US2088230A - Automatic volume control circuit - Google Patents

Description

- July 27, 1937. DI BURNSIDE AUTOMATIC VOLUME CONTROL CIRCUIT 5 Sheets-Sheet 1 Filed Jan. 27, 1934 r m A H H 0 U. 7 W T. LH 4 IF W F J W xx) m 0 0 n 4 R3 e n" a INVENTOR DON G.BURN$IDE ATTORNEY y 1937. D. G- BURNSIDE 2,088,230 I AUTOMATIC VOLUME CONTROL CIRCUIT Filed Jan. 27, 1934 3 Sheets-Sheet 3 INVENTOR DON G.BURNSIDE Q, R BY aw QQIQ v ATTORNEY Patented July 27, 1937 AUTOMATIC VOLUME CONTROL CIRCUIT Don G. Burnside, East Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 27, 1934, Serial No. 708,564

15 Claims.

My present invention relates to automatic gain control circuits for radio receivers, and more particularly to improved methods of, and means for, automatically regulating the volume of a radio receiver.

One of the main objects of my present invention is to provide an automatic volume control arrangement for a radio receiver, which receiver includes a radio frequency amplifier whose gain is to be controlled, the control arrangement comprising a signal rectifier adapted to vary the conductivity of an electron discharge tube in such a manner that a positive control electrode of the controlled amplifier is varied in potential as the conductivity of the rectifier control tube is varied.

Another important object of the invention is to provide in a radio receiver which includes a multi-grid signal amplifier and at least two diode signal rectifiers, an arrangement wherein one ofthe diode rectifiers is utilized to vary the negative potential on one of the grids other than the signal grid of the said signal amplifier, while the other diode rectifier is employed to vary the positive potential on another grid of the signal amplifier.

Another object of the invention is to provide an automatic volume control system for a radio I receiver wherein the controlled amplifier includes a negatively biased grid and a positive grid, the potentials of the two grids being varied automatically to control the volume of the receiver, and the effective plate resistance of the controlled amplifier being additionally varied to control the fidelity of the receiver.

Still another object of the invention is to provide in a radio receiver of the type including a background noise suppressor system for rendering inefiicient the amplification of undesired noise impulses, an automatic volume control system of the type wherein the negative bias of a suppressor grid and the positive potential of a screen grid are varied in accordance with changes in amplitude of received signals.

Still another object of the invention is to improve generally the simplicity and efficiency of radio receivers employing automatic volume control arrangements, and particularly to provide a radio receiver provided with an automatic volume control system which is not only reliable in operation, but economically manufactured and tion 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 drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings;

Fig. l diagrammatically shows a receiving arrangement embodying one form of the invention;

Fig. 2 diagrammatically shows a modification;

Fig. 3 is a graphic representation of the operation of the circuit modification in Fig. 2;

Fig. 4 shows a circuit diagram of still another modification.

Referring now to the accompanying drawings, wherein like reference characters in the different figures designate similar circuit elements, in in Fig. 1 there is shown the networks associated with the demodulator circuit of a radio receiver. The demodulator in this case is a tube known as a duplex diode triode, commercially designated as a 55 type tube, and referred to herein by the numeral 55. Such a tube includes usually a triode section and a pair of independent diode sections, these three sections utilizing a common cathode, the diode anodes D1 and D2 being positioned outside the electron stream to the grid and plate of the triode section. Such a multiple duty tube is well known at the present time to those skilled in the art, and need not be described in any further detail.

In Fig. 1 the diode anodes are strapped together and are connected to the ground side of the cathode resistor R5 through a path which includes in series the resonant circuit 1 and the diode load resistor R1, the latter being shunted by the radio frequency by-pass condenser C1. The audio frequency component of the voltage developed across the diode load resistor is impressed upon the control grid 2 of tube 55 through apath which includes the resistor R3, one side of which resistor is connected to the diode anode side of resistor R1. The plate of tube 55 is designated by the numeral 3 and is connected to a point of positive potential on the B voltage supply source of the receiver through an audio frequency coupling coil 4. The amplified audio frequency current flowing in the plate circuit of tube 55 is transmitted to a succeeding audio frequency network through the coupling condenser 5, the audio network comprising one or more stages of audio frequency amplification and a final reproducer.

