US2093565A - Automatic gain control circuit - Google Patents

Description

Sept. 21, 1937. I w. R. KOCH 2,093,565

AUTOMATIC GAIN CONTROL CIRCUIT Filed May 8, 1954 l I I l ME: I i.

Qk INVENTOR WINFIELD R. KOCH SQ BY/ gg ATTORNEY Patented Sept. 21, 1937 NHTED STATES AUTOMATIC GAIN CONTROL CIRCUIT Winfield lit. Koch, Camden, N. J assignor to Radio Corporation of America, a. corporation of Delaware Application May 8, 1934, Serial No. 724,496

Claims.

My present invention relates to automatic volume control circuits, and more particularly to such volume control circuits wherein the control bias voltage is amplified prior to control utiliza- 5 tion.

,It often happens that the control bias voltage obtainable from the volume control tube of a radioreceiver is not sunicient to keep the audio output, characteristic fiat for all signal inputs.

; For example, in the case wherein a diode is utilized asthe volume control tube, the aforementioned type of fiat characteristic may be somewhat dimcult to obtain. It may therefore be stated. that is one of the main objects of the 151 present invention to provide an automatic volume control circuit for a radio receiver, the circuit amplifying the control bias voltage and adding the amplified voltage to the original control bias, so that the sum of the two bias voltages are effective in controlling the amplification of the controlled tubes of the receiver.

While amplification of a volume control bias voltage has been considered in the past, additional direct current energy sources have been found necessary to operate the control voltage amplifier. This is not economical in compact receivers, or receivers of the A. 0-D. C. type. Hence, it may be stated that it is an additional, and important, object of the present invention 430 to utilize the voltage dropin the grid leak of the localpscillator network of a superheterodyne receiver for furnishing the direct current voltages required to secure amplification of the automatic volume control bias voltage.

. .35 Still another object of the invention is to provide a radio receiverof the superheterodyne type, the receiver utilizing a composite local oscillatorfirst detectortube, and the voltage drop devel- I oped across the grid leak of the local oscillator portion of the composite tube being used to energize aportion of the automatic volume control network which functions to amplify control bias voltage developed by the said volume control network; 7

Still other objects of the invention are to improve generally automatic volume control circuits for radio receivers, and more especially to provide such circuits which are not only reliable in operation, but economically manufactured and assembledin radio receivers.

.1 The novel features which I believe to be characteristic; ofmy invention, are set forth in particularity in the appended claims. The invention itself, however, both ,as' to its organization and 5 5\method of operation, will best be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:-

Fig. 1 diagrammatically represents a superheterodyne receiver embodying the invention,

Fig. 2 schematically shows a modified form of the invention.

Referring now to the accompanying drawing, wherein like reference characters in the different figuresdenote similar circuit elements, there is shown in Fig. 1 a superheterodyne receiver of a conventional type. This receiver includes the usual signal collector I, which may be a grounded antenna circuit, and a tunable radio frequency amplifier 2 having its tunable input circuit coupled to the signal collector. The numeral 3 denotes a composite local oscillator-first detector tube of the pentagrid type. The tube 3 and its associated tunable circuits has been fully described by, and is claimed in, application Serial No. 654,421, filed January 31st, 1933, in the name of J. C. Smith.

Briefly, the tube 3, of the 2A7 type, comprises a cathode, a plate, a signal grid, an oscillator grid, an oscillator anode, and a pair of screen grids between which is disposed the signal grid. The tunable local oscillator circuit includes a coil 4 and variable turning condenser 5, one side of the condenser being. grounded, and the grounded side of the condenser being connected to the low alternating voltage side of coil 4 through a condenser 6. The ungrounded sides of coil 4 and condenser 5 are connected to the oscillator grid 6 of tube 3, and the oscillator anode l is connected to a source of positive potential 0 (not shown) through a coil 8 which is magnetically coupled with the coil 4. The two screen grids, between which the signal grid is disposed, are connected to a point of positive potential S. i .The plate of tube 3 is connected to a point of positive potential B, the resonant network 9 tuned to the operating intermediate frequency being connected in the plate circuit of tube 3. A radio frequency by-pass condenser I0 is connected between the plate circuit lead oftube 3 and ground, while the circuit 9 is magnetically coupled to the resonant input circuit of the following I. F. amplifier E2; the circuit ll, also, being tuned to the intermediate frequency. The condenser rotors of the tunable signal input circuits of amplifier 2, and tube 3 are mechanically coupled, and the rotors of condenser 5 are also mechanically coupled to the aforementioned rotors, this being denoted by the dotted lines I 3.

