US2185612A - Automatic gain control - Google Patents

Description

Patented Jan. 2, 1940 UNITED s'mrssA AUTOMATIC GAIN CONTROL Bertram Trevor.- Riverhead, N. Y., assigner -to Radio Corporation of America,4 a corporation of Delaware Application May 24, 1937, Serial No. 144,443

8 Claims.

- My present invention relates to an automatic gain or volume controlled circuit.

The conventional method of controlling the gain of radio frequency or intermediate irequency amplier stages of a radio receiver is to change thecontrol or signal grid bias. If this scheme -is used iny connection with Wide band receivers, such as are used for the reception of ultra short Wave `television modulated radio waves, the changes in control or signal grid bias produce harmful changes in grid input capacitance. rThese changes detune, for example, the intermediate frequency amplifier, producing distortion and this' effect is particularly objection# able in Wide band television intermediate fren fluency amplifiers because of the small tuning capaeitances used. Accordingly, one object of my present invention is to provid-e an automatic control circuit Which does not cause ciable detuning. In carrying this object into effect, I provide tube amplifying stages which are provided with self-biasing cathode resistors` In addition, the automatic gain controlling voltare applied to the screen grids of the tubes. Ey virtue of this arrangement, the control grid bias voltage changes with the screen voltage because of the self-biasing feature, as a conseduence of which large changes of grid input capacitance with change of gain of the volume or gain controlled tubes is substantially elii .iuated A further object of my present invention is to provide an improved circuit for applying automatic gain or volume controllingr voltages to screen grids of amplifying tubes. Brieiiy, I ac complish this by placing an automatic gain or regulator tube in series with the screen grid supply voltage lead. The conductivity or resistivity of this tube is varied in such a way that automatic volume controlling action takes place by virtue of the controlled voltages applied to the screen grid or grids of the tube or tubes employed in 'the system.

The accompanying drawing is typical of the manner in which the principles of my present invention may be carried into effect. In brief, the drawing shows my improved automatic gain control circuit applied to the intermediate frequency stages of a Wide band television receiver.

Received television modulated `ultra short Waves after being heterodyned to a suitable intermediate frequency are fed to the grid 2 of an intermediate frequency amplifying tube or stage il. The output of tube @l is fed through the tuned coupling circuit 6, 8 broadened by the ac tion of resistors l0, I2 to va second intermediate lfrequr-:ncy amplifier tube 4.

(Cl. Z-20) frequency amplifier tube or stage le. The outfput of this tube is fed to a driving transformer "system lli for applying the amplified intermeplifler 22 of which only one tubel or stage is illustrated for the sake of simplicity.

Tube 4, upon the screen gridlil `of which is impressed variable screen grid voltages for ane 10 tomatic gain control, is provided with selfubiaring cathode resistor 2li, so chosen that the bias voltage upon, the control grid 2 is normal or slightly above normal with maximum screen Voltage. In this Way, large changes ofcgrid input capacitance with changes of gain of the intermediate frequency tube 4 is largely prevented or substantially eliminated.

In the event that it is desired to switch. from automatic volume control to manual volli-.me control, switch l'l is thrown to its levier Contact 49. Manual volume control may then bc eifected by adjustment 'of potentiometer tap il upon resistor 53, in turn connected to lead 5t lthrough resistor 55.

Normally, with. switch ll closed against contact 5l, the secondary 32 of the diode transformer system l@ and the associated diode load resistor 34 are maintained at a potential of minus ninety volts with respect to ground with the condition oi no signal. In the presence signal voltages eventually applied to the control grid 2 of the rst stage il, the high side of the diode load resistor 35i becomes relatively more positive in potential by, for example, five volts. Under this condition, the control grid til of the autc matic gain control direct current amplier tube 35 assumes a lpotential of, for example, minus eighty-five volts. With proper grid bias of tube, 365, set by means of the gain control potentieine` ter l and cathode tap 42, plate current will dos; through the tube t and kthrough the plateresistor llt which may, for example, be ilve megoluns in value. The current is adjusted to just the right value to bias the control grid lit of the automatic gain regulator tube it. This tube is connected in series between` the leads iii?. 52, leading respectively to a source of screen voltage and to the screen grid 2i of intermediate Accordingly, tube 48 acts as a controlled variable resistance or conductance in series with the screen grid supply lill for the intermediate frequency ampliiier tube il.

An increase in signal level will cause a larger voltage drop across the diode load resistor 3ft which, in turn, will cause the grid 38 of the automatic gain control D. C. amplifier tube 36 to go relatively more positive. This small change in potential is then amplified by the direct current amplier tube 36, giving a large negative swing to the grid 46 of the regulator tube 48, tending to make this tube approach cutoff. The effect of this is to merely reduce the current feed and also the voltage supplied to the controlled intermediate frequency screen grid 24 from the source 50. Obviously, the process is reversed if a decrease in signal level appears at the input terminal 2.

Although the lead 52 has been shown as leading to only the screen grid 24 of the intermediate frequency amplifier tube 4, it 'may, of course, be connected by way of conductor 54 to the screen grids of other gain controlled intermediate frequency stages and/or radio frequency stages. Also, if desired, the screen grid 25 may be connected to lead 52should it be desired to automatically volume control tube Id.

In shunt to the cathode bias resistors 28, 30, I have provided by-passing condensers 56. Bypassing condensers 58, 60 are provided for the screen grid 2li and voltage dropping resistor 62. Other condensers 64 are provided to by-pass the leads to which they are connected.