The cathode circuit of tube 55 is completed by connecting the resistor R5 to the negative side of the voltage source B through a resistor R2. The voltage developed across this resistor R2 is utilized for the automatic volume control function. The gain of one or more preceding radio frequency amplifier stages may be regulated, and for the purposes of this application, as well as to preserve simplicity of disclosure, there is shown in l the circuit connections required for automatically regulating the gain of one stage of radio frequency amplification ahead of the demodulator tube 55. The controlled amplifier is designated by the numeral 58 and is a pentode tube of the well known 58 type. Between the control grid and cathode there is connected a path which ineludes in series the well known resonant circuit 6 and a grid biasing source 'l.

The grounded cathode of tube 58 is connected by a lead 8 to the negative terminal of source B and the negative side of resistor R2. The anode of tube 58 is connected to the positive terminal of the source B through a path which includes the primary 9 of the coupling transformer T and the lead it. The coil 9 is magnetically coupled to the secondary of transformer T. The screen grid electrode II of tube 58 is connected by a lead I2, which includes the radio frequency choke coil L4, to the positive side of resistor R2. A radio frequency by pass condenser C2 is connected between the cathode and screen grid lead of tube 58, while a radio frequency by-pass condenser I3 is connected across resistor R2.

The radio frequency amplification stage including the tube 58 may be variably tuned by the condenser 6', and in that case the condenser i would also be variable. In such a tuned radio frequency type of receiver the source of signals, conventionally designated in Fig. l and represented by the numeral I 5, would comprise one or more stages of tuned radio frequency amplification. The invention is, of course, not limited to such a type of receiver, but may also be applied to a receiver of the superheterodyne type. In that case, the stage including tube 58 would be fixedly tuned to the operating intermediate frequency, and the stage including tube 55 would function as the second detector and its input circuit I similarly be tuned to the intermediate frequency. The network M preceding the tuned circuit 6 would then comprise the usual radio frequency amplifier, first detector and one or more intermediate frequency amplifier stages.

In actual operation, the lead It functions as the automatic volume control lead between the controlled amplifier 58 and the control diode circuit. Potential variations across resistor R2 result in changes in the mutual conductance of tube 58. This is accomplished by changing the screen grid voltage of tube 58. The voltage drop across resistor R2. furnishes the screen voltage for the grid I I.

The voltage drop across resistor R2 is a function of the space current flow through the triede section of tube 55. The two diode anodes connected to the cathode of tube 55 provide half Wave rectification when the anodes are swung positive with respect to the cathode by reason of signal voltage applied to the input circuit I. Of course, full wave rectification could be used as well. Where the triode section of tube 55 is employed to transmit the audio component of voltage developed across resistor R1 the magnitude of resistor R3 should be designed so as to permit only the direct current component and audio component of rectified voltage to be applied to grid 2. In other words, the grid 2 is biased. increasingly negative as the signal input to the diode rectifier increases. This results in a diminution of the space current flow through resistor R2 and a resultant decrease in the positive potential value of the point on resistor R2 to which the screen grid Ii is connected.

As the positive voltage of the screen grid of tube 58 decreases, the mutual conductance of the tube 53 falls, and consequently the amplifier gain decreases. In other words, with minimum signal input to tube 55 the sensitivity of tube 58 is a maximum. The filter L4C3 functions to remove any radio or audio voltage from the screen grid I I. The condenser C3 should be of such size and quality that the impedance between screen grid It and the cathode of tube 53 is low. Should the carrier voltage impressed on circuit I cause a larger direct voltage across resistor R1 than is desired, a full wave connection could be used, or the control grid 2 connected to a tap on resistor R1.

Without any signals applied to circuit I there is no drop across resistor R1 and the control grid is at approximately zero bias. In this case the plate current that fiows through resistor R2 is a maximum, and depends on the value of the resistor and on the plate supply voltage. Under the influence of signals applied to circuit I the control grid 2 is biased negatively, and as a consequence the screen grid I I has its positive voltage decreased.

The variation of mutual conductance of tube 58 with change in screen grid voltage is fairly rapid, and gives good control. In order to delay the development of volume control Voltage; that is, a change of voltage drop across resistor R2, a resistor R5 can be inserted in the cathode circuit of tube 55. The drop across this latter resistor puts a negative voltage on the diode anodes to delay rectification. With an increase in signal, the delay bias decreases when this arrangement is used so that an improved form of automatic volume control curve results. In other words, the magnitude of resistor R5 is chosen so as to produce a negative bias on the diode anodes until the signals impressed on circuit I attain a magnitude at which volume control action is desired. When this magnitude is reached the signals render the diode anodes positive and overcome the delay bias.