' It will be understood that the reference characters O, S and B may denote appropriate voltage points on a common voltage supply potentiometer, and such a common supply potentiometer would also be used to feed the various electrodes of the different tubes of the system, the potentiometer being omitted in order to preserve simplicity of description.

The intermediate frequency amplifier I2 may be of the conventional type, and its output is coupled to the input electrodes of the following second detector tube I3 which is of the pentode type. The coupling between the amplifier I2 and second detector I3 is provided by a transformer I4 whose primary and secondary are tuned to the operating intermediate frequency, and it will be noted that the present invention is not limited to the utilization of only a single stage of intermediate frequency amplification, but more than one stage may be utilized. One or more stages of audio amplification may follow the second detector I3, and a reproducer of any desired type will 1 terminate the receiving system. The circuit details following the tube I3 are not shown, because those skilled in the art are well acquainted with such details.

The automatic volume control system employed to maintain the signal intensity level to the second detector substantially constant while the receiver is in operation will now be described. The numeral I5 denotes the automatic volume control tube, and it will be observed that this tube is of the 55 type and embodies, in addition to a cathode, control grid and plate, a pair of diode anodes disposed outside the electron stream flowing from the cathode to the plate. Such a tube and its properties are well known to those skilled in the art at the present time, and it will suifice to state that the two diode anodes are connected together, a lead It connecting them to the signal grid circuit of second detector tube I3. The low alternating voltage side of the signal input circuit of tube I3 is connected to the cathode of the multi-function tube I 5 through a path which includes resistor R1 and lead H, the cathode of tube I5 being grounded through a condenser I8.

The control grid of tube I5 is connected by a lead I9 to the negative direct voltage side of resistor R1, and condenser 20 is connected across the last-named resistor. The plate of tube I5 is connected to ground, and the cathode of the tube is additionally connected to the grounded side of the local oscillator tuning condenser 5 through a path which includes lead 2 I, resistor R2 and resistor R3. The low alternating voltage side of coil 4 is connected to point A intermediate the resistors R2 and R3. The volume control bias Voltages are applied to the signal grids of amplifier 2, tube 3 and amplifier I2 through a path which includes lead 22, and branch leads 23, 24, and 25. Each of these leads includes appropriate filter resistors, the lead 22 being connected to the negative side of resistor R1, and each of the branch leads 23, 24 and 25 being connected to the grid circuits of the controlled stages. Reference letters AVC are used to denote the automatic volume control path.

The resistor R3 is so chosen that the point A is normally about volts negative with respect to ground due to the grid current flow of the local oscillator portion of the tube 3. For conditions of weak signal there will be a very small voltage drop across the resistor R1, and it will be noted that the resistor is included in the diode circuit connected between the diode anodes and the cathode of tube I5. Since the grid of tube I5 is connected to the negative side of resistor R1, the bias on the grid will be relatively small when little or no signal energy is impressed upon the input circuit of detector I3. At such minimum bias on the grid of tube I5 the plate impedance of the tube is relatively low compared with resistor R2. The voltage drop across resistor R2, plus the voltage between cathode and anode of tube I5, must equal thevoltage across resistor R3. Because the tube resistance is much smaller than R2, the cathodeanode voltage will be small, and the cathode will be at only a small negative voltage to ground. The A. V. C. voltage will be this voltage drop in the tube plus the voltage drop in R1. Both of these are small, and the A. V. C. voltage will be small.

When a strong signal is tuned in the increased voltage drop across resistor R1 results in an increased bias on the grid of tube I5, so that this plate impedance is high compared with resistors R2 and R3. The bias for the controlled tubes is then equal to substantially the voltage drop across resistor R3 plus the voltage drop across resistor R1. It will, therefore, be seen that the control bias for increasing signal intensity is increased many times as much for the same signal level increase as when only the diode is used as a control mechanism. The controlled characteristic will, therefore, be much more complete and fiat.

