US2072283A - Automatic gain control - Google Patents

Description

March 2, 1937. H. A. SNOW 2,072,283.

AUTOMATI 0 GAIN CONTROL Filed July 23, 1952 INVENTOR HAROLD A. SNOW BY r ATTORNEY Patented Mar. 2, 1937 PATENT OFFICE AUTOMATIC GAIN CONTROL Harold A. Snow, Mountain Lakes, N. J., assignor to Radio Corporation of America, a. corporation of Delaware Application July 23, 1932, Serial No. 624,254

'7 Claims.

My present invention relates to amplifier gain control arrangements, and more particularly to an automatic volume control circuit adapted for use in connection with a radio receiver.

As is well known in the prior art the ideal automatic sensitivity, or volume control, arrangement for a radio receiver should be simple in construction and independent of the type of tubes used in the receiver. Furthermore, an automatic 10 volume control circuit for a radio receiver should be flexible with regard to the control voltage source.

Accordingly, it is one of the main objects of my present invention to provide an automatic volume control circuit fora radio receiver wherein the grid current of any tube, be it an amplifier or detector, can be utilized to furnish control bias for one, or more, preceding amplifier stages,

the system being independent of the type of tube used, and being capable of eflicient use without the employment of auxiliarytubes.

Another important object of the present invention is to provide a control circuit for auto matically controlling the amplification characteristic of one, or more, radio frequency amplifier 25 stages, the source of control voltage comprising an impedance disposed between the grid and cathode of a succeeding amplifier or detector tube, and the control voltage being provided by 3 the potential drop across this impedance which arises in virtue of the flow of grid current through the impedance.

Other objects of the invention are to improve generally the simplicity and efficiency of automatic gain control circuits, and to provide a circuit of this kind whichis not only durable and reliable in operation, but economicallymanufactured and installed.

,The novel features which I believe to be char- 40 acteristic 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 drawing in which I have indicated diagrammatically one circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing,

50 wherein there is shown in diagrammatic manner a conventional type of radio receiver, it will be i noted that the said receiver comprises the usual signal energy collecting circuit A, G. This collecting circuit may, of course, be a grounded an- 55 tenna circuit, or;even a portable loop antenna.

The signal collecting circuit, whatever its construction, is coupled, as at M, to the tunable input of the first radio frequency amplifier tube I, it being understood that the said amplifier tube is preferably one of a plurality of radio frequency amplifier tubes of a multi-stage radio frequency 5 amplifier. In order to preserve simplicity of description, the present invention will only be described in connection with the stage including tube I, it being clearly understood, however, that the present invention is also to be considered as applicable to the tube 2 of the multi-stage amplifier. The input circuit of tube l includes the usual variable tuning condenser 3, and a grid bias resistor 4 in the grounded leg of the cathode, the resistor 4 being shunted by a radio frequency by-pass condenser 5. l

The rotor of the variable condenser 3 is shown grounded, and a radio frequency by-pass condenser 6 is connected between the low potential end of the inductance coil 1 and the grounded side of the rotor of the variable condenser 3. The anode of tube I is connected to the positive terminal of a source of anode potential B, the potential source not being shown, and a trans former coupling M; is arranged to couple the out put of the multi-stage radio frequency amplifier to the tunable input of the detector tube T.

The input circuit of the detector tube T includes a variable tuning condenser 8, the rotor thereof being grounded and mechanically connected, in any well known manner, as shown by the dotted line 9, to the rotor of variable condenser 3 for uni-control adjustment. The grid of the tube T is connected to the high potential side of the tunable input circuit of the tube by a coupling condenser C1. The detector tube T is shown as comprising a tube of the indirectly heated cathode type, the cathode including in its grounded lead a grid bias resistor R3, which is shunted by a radio frequency by-pass condenser 03. The resistorRz is shown as adjustable in order to vary the initial and normal bias applied to the grid of the detector tube. The anode of detector tube T is arranged for connection to a source of anode potential B, the source not being shown, and it is to be understood that the sources B and B may be a single rectified alternating current source, as is well known to those skilled in the art.

The amplifier gain control path is arranged in shunt with the input electrodes of the detector tube T, and thispath comprises a pair of resistors R1 and R2 arranged in series, the upperterminal of the resistor R1 being connected 55 intermediate the grid of detector tube T and one side of the coupling condenser C1, while the lower terminal of resistor R2 is grounded. The resistor R1 preferably has a magnitude of one megohm, while the resistor R2 is given a magnitude of two megohms or more. The common terminal of resistors R1 and R2 is connected by a lead ID to the radio frequency amplifier grids, the connection to the grid of tube l including a filtering resistor II, it being clearly understood that the connections to the other grids of the multi-stage radio frequency amplifier would be arranged in a similar manner. A radio frequency by-pass condenser C2 is connected in shunt with the resistor R2.

