US2237420A - Amplifier gain control circuit - Google Patents

Description

Aprl 8, 1941. w. R. FERRls 2.237.420 v AggpAIEIERA-IN YCONTROL.' CIRCUIT Filed June 4, 1938 A TTORNE Y.

Patented Apr. 8, 1941` OFFICE MWLIFIER GAIN CONTROL CIRCUIT Warren R. Ferris, East Orange, N. J., assigner to Radio Corporation of America, a corporation of Delaware Aminos-uen .rune 4, 1938, serial No. 211,724

6 Claims.

`ll/iy present invention relates to gain control circuits for wave amplifiers, and more particularly to a novel method of, and means for, varying'the gain of a Wave amplier without aifecting the input capacity thereof.

One of the main objects of my present invention is to eliminate shift in the input capacity of a wave amplifier While adjusting the gain thereof; the capacity shift being eliminated by controlling the amplifier electrode voltages in such a manner that the ratio of the transit time between cathode and control grid and that between control grid and the following electrode has a constant value.

Another important object of this invention is to provide a method of varying the gain ci an amplier which includes the step of maintaining the plate voltage to effective grid bias voltage ratio constant thereby to keep the input capacity value constant.

. Another object of my invention is to provide a method of eliminating change in the input capacity of an amplier as the voltage of a positive electrode is varied by a factor N; the method comprising varying the grid voltage until the plate current varies by NW2.

Still other objects of my invention are to provide an amplifier of the screen grid type wherein gain control is had by adjustment of the control grid and screen grid voltages in such relative senses that the change in gain is effooted without varying the amplier input capacity.

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, as to both its organization and method oi operation will best be understood by reference to the following description taken in connection with the drawing in which I havel indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

, Referring now to the drawing, there is shown a tube l of the screen grid type. The input circuit 2 thereof comprises a coil 3 and condenser li which are resonant to a desired Wave frequency. The coil 3 can be coupled to any desired source of signal Waves; for example, these waves may be in broadcast band of 500 to 1550 kc. The present invention is employed with advantage in the 6o to 20G megacycle band. The tube l includes a cathode 5, a signal control grid li, a screen grid 'l' and a plate 8. The signal output circuit 9 may feed further amplifier stages; in fact, the use of a large number of amplifier stages is desirable with this arrangement since the amplification in each stage varies only as the square root of the applied direct current voltage.

i plier.

Thenumeral lll denotes a source of direct current employed to energize the various electrodes; the resistor Il is shunted across source li) to provide a low resistance bleeder. The cathode 5 is connected to an intermediate ground point on the bleeder, While plate 8 may be connected to the positive end thereof. Condensers l2, i3 and I4 are the usual radio frequency bypass condensers. The control grid E is connected to the high alternating potential side of input circuit 2, While the low potential end of the input circuit 2 is connected to a tap l5 which is slidable usually along that portion of bleeder il which is negative with respect to ground.

The control grid voltage is denoted by the symbol Eg. The screen grid l is connected by tap I6 to any desired point of bleeder Il which is positive with respect to the grounded cathode. The symbol Es denotes the voltage applied to the screen grid electrode. The control grid to cathode capacity Ci, or input capacitance, is shown by dotted lines, and is eifectively in shunt to the tuning condenser il. The control grid to screen grid capacity Co is shown by dotted lines. The fundamental improvement in gain control provided by this invention resides in the fact that the frequency of input circuit 3-4 is independent of amplifier gain adjustment if the ratio of Eg to Es is kept constant; this follows from the fact that in the latter case Ci-l-Co is constant.

It is Well known that the input and the output capacity of an electron discharge tube depends upon both the geometry and the potential distribution of the elements. The usual volume control circuits operate by applying a variable negative bias to the signal control grid of an amplifier tube, usually with the other voltages held substantially constant. This results in a serious change in the input capacity of the am- For example, in the case of a 57 type tube, the change in input capacity is about 1 mmf., as the bias is varied from its normal value to cut-olf. This necessitates the use of large padding condensers in receiver circuits, which reduce the possible gain to less than onehalf of its value if such padding were not necessary. Particularly in the case of receivers equipped with automatic volume control is the change in input capacity of the controlled tube a disadvantage. In the case of the high transconductance tubes intended for television use the change in capacity is several micro-microfarads.

It can be demonstrated that the change in input capacity of a tube of the screen grid type may be expressed by the following relationship:

Cc r. n 1 ^C[1+4n 1k+1+3o+1 2)] where AC is the change in capacity from` its value at cut-off to its value at any given set of operating voltages; T2 is the transit time from control grid to screen grid; T1 is the transit time from cathode to control grid; 1c is the square root of the ratio of the effective voltage at the screen grid to that at the control grid; and Cc is the capacity between such sections of control grid and cathode as are in the electron stream.

