US2358325A

US2358325A - Gain control circuits

Info

Publication number: US2358325A
Application number: US415919A
Authority: US
Inventors: George W Fyler
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1941-10-21
Filing date: 1941-10-21
Publication date: 1944-09-19
Anticipated expiration: 1961-09-19

Description

Sept. 19, 1944.

GAIN- G. W. FYLER GAIN CONTROL CIRCUITS Filed Oct.

GAI

FREQUENGY FREQU ENCY-- F IQ. 4. 70'

GAIN- FREQUENCY Inventor: George N. Fyler;

His Attorney.

Patented Sept. 19, 1944 GAIN CONTROL CIRCUITS George W. Fyler, Stratford, Conn, minor to General Electric Company, a corporation of New York Application October 21, 1941, Serial No. 415,919

9Claims.

My invention relates to gain control circuits for signal amplifiers and has for one of its objects the provision of improved manual gain control circuits for high-frequency wide-band amplifying systems.

Another object of my invention is to provide improved means for varying the gain of a highfrequency amplifier without affecting the symmetry of its band pass characteristic. v A further object of my invention is to provide an improved manual gain control for a highfrequency thermionic amplifier wherein adjustment of a single movable contact member simultaneously varies the potential on, one control electrode in order to vary the gain of the device and also the potential on another control electrode in order to maintain the input capacitance of the device substantially constant at all gain control settings.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, together with further objects and advan-' tages thereof, may bestbe understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. l is a circuit diagram of a high-frequency amplifier embodying my invention; Figs.

2, 3 and 4 are curves illustrative of certain operating characteristics of the apparatus of Fig. 1; and Fig. 5 is a circuit diagram illustrating a modified form of my invention.

winding plus any other shunt cincuit capacities, all indicated by the dotted capacitor IS. The secondary winding I2 is similarly tuned by the distributed capacity and other shunt capacities across the transformer winding, represented by the dotted capacitor ii, in conjunction with the input capacitance of the amplifier ll, represented by the dotted capacitor l8 connected between the control grid l3 and cathode IS. The cathode I3 is connected to ground through a resistor 20 shunted by a bypass capacitor 2|. The capacitor 2| is of low impedance at the carrier and sideband frequencies so that the input capacitance l8 of the device It is effectively connected directly in shunt to the secondary circuit I2. When a wide-band frequency characteristic is desired, either or both of the circuits Ill and I2 may be shunted in a. conventional manner bydamping resistors. Such a damping resistor 22 is shown connected in shunt to the secondary winding i2.

The anode 23 of the device It is connected to ground through a circuit including the primary winding 24 of an output coupling transformer 25, a resistor 26 and a suitable source of oper- Referring now to Fig. 1, high-frequency electrlcal waves are impressed upon the primary winding ill of an input coupling transformer II from any suitable source, not shown. While my invention is not limited thereto, it has particular utility when these waves are of relatively high frequencies, of the order of several megacycles ing device l4 represented as a pentode, and the other terminal connected to ground by a conductor 15. The primary and secondary windings l0 and I2 may be tuned to the mean operating frequency in any suitable manner. primary winding I0 is represented as being tuned by the distributed capacity of the transformer ating potential represented conventionally by the battery 21. The resistor 26 and potential source 21 are bypassed by a large capacitor 23. The secondary winding 29 of the transformer 25 is coupled to any suitableutilization circuit or load device, not shown. This may, forexample, comprise another stage of amplification similar to that represented in Fig. 1. The transformer 25 is tuned to the mean operating frequency in substantially the same manner as the trans former II, the total effective capacity across the primary winding 24 being represented by the dotted capacitor 30 and the total effective capacity across the secondary winding 29 being represented by the dotted capacitor 31. The secondary winding 29 is also preferably shunted by a damping resistor 33 to assist in obtaining a wide-band characteristic.

The upper terminal 34 of the resistor 26 is connected to the upper terminal 35 of the resistor 20 through a potentiometer resistor 36.

The screen grid 3'! of the pentode is connected to a tap 38 which is adjustable along the resistor 36. The tap 33 is preferably in the form of a sliding contact providing a smooth range of ad- Thus the justment between the

points

34 and 35. The screen grid 3'! is also bypassed to ground for operating frequencies by the capacitor 33 and is connected directly to the point 34 through a resistor 40.

The suppressor grid ll is internally connected istic at all settings.

to the cathode It in the type of pentode amplifier represented in. Fig. 1.

