US2118287A - Automatic gain control circuit - Google Patents

Description

May 24, 1938. w KOCH AUTOMATIC GAIN CONTROL CIRCUIT Original Filed May 8, 1934 KOCH W Li W MM WUUE c vx INVENTOR WINFIELD R BY ATTORNEY Patented May 24, 1938 UNlTED s'rsrss Z,ll8,28l rarest crease AUTOMATIC GAIN CONTROL CIRCUIT Winfield R. Koch, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Original application May 8, 1934, Serial No. 724,496. Divided and this application June 20, 1936, Serial No. 86,224.

'7 Claims.

V 1934, Patent No. 2,093,565, September 21, 1937.

It often happens that the control bias voltage obtainable from the volume control tube of a radio receiver is not sufficient to keep the audio output characteristic fiat for all signal inputs. For example, in the case wherein a diode is utilized as the volume control tube, the aforementioned type of flat characteristic may be somewhat difficult to obtain. It may therefore be stated that it is one of the main objects of the 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. C.D. C. type. Hence, it may be stated that it is an additional, and important, object of the present invention to utilize the voltage drop in the grid leak of the local oscillator network of a superheterodyne receiver for furnishing the direct current voltages required to secure amplification of the automatic volume control bias voltage.

Still another object of the invention is to provide a radio receiver of the superheterodyne type, the receiver utilizing a composite local oscillator-first detector tube, and the voltage drop developed across the grid leak of the local oscillator portion of the composite tube being used to energize a portion of the automatic volume control. network which functions to amplify control bias voltage developed by the said volume control network.

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 assembled in radio receivers.

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, both as to its organization and 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. l diagrammatically rep-resents 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 figures denote 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 de scribed by, and is claimed in, application Serial No. 654,421, filed January 31, 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 tuning 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 it through a condenser 6. The ungrounded sides of coil 6 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 ll (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.

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 H3 is connected between the plate circuit lead tube and ground, while the circuit 9 is magnetically coupled to the resonant input circuit of the following I. F. amplifier IZ; the circuit l l,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 afore-mentioned rotors, this being denoted by the dotted lines Hi.

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 52 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 !3 is provided. by a transformer [4 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 terminate the receiving system. The circuit details following the tube is 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 embodied, in addition to a oath" ode, 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 sufiice to state that the two diode anodes are connected together, a lead It connecting them to the signal grid circuit of second detector tube iii. The low alternating voltage side of the signal input circuit of tube 33 is connected to the cathode of the multi-function tube 85 through a path which includes resistor R1 and lead H, the cathode of tube l5 being grounded through a condenser H8.

The control grid of tube i5 is connected by a lead I9 to the negative direct voltage side of resistor R1, and condenser 28 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!, 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 82 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 30 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 15 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 it. At such minimum bias on the grid of tube 55 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 I 5, must equal the voltage across resistor R3. Because the tube resistance is much smaller than .R2, the cathode-anode 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 85, 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 flat.

V The voltage drop across the grid resistor oi 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 38. The pentode tub-e 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 tii'is connected by lead 33 to an intermediate point on resistor R3. The point A of resistor R3 is connected to the cathode of tube 30 through a path which includes resistor R2, lead 3% and condenser 35, the lead 3! being connected to lead 3% and condenser 35 through a resistor 36. ihe screen grid of tube 3 is grounded, and also is connected to the automatic volume control lead 3? through a condense 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 38 may be a 5? 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 eiiiciency. 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 38 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. Of 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 3D is biased substantially to cut-oi? by the position of the tap on resistor R3. The voltage drop in resistor 38 due to plate current of tube 38 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 30, causing an increase in average plate current, and an increase in voltage drop across 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 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 radio receiver provided with a local oscillator including a resistor in its grid circuit, an amplifier whose gain is to be controlled, a rectifier, tube having grid and cathode input electrodes and a plate and including means for impressing signals upon its input electrodes, direct current voltage connections from said resistor to the grid and cathode of the rectifier, said connections applying to said grid and cathode a no-signal cutofi bias developed by the flow of oscillator grid current through said resistor, a gain control connection from the plate circuit of the rectifier to said amplifier and a diode connected to the control connection for delaying the gain control of the amplifier.