The output of the detector tube T is impressed, through an audio frequency coupling M3, upon the input of an audio frequency amplifier, which amplifier may comprise a single stage, or a plurality of stages. Of course, if the detector tube T is of the power detector type, then a single stage of audio frequency amplification will be sufficient to operate a utilization means such as a loud speaker, which is coupled to the output of the audio frequency amplifier. Preferably, it is desirable to have the detector tube T operate at high voltages, that is operate as a power detector. In that case, of course, the multi-stage radio fre quency amplifier would be sufficient to supply a radio frequency voltage of the order of at least 20 volts to the grid of tube T while the source B should be capable of applying a positive potential of the order of 350 volts to the anode of tube T. The resistor R3 would then be adjusted to maintain a normal negative bias voltage on the grid of tube T of the order of about 20 volts.

Considering, now, the operation of the present arrangement it is to be understood that when the receiver shown in the diagram is operated, the resistor R3 would be adjusted, since it operates in the manner of a manual volume control, to that magnitude which will result in a predetermined desired volume of loud speaker output when the uni-control tuning device 9 is adjusted to different station settings. The magnitude of resistor R3 should be such that the grid of tube T is maintained normally biased to a point on its grid voltage-plate current characteristic such that the signal voltage which will produce the desired volume level will result in a slight flow of grid current through the path R1, R2. This slight fiow of grid current through the resistor R2 will result in the application of a control bias to the grids of the multi-stage radio frequency amplifier such that the radio frequency amplification will be constantly maintained at a level such that the output of the radio frequency amplifier will deliver to the input of the power detector the desired radio frequency voltage.

Should the input to the radio frequency amplifier, for one reason or another, diminish or fade, then there will obviously be a decreased input to the power detector T with the result that there will be a decreased flow of grid curent through the resistor R2. This will naturally result in a decreased control voltage applied to the grids of the radio frequency amplifier, and the radio frequency amplification will accordingly be increased, thus bringing the radio frequency voltage input to the power detector to the desired level. On the other hand, if the, input to the radio fre quency amplifier increases forsome reason, then the radio frequency input to the power detector will increase, and the grid of the detector tube T will draw more current, with the result that the potential drop across the resistor R2 will increase, and due to the increased bias voltage applied to the radio frequency amplifier grids, the radio frequency amplification will be cut down to the desired radio frequency input level of the power detector.

It will thus be seen that a very effective and simple type of automatic volume control circuit has been provided by the present invention. It should be particularly noted that the present system is independent of the type of tube used, provided small grid currents can be taken by the tubes. Additionally, it will be noted that the present invention may be used without the need for auxiliary tubes. It is to be particularly noted that since the tube T is operated at high voltages, that is as a power detector, the slight increase in negative bias on the grid of tube T due to the resistor R1 in series with the resistor R2, is governed so as not to become too great for the efficient use of the power detector.

Again, it is pointed out that the arrangement permits the ready uni-control of the variable tuning condensers and the grounding of the rotors of these tuning condensers, this being a desirable mode of construction in modern broadcast receivers. It is obvious that with the present system the detector is drawing grid current during the entire control period, and there necessarily exists some distortion. However, this distortion will not be objectionable or noticeable if the control range is so adjusted that the grid current variation range is such that the resulting distortion is not audibly noticeable and is kept at a minimum value. One of the essential virtues of the present arrangement resides in its economy of space, construction and cost, and therefore provides an efficient automatic volume control arrangement even though the grid current utilization results in a certain amount of distortion.

Of course, the present invention is not limited to the use of the input of a detector stage as the source of control voltage. The tube T and its associated circuits, can readily be the last stage of r the radio frequency amplifier. In that case, of course, a detector is provided between the tube T and the input of the audio frequency amplifier, the control voltage for one or more preceding amplifier stages be obtained from the tube T in the same manner. As a matter of fact, since the detector tube T has been described as a power detector, it will be realized that the tube T functions as an amplifier circuit, and simultaneously as a detector circuit. Therefore, it is equally true of the present description that the input of a radio frequency amplifier is utilized to control the gain of preceding radio frequency amplifier stages as it is to say that the input of a detector circuit is utilized for the same purpose. Again, the tube T can be an audio frequency amplifier stage, and furnish amplifier control grid bias in the same Way.