It is known that the input capacitance of an electron discharge amplifying device depends not only upon the tube .structure but also upon the operating potentials applied to the various electrodes. It has heretofore been common practice to vary only the control grid bias in order to change the over-all amplifier gain. This may provide satisfactory control in amplifying circuits operating at lower frequencies, where the input capacitance of the amplifieris negligible as compared to externalcircuit capacities. However,- serious difilculties have been encountered in attempts to apply this form of control to highfrequency' amplifying circuits, such as the intermediate frequency circuits of television apparatus, for example. The reason for this apparently lies in the fact that the input capacitance between the control grid and cathode of tubes available for this type of work may change by as much as one or two micro-microfarads as the control grid bias is varied between its normal operating value and a value corresponding to anode current cutofl. As the control grid bias becomes more negative the input capacitance decreases. In a high-frequency, wide-band amplifier of the type shown in Fig. 1 this effect is circuit in a sense to give increase relative low frequency response within the pass-band.

Fig. 2 illustrates typical response curves for a wide-band intermediate frequency amplifier for three different values of control grid bias, the other operating potentials remaining fixed. Assume first-that the amplifieris adjusted to have a fiat frequency response curve at one value of operating bias, as represented by the curve 58. Then as the grid bias is made more negative, the change in input capacitance renders the response curve unsymmetrical. Thus the dotted curve 6| is typical of the response obtained with one value of increased grid bias and the dotted curve 52 is representative of the response obtained with a still higher value of negative grid bias.

Conversely, it has been found that if the amplifier gain is varied by controlling the. screen grid potential, other operating potentials remaining constant, then the opposite type of detuning occurs. Thus if the curve 60 in Fig. 3 is taken to represent the response curve for a given value of screengrid potential, then the dotted curves 6| and 62 illustrate the effect of progressively 'decreasing the screen grid potential. It will be observed that these curves are almost exactly complementary to the curves 6! and 62 of Fig. 2,for certain potential values.

In accordance with my invention these two effects just described are combined in such a manner that a wide range of gain control may be effected while yet retaining a fiat frequency response characteristic, It will be observed that the

resistors

20, 26, 36, and 40 of Fig. 1 form the component parts of a voltage divider connected directly acros the potential source 21. It has been found that by a suitable choice of these resistances, with regard to the type of amplifier employed and the operating voltages and frequencies, it is possible to obtain a wide range of gain control by moving the tap 38 while yet maintaining a substantially symmetrical response character- Thus the curve 10 of Fig. 4 is representative of operating conditions with the tap 88 connected directly to the point 36, while tain particulars.

the curves 1| and 12 correspond to positions of the 16 tap l8 progressively nearer the point 36. The ad- Just-ment and operation of the circuits, whereby these results are achieved, will now be explained in greater detail.

First assume that the tap 38 is connected directly to the point 34. The control grid I8 is selfbiased by a predetermined value of anode and screen grid current flowing through the resistor 20. This self-bias is also assisted by a predetermined value of current flowing directly through the voltage divider resistors 26,. I6, and 20, the resistor 40 being short-circuited in thi position of the tap 38. Consequently, the total efiective bia upon the control grid I3 is determined by the total voltage drop across the resistor 20 due to all these currents. The potential between the screen grid 31 and cathode i8 is equal to the voltage drop across that section of the voltage divider between the tap 38 and the point 86, i. e., across the entire resistor 36 in this case.

Assume now that the tap 88 is moved toward the point to some intermediat point such as that represented in Fig. 1. It will be apparent that the resistor 40 is now connected in parallel to the right-hand portion of the potentiometer resistor 36 so that the current drawn through the voltage divider network is somewhat greater than before. In accordance with the invention the resistors are so proportioned that although the screen grid. potential now decreases, causing the anode and'fscreen grid currents through the amplifier l4 and resistor 20 to decrease, nevertheless the efiective bias potential between the control grid I3 and cathode I8 is caused to increase by virtue of the increased current drawn through the voltage dividing-network from the potential source 21. In other words, the circuit constants are so selected that the voltage divider current through the resistor 20 increases faster than the sum of anode and screen grid currents decreases. The proper values for these resistors are most easily determined by actual tests. Proper adjustment is secured when the detuning effect resulting from the decrease in screen grid potential is just compensated by a corresponding increase in the net bias potential applied to the control grid iii. In actual practice it has been found that most of the gain control action is accomplished by variation of the screen grid potential and that only a relatively small variation in th control grid potential is required to maintain the input capacity l8 substantially constant.

In the arrangement of Fig. 1 it will be observed that the screen grid potential eventually decreases to zero as the tap 38 reaches the point 35 and that the voltage divider current is a maximum for this adjustment, since the resistor 40 is then connected in parallel with the entire resistor 36.