2. In a radio receiver provided with a local oscillator including a resistor in its grid circuit, an amplifier whose gain is to be controlled, a rectifier tube having grid and cathode input electrodes and a plate and including means for impressing signals upon its input electrodes, direct current voltage connections from said resistor to the grid and cathode of the rectifier, said resistor developing a voltage applied through said connections as a rectifier no-signal cut-off bias, a gain control connection from the plate circuit of the rectifier to said amplifier, the direct current connection to the rectifier cathode being from an intermediate tap on the said resistor and a diode connected to the control connection for delaying the gain control of the amplifier.

3. In a wave transmission system, a wave trans mission tube, an automatic gain control network for the tube comprising a space discharge device, a resistor and a source of direct current voltage all arranged in a series circuit, a direct current voltage connection between a gain control electrode of the tube and a point on the resistor, means maintaining the internal impedance of said device sufiiciently high to substantially prevent current fiow through said resistor, and additional means responsive to an increase in wave amplitude for decreasing the magnitude of said impedance whereby a direct current voltage is developed across said resistor to serve as a biasing voltage for said gain control electrode and means operatively associated with said resistor for delaying the impression of said biasing voltage on the gain control electrode ifor waves of less than a desired amplitude.

4. In a wave transmission system, a wave transmission tube, an automatic gain control network for the tube comprising a space discharge device, a resistor and a source of direct current voltage all arranged in a series circuit, a direct current voltage connection between a gain control electrode of the tube and a point on the resistor, means maintaining the internal impedance of said device sufiiciently high to substantially prevent current fiow through said resistor, additional means responsive to an increase in wave amplitude for decreasing the magnitude of said impedance whereby a direct current voltage is developed across said resistor to serve as a biasing voltage for said gain control electrode, and a diode device connected to said resistor point for delaying the impression of said biasing voltage on the gain control electrode for waves having less than a desired amplitude.

5. In a wave transmission system, a wave transmission tube, a source of biasing voltage for the input electrodes of the tube which comprises a space discharge device having a fixed resistor connected in series with its space current path, said series path including a source of direct current voltage, direct current voltage connections between said input electrodes and said device, and means for decreasing the internal impedance of said device in accordance with wave amplitude increase thereby to increase the bias voltage applied to said input electrodes and additional means for delaying the impression of said bias voltage on said input electrodes for wave amplitudes of less than a predetermined value.

6. In combination in a superheterodyne receiver, provided with a local oscillator network, said network including a tube having a grid leak resistor, an automatic gain control network for at least one signal transmission tube of said receiver, said network comprising an electron discharge tube provided with at least a cathode, a control electrode and a plate, means utilizing direct current voltage developed across said leak resistor by oscillator grid current flow for biasing said control electrode, in the absence of received signals, in a sense to substantially prevent space current fiow through said control network tube, a resistor disposed in the space current path of said control tube, means for connecting a gain control electrode of said one transmission tube to a point on said resistor which assumes a negative direct current potential when signals are received, and means for impressing received signals on the control electrode of said control network tube.

7. In a signal wave receiver, a signal transmission tube arranged to feed signals to a demodulator, an automatic gain control circuit comprising a tube having a resistor in its space current path, means biasing said control tube in a sense to reduce the space current fiow through the resistor in the absence of signal waves, means for impressing signal waves on said control tube, a gain control connection between a gain control electrode of said transmission tube and a point of said resistor, a conductive diode including said resistor in its space current path and developing a direct current voltage thereacross which is applied to said gain control electrode in a negative sense, and the anode of said diode being connected to a point on said resistor suchthat the diode is rendered non-conductive when the space current of said control tube is increased by a signal wave of a predetermined amplitude.

WINFIELD R. KOCH.

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