Automatic adjustment of the gain of amplifier tube 58 causes an increased plate resistance (13;) in the controlled amplifier. This increased plate resistance provides increased selectivity for strong carrier signals, thus tending to increase distortion for strong signals on account of side band cutting. Actually, however, so long as the plate resistance of tube 58 is above about 0.5 megohm, the effect on selectivity is negligible. Curve I of Fig. 3 shows how the plate resistance of a 58 type tube increases with decreasing screen grid voltage. Since selectivity requirements, aside from fidelity considerations, are most severe when receiving distant, or weak, stations, it is evident that the plate resistance change is opposite to the desired effect.

When triple grid tubes, as the 58 type, are employed with the outer or suppressor grid available for additional gain control, the above effect of increased r due to lowering the screen voltage can be compensated for, or even decreased, by supplying negative voltage to the suppressor grid. An examination of suppressor grid voltageplate resistance curves shows that when the suppressor grid is made negative a reduced plate resistance results.

The effect of negative suppressor voltage on the plate resistance of a. 58 type tube is shown by curve 2 of Fig. 3. .When the screen grid voltage is reduced and a negative voltage put on the sup pressor grid at the same time the plate resistance, instead of being the mean of curves l and 2, is less than the mean. This is represented by curve 3 of Fig. 2. The effect then of suppressor and screen grid control is to reduce the plate re sistance sufliciently to give improved fidelity to a certain point. In addition to lowering the plate resistance, a negative suppressor voltage acts to reduce the mutual conductance so that a greater gain control is obtained when both variable suppressor voltage and screen grid voltage are available.

In order to secure both screen Voltage and suppressor voltage for control purposes, one of the diode anodes is operated as an independent rectifier in the manner shown in Fig. 2. In this figurethe diode anode D2 is coupled to the diode anode Drthrough the capacitor C4, so that the same signal voltage on anode D1 is put on the other diode anode with but little attenuation. The resistor R6 comprises the load for the diode circuit including the anode D2, and it will be observed that resistors Re and R2 are connected in series between anode D2 and the cathode of tube 55. The supressor grid l5 of tube 53 is connected to the negative side of resistor R6 through a path which includes the radio frequency choke coil L7, the condenser C5 being connected between the cathode of tube 58 and suppressor grid l 5 to serve as a radio frequency by-pass condenser.

Otherwise the circuit arrangement of Fig. 2 is substantially the. same as that shown in Fig. 1. The control grid 2 is connected to the negative side of resistor R1. Under the influence of signals impressed on circuit I, the action that takes place in this circuit is the same as that explained for Fig. 1, except that the voltage drop across resistor Re 'is used 'to bias the amplifier tube suppressor grid negatively.

Control by the suppressor grid does not begin immediately, however, since the drop across resistor R2 represents a delay bias for the D2 circuit. This bias is, as indicated 'by the circuit diagram, thetotal drop across resistor R2 when there is no carrier signal present. Furthermore, the suppressor grid, in the no-signal condition, is at the amplifier tube cathode potential, the ground side of resistor R6 being connected by lead 8'to the cathode of tube 58.

When a carrier signal causes a reduction in the voltage dropacross resistor R2, the bias on the diode anode D2 is" reduced. As soon as a peak carrier voltage on the diodes reaches the value of the drop across resistor R2, 2. negative voltage is impressed on the suppressor grid I 5. This voltage builds up rapidly with increase of carrier, so that a sharp increase in control is obtained. This. then, constitutes automatic volume control by lowered screen grid voltage and negative bias on the suppressor grid, while simultaneously obtaining automatic regulation of selectivity and fidelity. The audio component of demodulated signal is taken 01f from across resistor R2 and transmitted to the succeeding audio network through the condenser 5.

In addition to the combined action on the amplifiergain and fidelity, there may be provided inter-station noise suppression. Fig. 4

shows one method of introducing into the system of Fig. 2 an inter-channel noise suppression circuit. Elements of the circuit arrangement of Fig. 4 used for automatic gain control and fidelity are shown to be the same as in Fig. 2 and bear the same designations. However, a power sup-ply bleeder is used in place of batteries, and the load resistor R1, instead of being a fixed resistor, is shown as being variable so that it may be used as a manual volume control to put any desired audio voltage on the grid of the tube 51, which is a pentode of the 57 type. The tube 58, as well as tube 58', has its gain controlled by variation of the positive voltage on the screen grid II and the negative bias on the suppressor grid E5.

The resistor R6, connected in series with the resistor R2, has its negative side connected by lead 2B to the suppressor grids of both controlled amplifiers, the positive side of the resistor being connected by lead 8 to the cathodes of the amplifiers and ground. The signal grids of the two amplifiers are connected by a common lead 22 to a point on the bleeder resistor P, both cathodes of the controlled amplifiers being connected to a point on the bleeder resistor which is positive with respect to the point C to which the signal grids are connected. The stages including tubes 53 and 58 are cascaded, and the plates of both tubes are connected by a common lead 23 to the +3 side of the bleeder P.

