a coil split into substantially equal sections 20, 2|. The filament leads are connected to the outer ends of the coil sections, while the inner ends of the latter are connected to the ends of the secondary of transformer T. The carrier by-pass condenser 22 is connected in series relation to the coil sections 20, 2 I, but is in shunt relation to the secondary of transformer T. The negative terminal of source II is connected to the midpoint of the transformer secondary. Otherwise the circuit is similar to the arrangement of Fig. 1. The amplified signal energy canbe derived by the auxiliary coil 30. Condenser 22, in Fig. 2, is considered to be at practically zero potential with respect to ground. No radio frequency voltage will appear in the leads from transformer T. The voltages induced in coil sections 20 and 2| are assumed to be equal in magnitude.

Another modification of the invention is illustrated in Fig. 3, wherein the diode filament heating source T is utilized for the additional function of providing the diode anode voltage. Thus, a filament lead is connected to an intermediate point of the secondary of T, and the diode anode I0 is in series with the load resistor R as before. The coil end of the load resistor R is at ground potential, while the load is by-passed by a condenser 40. Condenser at has a fairly large capacity to smooth out the I20 cycle ripple from transformer T. In this circuit arrangement diode 5 rectifies only the low frequency supply voltage, and not carrier voltage. A filter F is desirable to prevent any low frequency voltage from reaching the signal grids of the controlled tubes and causing hum modulation of the carrier voltage. The operation of this arrangement is similar to that described in connection with Fig. 1.

While I have indicated and described several systems for carrying my invention into efiect, 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, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a signal receiver of the type including at least one amplifier of signals and a diode device, means applying signals to be amplified upon said amplifier, means establishing emission current flow from the diode filament, means deriving a uni-directional voltage from the normal filament current, means controlling the gain of the amplifier with said uni-directional voltage, and means varying the emission from the diode filament with amplified signal voltage.

2. In a signal transmission system, an automatic transmission control network comprising an electron discharge device which includes at least an electron emission element and a cold electrode, means for energizing said emission element to provide an electron stream to said cold electrode, an impedance in circuit with the cold electrode and emission element for developing a uni-directional voltage from the electron flow through the impedance, means for utilizing the uni-directional voltage to control the transmission through said transmission. system, and means for impressing signal voltage upon said emission element for varying the energization of the latter thereby to vary the magnitude of said uni-directional voltage.

3. In a radio receiving system of the type including at least one signal amplifier, a diode provided with a filament and an anode, means for heating said filament to provide emission current to said anode, an impedance connected between the filament and anode for developing a direct current voltage from said emission current flow,

means for applying the uni-directional voltage to said amplifier to control the gain thereof. means for applying signal voltage to the diode filament to vary the heating of said filament over a sufficiently wide range substantially to vary the magnitude of said direct current voltage.

4. In a radio receiving system of the type including at least one signal transmission tube, a

gain control circuit comprising an electron discharge device provided with at least a cathode and a cold electrode, means for heating the oathode to provide emission current to said cold electrode, an impedance connected between the cathode and cold electrode for developing a direct current voltage from the emission current, means for applying the direct current voltage to a gain control electrode of said transmission tube, means for applying signal voltage to said control device cathode to vary the heating of said cathode sufficiently to control the magnitude of said direct current voltage.

5. In a radio receiving system of the type including at least one signal transmission tube, a gain control circuit comprising an electron discharge device provided with at least a cathode and a cold electrode, means for heating the oathode to provide emission current to said cold electrode, an impedance connected between the oathode and cold electrode for developing a direct current voltage from the emission current, means for applying the direct current voltage to a gain control electrode of said transmission tube, means for applying signal voltage to said control device cathode to vary the heating of said cathode sufficiently to control the magnitude of said direct current voltage, said heating means additionally being connected to said cold electrode to provide a positive potential therefor.

6. In an alternating current transmission system, an electron discharge device provided with at least an electron emission element and an additional electrode, said emission element having a normal electron emission, means deriving a unidirectional voltage from space current flow or said device, means responsive to said voltage for controlling transmission of current through said system, and means applying alternating current transmitted through said system to said emission element for varying said normal emission.

7. In a high frequency current transmission system, a space discharge device provided with an electron emission element and a cold electrode, means establishing a normal flow of electrons between said element and cold electrode, means applying high frequency current from said system to said element to energize the latter thereby to supplement the electron flow therefrom, and means responsive to the space current flow of said device for controlling current transmission through said system.

8. In a high frequency current transmission system, a space discharge device provided with an electron emission element and a cold electrode, means establishing a normal flow of electrons between said element and cold electrode, means applying high frequency current from said system to said element to energize the latter thereby to supplement the electron flow therefrom, and means responsive to the space current flow of said device for controlling current transmission through said system, said energizing means additionally establishing said cold electrode at a positive potential relative to said emission element.

DON G. BURNSIDE.