The voltage drop across the grid resistor of the local oscillation portion of tube 3 may also be employed for automatic volume control using a circuit as shown in Fig. 2. In this modification only those circuit elements are shown which are essential to an understanding of the modified portions of the circuit. Thus the automatic volume control tube in this case is a pentode tube 30. The pentode tube gives a more complete cut-off, and works with lower applied signal voltages and lower tube voltages. The signal grid of the tube 30 is connected by lead 3I and condenser 32 to the high signal voltage side of the input circuit of tube I3, while the cathode of the tube 30 is connected by lead 33 to an intermediate point on resistor R1. The point A of resistor R3 is connected to the cathode of the tube 30 through a path which includes resistor R2, lead 34 and condenser 35, the lead 3I being connected to lead 34 and condenser 35 through a resistor 36. The screen grid of tube 30 is grounded, and also is connected to the automatic volume control lead 31 through a, condenser 38'. Resistor R2 and condenser 35 act as a filter to prevent any small radio frequency voltages existing across R3 from reaching the grid of tube 30.

For ordinary usage the tube 30 may be a 57 type tube with its plate connected to ground through a resistor. The pentode construction permits the plate voltage to become much less than the screen voltage without affecting the tube efficiency. If a delay action is desired, the bias on the tube can be made more than the amount required for cutoff. A second method of getting a delay action is to connect the plate resistor of tube 30 to a point which is positive with respect to ground, and uses a diode 39 to prevent the bias on the controlled tubes from becoming positive. 01' course, the grid currents of the controlled tubes may be made to keep the control bias from becoming positive, instead of using the diode 39.

The circuit shown in Fig. 2 operates in the following manner: When no signal is tuned in, tube 30 is biased substantially to cut-off by the position of the tap on resistor R3. The voltage drop in resistor 38 due to plate current of tube 30 will be small. The A. V. C. voltage tends to become positive with respect to ground, but current flowing through the diode 39 causes a voltage drop in resistors 31 and 38, and thus prevents the A. V. C. voltage from becoming positive to any substantial value. As the signal is tuned in, the I. F. voltage is impressed on the grid of tube 38, causing an increase in average plate currents, and an increase in voltage drop in resistor 38.

Because of the amplification in tube 30, the drop in the plate resistor 38 will be several times as large as the I. F. voltage applied to the grid.

When the drop in resistor 38 becomes large enough, the plate of the tube will become negative with respect to ground, the current through diode 39 will stop, and the A. V. C. voltage will become negative, biasing ofi the amplifier tubes. The action of the A. V. C. system is, therefore,

, such that no increase in A. V. C. voltage occurs until the signal exceeds a predetermined value, but increases rapidly for stronger signals.

While I have indicated and described several 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, 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 combination, an electron discharge device provided with a signal input circuit and including a gain control electrode, a local oscillator including an electron discharge device provided with an impedance in its grid circuit, a rectifier circuit including an impedance, means for coupling the rectifier circuit to said first named electron discharge device, a direct current potential connection between the rectifier impedance and said gain control electrode, said rectifier including at least a cathode and an anode, a plate and control grid associated with said rectifier cathode, the last named control grid being connected to said rectifier impedance, and connections between the cathode and plate of said rectifier and the impedance in the grid circuit of said local oscillator.

2. In a system as defined in claim 1, said connection from the control grid of the rectifier and the direct current potential connection to said gain control electrode being to a common point which is at a negative direct current voltage with respect to the rectifier cathode.

3. In a system as defined in claim 1, said oscillator impedance comprising the grid leak thereof, and said rectifier cathode and anode, and the plate and control grid associated therewith, being disposed within a common tube envelope.

4. In combination with a signal transmission tube of a radio receiver of the superheterodyne type and the local oscillator thereof, a signal rectifier including an impedance in its space current path for developing a direct current voltage whose magnitude is dependent on received signal amplitude, said oscillator including a grid leak resistor, a circuit including said rectifier and the space current path of an electron discharge device, a gain control connection between a gain control electrode of said transmission tube and a point on said impedance, a more positive point on the impedance being connected to said resistor, and means, responsive to voltage variations of said first point, for controlling the said space current path.

5. In a wave transmission system, a wave transmission tube, a source of biasing voltage for the input electrodes of the tube, said source comprising a space discharge device having a fixed resistor connected inseries with its space current path, said series path including a source of direct current voltage, connections between said input electrodes and said device, means for controlling the internal impedance of said device in accordance with wave amplitude variations thereby to control the bias voltage applied. to said input electrodes, said control means including a wave rectifier having a resistor in its space current path for developing a direct current voltage from rectified Wave currents, and said second resistor being connected in series with said fixed resistor between said input electrodes whereby the direct current voltage is added to said first voltage to provide an increased biasing voltage.

WINFIELD R. KOCH.

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