The resistor R2 has its positive side connected to the screen grids of the controlled amplifiers, and appropriate radio frequency by-pass con-.

densers are connected to the leads 26 and I2 where necessary. The filter combination R3-C2 functions to prevent the impression of the radio and audio components of the rectified voltage developed across resistor R1 from being impressed on the grid 2 of tube 55. The direct current component of the rectified voltage is impressed on grid 2, while the audio component of the voltage developed across resistor R1 is impressed upon the signal grid of tube 57 through the condenser Cs.

Instead of having a fixed minimum bias upon the audio tube 51, a variable bias is supplied by connecting the cathode of the tube to an intermediate tap on resistor R2, the tap being designated by the reference character A, the signal grid of tube 57 being connected to the negative side of the resistor through a path which includes the resistor Rs. Since the control grid of the audio tube is connected to the negative end of resistor R2 through the grid leak Rs, and the cathode is connected to point A, the effective bias on the audio tube will be the voltage drop across the resistor between the negative terminal and the point A.

The point A is selected so plied to the grid of the audio tube is sufficient to maintain the audio amplifier in cut-oil condition to give the desired suppression. Of course,the point A may be an adjustable tap so that the degree of suppression may be varied. When a carrier signal is impressed on the diode anode D1, the current flowing through resistor R2 decreases. This results in a decrease in the cut-off bias applied to tube 5'1, with the result that signals are normally transmitted through the audio amplifier 57. Should the carrier be strong enough to cause plate current cut-off in the triode section of tube 55, the bias on tube 5'! would fall to zero, except for the fact that the plate and screen current of the tube 5'! flows through the section of resistor R2 below point A, with the result that the initial bias apthe audio bias never falls beyond a certain value.

It will thus be seen that there has been provided in the circuit arrangement of Fig. 4 combinedregulation of screen grid voltage and suppressor grid voltage in the same voltage directions, with the result that the gain of the controlled amplifier is effectively regulated over a wide signal range and selectivity control simultaneously secured, the normal transmission of signals between the demodulator and reproducer being inhibited or suppressed when the received signal level drops below a predetermined intensity value.

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 not 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:

l. A method of regulating the amplification of signals in a high frequency amplifier of the type including a positive control electrode and a suppressor electrode which includes the steps of amplifying high frequency signals, rectifying the amplified signals, varying the conductivity of an electron discharge device, utilizing changes in said conductivity to regulate the positive potential of the positive electrode of said amplifier, and utilizing the rectified signals to regulate the potential of the suppressor electrode.

2. A method of regulating the amplification of a high frequency amplifier of the type including a positive screen grid and a suppressor grid, which includes the steps of rectifying a portion of the amplified output of said amplifier, rectifying a second portion of the amplifier output, varying the potential of the positive screen with the direct current component of one of said rectification steps, and varying the potential of the suppressor electrode with the direct current component of the other rectification step.

3. In a radio receiver, the combination of a high frequency amplifier provided with at least a positive control grid, a diode rectifier circuit coupled to the amplifier output circuit and including a load resistor across which a direct current voltage is developed as signals are impressed upon the diode rectifier circuit, an electron discharge device including an impedance in its cathode circuit for developing a control voltage from the space current fiow through the impedance, a direct current connection between the said positive grid and the positive side of said impedance, and a direct current connection between the negative side of said diode load resistor and a control electrode of said electron discharge device whereby the direct current bias for said last control electrode is provided solely by the voltage drop across said resistor.

4. In a receiver, as defined in claim 3, a suppressor grid disposed adjacent said positive grid, a second diode rectifier circuit including a load resistor, a signal coupling path between the two diode rectifier circuits, and a direct current connection between the said suppressor grid and the negative side of said second load resistor.

5. In a receiver, as defined in claim 3, said amplifier including a suppressor or grid, a second diode rectifier circuit including a load resistor, said impedance being connected in series with said second load resistor, and a direct current connection between the said suppressor grid provided in the controlled amplifier and the negative side of said second load resistor.

6. In a receiver, as defined in claim 3, a common tube envelope housing the electrode elements of said electron discharge device and the diode of said rectifier circuit, and said diode and electron discharge device having a common cathode.