It will, therefore, be seen from the above relationship that if the ratio of the effective voltage at the screen grid to that at the control grid is kept constant, the ratio is also constant, and therefore AC is maintained constant. This is realized, as shown in the drawing, by varying the voltages applied to the screen grid and control grid in such a manner as to satisfy the relations noted. It is assumed that the contact potential of the control grid is zero, or has been balanced out by a Xed bias battery or similar means. As a matter of fact, an auxiliary bias battery can be employed in the signal grid circuit to balance out the effect of contact potential, the initial velocity of electrons, thermo-couple eiTect-s, and gas ionization effects. With such effects balanced out, the eiective grid bias voltage of the grid 5 is then the value E.

The present gain control arrangement is applicable to any amplifier wherein the amplier tube employs a cathode, signal grid and at least one positive electrode adjacent the signal grid. The essential relation involves maintaining the positive electrode voltage to eiective grid bias voltage ratio constant thereby to maintain the input capacity constant. This is readily shown to be the case by observing the input capacity values and positive electrode current under a given set of conditions, and then increasing the positive electrode voltage by a factor N. The control grid voltage is then varied until the positive electrode current increases by N3/2. It can, also, be stated that the input capacity increases with the ratio of T2 to T1, these being the transit times noted above. In actual practice, assuming the input circuit 2 of the amplifier to be tuned to a given signal frequency, the gain of tube I is varied by adjusting taps I5 and I6 in such a way that the ratio of the transit time between cathode and control grid and that between control grid and the next positive electrode, that is the plate or screen grid, shall have a constant value. If the tap I il is adjusted away from the grounded cathode tap, then. the slider I6 is adjusted away from the cathode tap. The gain is increased with such adjustment whereas moving them towards the cathode decreases the gain. If both taps were set positive with respect to the cathode, adjustment of both taps to the right of the cathode increases gain without change in input capacity.

After the range of gain control which is desired has been decided upon, and assuming that adjustment of the control grid tap I5 is being depended upon for gain control variation, the adjustment of tap I6 can be made conjointly with adjustment of tap I 5, and in accordance with the relationships given above, so as to maintain the tuned input circuit independent in frequency of the amplication control.

While I have indicated and described a system 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 modications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. A method of controlling the gain of an alternating current amplifier of the type including a tube provided with at least a cathode, a control grid and a positive electrode, the method including the steps of varying the direct current voltage of the positive electrode, varying the voltage of the control grid in a sense 'such that the ratio of the positive electrode voltage t0 effective grid bias voltage is substantially constant whereby the capacity between control grid and cathode is constant.

2. A method of controlling the gain of a wave amplifier which includes varying the direct current voltage of a positive electrode of the amplifier Which has a predominating eiect on the space current ow of the tube, and varying the direct current voltage of the control electrode of the tube in a sense such that the ratio of the positive electrode voltage to effective control electrode bias voltage is substantially constant whereby the input capacity of the amplier is maintained constant.

3. In a wave amplifier of the type including a tube provided with at least a cathode, a wave input grid, a positive screen grid and a plate, the method which includes the steps of varying the screen grid and input grid direct current voltages in such relative senses as to maintain the ratio of the electron transit time between the cathode and input grid and that between input grid and screen grid substantially constant.

4. In a wave amplier of the type including a tube provided with at least a cathode, a signal grid and a positive electrode adjacent the signal grid which has a substantial eifect on the electron flow, the method of eliminating the input capacity shift of the amplifier during adjustment of the gain thereof which includes adjusting the grid and positive electrode voltages in such relative senses that the ratio of the transit time between cathode and signal grid and that between signal grid and the positive electrode has a constant value.

5. An amplifying system comprising a tube having a cathode, an anode, a control grid and a screen grid, a wave input circuit connected between the control grid and cathode, a wave output circuit connected between the anode and cathode, and means for controlling the gain of said tube without affecting the input capacitance between the control grid and cathode of the tubes, said means comprising a direct current voltage circuit for applying control voltages to the screen grid and control grid, and the control grid and screen grid voltages being constantly related'to prevent said input capacity shift.

6. In an amplifying system having a tube provided with at least a cathode, a control grid, and a positive electrode, a means of controlling the gain comprising a device arranged to vary the control grid and positive electrode voltages in such relative senses that an increase in the positive electrode voltage of N-fold is made to produce an increase in the positive electrode current of substantially NW2-fold thereby to prevent any change in the input capacity of the tube.

WARREN R. FERRIS.

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