While I have indicated and described one arrangement 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 organization 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 of the type including a radio frequency amplifier adapted to deliver a relatively high signal voltage and a following detector tube including at least a control grid, cathode and anode, means for coupling the control grid to the amplifier output, a control bias resistor connected between the said control grid and cathode, a direct current connection connected between the control grid side of said resistor and a gain control electrode of said amplifier whereby flow of grid current through the control grid circuit develops an amplifier control potential across said resistor, and means for oper ating the said electrodes of the detector tube at potentials such that the entire signal voltage output of the amplifier is handled by it without overloading and the effect of the increased negative bias on the detector due to said grid current flow is minimized.

2. A method of operating a radio receiver including a multi-stage radio frequency amplifier, a power detector and an audio frequency amplifier in such a manner that the output of the audio frequency amplifier is maintained at a uniform signal intensity level, which consists in amplifying collected signal energy in said radio frequency amplifier, impressing the amplified energy upon the input of the power detector, adjusting the detector so that it draws grid current only at signal inputs which exceed said level, and causing said grid current to control the amplification characteristic of said multi-stage radio frequency amplifier in such a manner that said level is maintained.

3. In combination, a radio frequency amplifier, including an electron discharge tube, means for impressing signal energy upon the input of said amplifier tube, a succeeding electron discharge tube detector having its input electrodes con nected to receive amplified signal voltage of a magnitude of the order of 20 volts from the output of said amplifier tube, means for adjusting ,the succeeding detector tube so that it draws grid current only at signal inputs which exceed a predetermined level, means for applying to the control grid of the succeeding tube a negative potential of the order of 20 volts and to the anode of the latter a positive potential of the order of 350 volts, and means in the input circuit of said succeeding tube for providing a control potential for an input electrode of said amplifier tube, said last named means being conductively connected to said amplifier input electrode.

4. In combination, a radio frequency amplifier, including an electron discharge tube, means for impressing signal energy upon the input of said amplifier tube, a succeeding electron discharge detector tube having its input electrodes connected to receive amplified signal energy from the output of said amplifier tube, means for adjusting the succeeding tube so that it draws grid current only at signal inputs which exceed a predetermined level, and means in the input circuit for said succeeding tube for providing a control potential for an input electrode of said amplifier tube, said last named means being conductively connected to said amplifier input electrode, said means including a resistor connected to the control grid of said succeeding tube and ground.

5. In combination, a radio frequency amplifier, including an electron discharge tube, means for impressing signal energy upon the input of said amplifier tube, a succeeding electron discharge detector tube having its input electrodes connected to receive amplified signal energy from the output of said amplifier tube, means for adjusting the succeeding tube so that it draws grid current when the signal energy input thereto exceeds a predetermined level, and means in the input circuit of said succeeding tube for providing a control potential for an input electrode of said amplifier tube, said last named means being conductively connected to said amplifier input electrode, said means including a resistor connected to the control grid of said succeeding tube and ground, said succeeding tube comprising a power detector, the cathode circuit thereof being grounded and including an adjustable means for maintaining the control grid of the detector at a value such that grid current will fiow through the control grid circuit of said detector tube only at signal inputs which exceed said predetermined level.

6. In a radio receiver, a radio frequency amplifier tube having a tunable input circuit, said input circuit including a variable tuning condenser having a grounded adjustable element, a succeeding combined amplifier-detector stage having a tunable input circuit, and including a variable tuning condenser having its adjustable element grounded, the adjustable elements of both variable condensers being arranged for mechanical uni-control, means for coupling the output circuit of said amplifier and the tunable input circuit of said combined amplifier-detector, a coupling condenser connected between the control grid of said combined amplifier-detector and the high potential side of its tunable input circuit, a path, including a pair of resistors in series connected between the control grid of said amplifierdetector and ground, and means for conductively connecting the junction of said resistors to the control grid of said amplifier tube.

'7. In combination with a receiver of the type including a radio frequency amplifier having a high signal voltage output and whose gain is to be automatically regulated and a detector tube, a resistive path connected between the detector grid and cathode and adapted to be traversed by grid current when the signal input to the detector increases above a predetermined carrier intensity level, a direct current connection between an input electrode of said amplifier and an intermediate point on said path for biasing the amplifier in a sense to decrease its sensitivity as the grid current fiow through said path increases, and means for applying voltages to said detector tube electrodes such that the detector is of the high voltage type whereby the effect of biasing the detector grid more negative than the amplifier input electrode with grid current fiow is minimized.

HAROLD A. SNOW.

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