The modified form of my invention represented in Fig. 5 differs from that of Fig. 1 only in cer- Corresponding elements have been designated by the same reference numerals and the function of these elements is essentially the same. Therefore they need not be repeated here. In this modification a slightly difierent typ of pentode amplifier I8 i shown wherein the suppressor grid 4| is externally connected to the grounded tube shield. However, the principal difference resides in the arrangement of the several sections of the voltage dividing network across the potential source 21. The section of this network between the cathode and ground, comprising the resistor 20, is the same as in Fig. l, but the section between the screen grid 31 and cathode is consists of a variable resistor 80, and the section between the screen grid 31 and the positive terminal of the power supply source 21 consists of a fixed resistor 8|. A separate decoupling resistor 82 is provided for the anode circuit of the amplifier in this embodiment of the invention, and a small filter resistor 83 is included in the connection between screen grid 3'! and tap 38,

The operation of the amplifier shown in Fig. 5

' Letters Patent of the United States, is:

does not difier essentially from that of Fig. 1.

As in the case of Fig. 1, it will be observed that the total resistance of the voltage divider network, measured across the three

resistors

20, 80 and BI, decreases when tap 38 is moved toward the left for the purpose of reducing the screen grid potential and thereby the gain. Likewise, the circuit constants are so proportioned that the tendency for the input capacitance to increase as the screen grid is made less positive is automatically.

compensated by a simultaneou variation of the total current through the resistor which causes the control grid l3 to become slightly more negatlve, and vice versa.

Any desired degree of compensation can readily be secured by properly proportioning the various circuit elements. It will also be appreciated by those skilled in the art that other equivalent forms of voltage divider networks maybe employed to provide the desired action.

For completeness of illustration only, and not in any sense by way of limitation, the following circuit constants are given as being typical for a high-frequency wide-band amplifier of the type represented in Fig. 5. These constants have been found to be satisfactory for the circuit elements of a signal amplifier adapted to operate over a frequency band of 42-48 megacycles. In this particular apparatus the pentode amplifier H was a type 7V7. Other circuit constants were as follows:

Resistor 20 ohms 4'7 Capacitor 2| mmf 3900 Capacitor 28 mmf 3900 Capacitor 39 mmf 3900 Resistor 80 ohms (variable) 20,000 Resistor 8| ohms 15,000 Resistor 82 do 470 Resistor 83 do 470 tials vary in the manner of my invention. 1

eliminate the need for any such mechanical complications by utilizing only a single movable contact member.

the potentials on the two difierent electrodes to vary in unison and to remain in proper ratio over a wide range of volume control adjustments.

While I have shown particular embodiments of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may be made, nd I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

This not only assures a quieter volume control action but automatically causes 1. In a system for amplifying currents having frequencies extending over a wide band, an amplifier including a pair of control electrodes, one of said electrodes being a current carrying electrode, a potential source, means for impressing operating potential on said one of said electrodes, said means comprising a connection from said one electrode to a variable point on said-source, means for impressing operating potential on the other of said electrodes, the gain of said amplifier being dependent upon both said potentials, means for varying the position of said point to vary the potential on said one electrode, and entirely electrical means responsive to variation in the position of said point for varying the potential on said other electrode in the same gain-varying sense, the potentials of said electrodes being varied in such amounts as to maintain the input capacitance of said amplifier constant.

2. In a system for amplifying currents having frequencies extending over a wide band, an amplifier including a pair of control electrodes, a potential source, means for impressing operating potential on one of said electrodes, said means comprising a connection from said one electrode to a variable point on said source, means for impressing operating potential on the other of said electrodes, the gain of said amplifier being dependent upon both said potentials and the frequency response characteristics of said amplifier being oppositely afiected by potential variations on the respective electrodes which produce changes of gain in the same sense, means for varying the positionof said point to vary the potential on said one electrode, and entirely electrical means responsive to variation in the position of said point for varying the potential on said other electrode in the same gain-varying sense and in such .relationship as to maintain the frequency response characteristic substantially constant.

3. In a high-frequency wide-band amplifying system, an amplifier having a cathode,.a pair of control electrodes and an anode, one of said electrodes being a current carrying electrode, input and output circuits operatively associated with said device, a source of current, means responsive to said current for impressing an operating potential on said one of said control electrodes, the gain of said amplifier being dependent upon said potential andalso upon the potential of said other control electrode, the input capacitance of said amplifier being oppositely afiected by potential variations on the respective electrodes which produce changes of gain in the same sense, means for varying said current to vary the potential on said one electrode and the amplifier gain, and electrical means responsive to said current variations for varying the potential on said other electrode in the same gain-varying sense and in an amount suflicient to maintain said input capacitance substantially constant.