'7. In combination with a signal amplifier provided with a positive screen grid and a suppressor grid in addition to the usual cathode, signal grid and anode, a signal rectifier including an impedance for developing a direct current voltage varying in response to signal amplitude, an electron discharge device including at least a cathode, control grid and anode, a direct current connection between the last named control grid and the negative side of said impedance, an impedance in the cathode circuit of said electron discharge device, a second diode signal rectifier including a third impedance in its circuit, the positive side of said third impedance being connected to the negative side of said second impedance, a direct current connection between the suppressor grid and the negative side of said third impedance, and a directcurrentconnectionbetweenthe screen grid and the positive side of said second impedance.

8. In a system as defined in claim 7, means for transmitting the audio component of the space current in said electron discharge device to a succeeding audio frequency network, and a single tube envelope housing the electrodes of said two signal rectifiers and said electron discharge device.

9. In a system as defined in claim '7, a single tube envelope housing the electrodes of said two signal rectifiers and said electron discharge device, said electrodes having a common cathode, an audio frequency amplifier having its input electrodes connected across a portion of said second impedance, and said impedance portion having a magnitude such that the audio frequency amplifier is biased to cut off in the absence of signals.

10. In a system as defined in claim '7, said second and third impedances having magnitudes such that the screen grid voltage of the signal amplifier is reduced and the negative bias on the suppressor grid of the latter is increased as signals are impressed upon said signal rectifiers whereby the plate resistance of the signal amplifier is reduced.

11. In a receiver including a signal amplifier of the pentode type provided with a positive screen and a suppressor grid adjacent the screen and plate, an output circuit connected to the plate, an automatic gain control circuit, responsive to signal carrier amplitude variations in the output circuit, for decreasing the positive voltage of the screen upon signal carrier amplitude increase, and a second automatic gain control circuit, responsive to said variations in the output circuit, for biasing the suppressor grid increasingly negative upon said carrier amplitude increase and to an extent sufficient to reduce the plate resistance of the output circuit.

12. In a receiver including a signal amplifier of the pentode type provided with a positive screen and a suppressor grid adjacent the screen and plate, an output circuit connected to the plate, an automatic gain control circuit, responsive to signal carrier amplitude variations in the output circuit, for decreasing the positive voltage of the screen upon signal carrier amplitude increase, and a second automatic gain control circuit, responsive to said variations in the output circuit,

for biasing the suppressor grid increasingly negative upon said carrier amplitude increase and to an extent sufficient to reduce the plate resistance of the output circuit, a signal transmission tube following said output circuit, and connections between the input electrodes of the last tube and said first automatic gain control circuit whereby substantially a cut ofi bias is applied between the input electrodes upon decrease of the carrier amplitude below a predetermined intensity level. 13. In a receiver including a signal amplifier of the pentode type provided with a positive screen and a suppressor grid adjacent the screen and plate, an output circuit connected tothe plate, an automatic gain control circuit, responsive to signal carrier amplitude variations in the output circuit, for decreasing the positive voltage of the screen upon signal carrier amplitude increase, and a second automatic gain control circuit, responsive to said variations in the output circuit, for biasing the suppressor grid increasingly negative upon said carrier amplitude increase and to an extent sufiicient to reduce the plate resistance of the output circuit, and connections common to said two gain control circuits whereby the second control circuit is rendered operative subsequent to said first control circuit.

14. In combination with a signal transmission tube provided with a cathode, a cold signal input electrode, output electrode, and at least one additional positive col-d electrode, a diode rectifier network having an input circuit coupled tosaid output electrode, a resistor in the rectifier network across which is developed a rectified signal voltage varying directly in magnitude with the received signal amplitude, an electron discharge de vice including input electrodes connected to said resistor, the space current path of said device including an impedance, a gain control connection between said positive cold electrode of said transmission tube and a point on the impedance which is positive with respect to the potential of the transmission tube cathode, and a second impedance in the space current path of said device, the cold electrode of the rectifier being connected to a point on the second impedance which is negative with respect to the diode cathode potential.

15. In combination with a signal transmission tube of the type including a cathode, a signal input electrode, an output electrode, and at least one auxiliary cold electrode, a demodulator network including a signal input circuit coupled to the output electrode, and an electron discharge device having an impedance in its space current path, the space current flow through the impedance being dependent upon the received signal amplitude, a direct current connection between the auxiliaryelectrode and apotential point on the impedance which is positive with respect to the cathode potential of said tube, a demodulated signal transmission tube provided with a cathode, anode and cold input electrode, and direct current connections between the input electrode and cathode of the last tube and said impedance, the connection point of the last named input electrode having a potential which is negative with respect to the cathode potential of the second tube and negative with respect to the first named potential point.

DON G. BURNSIDE.

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