Condenser 56 functions as a by-pass condenser. Condenser 68 may be of 0.01 microfarad in value and condenser 'Hl may be one microfarad in value. Both ccndensers 68 and 'l0 serve as both by-passing and time constant condensers.

To restate matters briefly, intermediate frequency waves are fed through the tuned transformer l to the grid 2v of the first intermediate frequency tube stage 4. The primary and secondary of the transformer should be tuned by way of condensers 3 and 5 and to adapt the system for wide band reception, broadening resistors 1 and 9 are provided. If the tube 4 were volume controlled in the ordinary way as, for example, by applying automatic volume controlling or gain controlling voltages to the grid 2, the grid input capacitance or the capacity across the grid 2 and cathode il would vary, causing detuning of the input circuit l, 3, 5.

As explained hereinabove, the automatic volume controlling voltages are applied by way of tubes 35, 48 to the screen grid 24. Application of the automatic gain controlling voltages to the screen grid 2li rather than to the control grid 2, together with the cooperative action of the cathode biasing resistor 28, prevents and substantially eliminates variations in input grid 2- cathode H capacitance. It, therefore, follows that with my system automatic volume control is obtained Without the heretofore concomitant disadvantage of changes in input capacitance which, I have found, prove exceedingly troublesome in wide band television receivers.

Also, from a qualitative point of View, when automatic gain control is supplied to the control grid in the conventional way, variations in grid Voltage vary the cloud of electrons in the space between grid and cathode, thereby varying the input capacity. With the system described herein, the distribution of electrons in the space between grid and cathode remains substantiallyv constant because any tendency towards a change produced by variation in screen grid voltage is compensated in the reverse manner by the resistor in the cathode return lead providing control grid bias.

Having thus described my invention, what I claim is:

1. An amplifying system comprising a tube having a cathode, an anode, a control grid, and a screen grid, a tuned signal input circuit connected between the control grid and cathode, an output circuit connected between the anode and cathode, and means for automatically controlling the gain of said tube without substantially affecting the input capacitance between the control c grid and cathoclevof said tube, said means comprising a signal-responsive circuit for applying an automatic gain control voltage to the screen grid of said tube, and a resistor connected in the return lead to said cathode, said resistor having a value such as to provide a normal lnegative control grid bias for the control grid of said tube when the screen grid voltage is amaximum, the screen grid and control grid voltages being so related over the operating range of said signalresponsive circuit that said input capacitance remains substantially constant thereover.

2. In a wide band television receiver, an amplier tube of the type having at least a cathode, control grid, screen grid and anode, a resonant input circuit being connected between the cathode and control grid, said circuit being tuned to a modulated carrier frequency lying in the ultra-short wave range, an output circuit coupled to the cathode and anode; an automatic gain control arrangement for said amplifier tube which includes means, responsive to an increase in carrier amplitude, for decreasing thepositive voltage of said screen grid, and additional means, re-

sponsive to the said screen grid voltage decrease,

control grid, screen grid andanode, a resonant input circuit being connected between the cathode and control grid, said circuit being tuned to a modulated carrier frequency lying in the ultrashort wave range, an output circuit coupled to the cathode and anode; an automatic gain control arrangement for said amplifier tube which includes means, responsive to an increase in carrier amplitude, for decreasing the positive voltage of said screen grid, additional means, responsive to the said screen grid voltage decrease, for adjusting the potential difference between the control grid and cathode in a sense, and to an extent, such as to prevent changes in inter-electrode capacitance of the said tube which aiect the tuned frequency of said resonant input circuit, said additional means consisting of a resistor disposed in the space current path of said tube, and the resistor having a magnitude suflicient to bias the control grid to its maximum negative value when the potential of said screen grid a maxirier amplitude, for decreasing the positive voltage of said screen grid, and additional means, responsive to the said screen'grid voltage decrease, for

adjusting the potential difference between the control grid and cathode in a' sense, and to an extent, such as to prevent changes in inter-electrode capacitance of the said tube which affect the tuned frequency of said resonant input circuit, said decreasing means including `a rectifier, direct current amplier and anelectronic impedance arranged in cascade.

5. In a receiving system comprising a plurality of tuned signal amplifier tubes arranged in cascade, a diode having a signal input ycircuit coupled to the last of said tubes, a load resistor in circuit with the diode, a tube having its input electrodes connected to said load resistor in such a manner that an increasing positive voltage is impressed on the last named tube as 'the signal carrier amplitude increases, at least one of the cascade tubes including a gainy control electrode connected to a source of positive voltage, and said connection including a tube, functioning as a variable impedance, having ar control electrode thereof connected to said last named tube output.

6. In a receiving system comprising a plurality of tuned signal amplifier tubes arranged in cascade, a diode having a signal input circuit coupled to the last of said tubes, a load resistor in circuit with the diode, a tubehaving its input electrodes connected to said load resistor in such a manner that an increasing positive voltage is impressed on the last named tube as the signal carrier amplitude increases, at least one of the cascade tubes including a gain control electrode 4 connected to a'source of positive voltage, said connection including a tube, functioning as a.

variable impedance, having a control electrode thereof connected to said last named tube output, and said one tube having a self-biasing retials being proportionedto maintain substantially constant the input capacitance of said input electrodes throughout a substantial range of adjustment of the transconductance of said tube.

8. A signal-translating stage comprising a vacuum tube having input and output electrodes and an auxiliary control electrode, signal-responsive means for adjusting simultaneously the potentials of said input and auxiliary control electrodes, thereby to vary the transconductance of said tube, said adjustable potentials being proportioned to maintain substantially constant the input capacitance of said input electrodes throughout a substantial range of adjustment of the transconductance of said tube.

BERTRAM TREVOR.

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