4. In a high-frequency amplifying system, an electron discharge device including a cathode, an anode, an input electrode and a current carrying electrode, input and output circuits operatively associated with said device, means responsive to discharge current flowing in said device for im pressing a self-biaspotential on said input electrode, said means comprising an impedance com mon to said circuits, means for causing an additional current to flow through said impedance in a direction to assist said self-bias potential,

means for impressing operating potential between said current carrying electrode and cathode, and means for simultaneously varying said operating potential and said additional current in a predetermined relationship such that the net bias on said input electrode becomes more negative as said operating potential is decreased, and vice versa.

5. In a high-frequency amplifier including an electron discharge device having a cathode, a signal input electrode, a positive gain-control electrode and an output electrode, means responsive to current flowing in said device for impressing a negative bias voltage on said input electrode, said means comprising an impedance common to the input and output circuits of said device, means comprising a source of potential for passing an additional current through said impedance to provide additional negative bias voltage for said input electrode, means for impressing a positive potential on said gain-control electrode, means for varying said positive potential to adjust the gain of said device, whereby the input capacitance of said device tends to vary, and means for simultaneously varying said additional current in the opposite sense and in an amount sufiicient to produce a change in the net bias voltage on said input electrode opposing said tendency.

6. In a high-frequency signal amplifying system including a thermionic amplifier provided with at least a cathode, a control grid, a positive grid and an anode, means comprising a fixed resistor bypassed for signal frequencies and common to the control grid and anode circuits for impressing a self-bias on said control grid in response to current in said device, a source of potential, a voltage divider connected across said source, said divider having a plurality of resist- ,ance sections, one of said sections comprising said resistor and being connected in such polarity that current through said divider assists said self-bias, means for impressing an operating potential between said positive grid-and cathode in response to the voltage drop produced by said current in another section, means for varying the total resistance of said divider comprising means for varying the resistance of at least said other section, the resistance values of the sections of said divider being interrelated to produce an increase in the net control grid bias as said positive grid potential decreases and vice versa.

7. In combination with a high-frequency wideband signal amplifier including a cathode, control grid, screen grid and anode, a potential source having positive and negative terminals, a voltage dividing resistance network connected across said terminals and drawing a predetermined current, said network having a first section between said positive terminal and screen grid carrying screen current and said predetermined current, a second section between said screen grid and cathode carrying said predetermined current and a third section of fixed resistance between said cathode and negative terminal carrying said screen current, said predetermined current and the anode current of said device, means for biasing said control grid in response to the currents in said third section, means for varying the resistance of said second section, whereby the gain and input capacitance of said device are varied, and means for simultaneously varying the series resistance of said first and second sections in the same sense as said second section and in such ratio thereto that said predetermined current varies at a faster rate than the sum of said screen grid and anode currents, whereby an opposite and compensatory change in input capacitance is produced by variation of said control grid bias.

8. In a high-frequency signal amplifying system, a thermionic device having a cathode, a control grid, a screen grid and an anode, a fixed bias resistance connected between said cathode and a reference point, input and output circuits respectively connected from said control grid and from said anode to said point, a source of screen grid potential having a negative terminal connected to said point and a positive terminal connected to said cathode through a second resistance, a third resistance, connected between said screen grid and a connection point on said second resistance positive with respect to said cathode, a connection from said screen grid to a tap on that portion of said second resistance between said connection point and cathode, means for adjusting the position of said tap on said portion to vary the screen grid potential and gain 'of said device, whereby the input capacitance of said device tends to change, said resistances being proportioned to maintain the control grid bias automatically related to said screen grid potential for all adjustments of said tap in such ratio that changes in input capacitance are substantially prevented.

9. In a high-frequency signal amplifying system, a thermionic device having a cathode, a control grid, a screen grid and an anode, a fixed bias resistance connected between said cathode and a reference point, input and output circuits respectively connected from said control grid and from said anode to said point, a source of screen grid potential having a. negative terminal connected to said point and a positive terminal connected to said screen grid through a second resistance, a variable resistance connected between said screen grid and cathode, means for adjusting said variable resistance, thereby to vary the screen grid potential and the gain of said device,

whereby the input capacitance of said device tends to change, said resistances being proportioned to maintain the control grid bias automatically related to said screen grid potential for all adjustments of said variable resistance in such ratio that said changes in input capacitance are substantially prevented.

GEORGE W. FYLER.