US2525632A - Low-frequency amplifier - Google Patents

Description

Oct. 10, 1950 w. A. ANDERSON LOW-FREQUENCY AMPLIFIER Filed April 18, 1946 INVENTOR WARREN A. ANDERSON ATTO RN EY indicated at +32.

i atentecl i0,

LOW-FREQUENCY AMPLIi I'fifi Warren A. Anderson, West Brighton, N. Y as signor to Radio Corporation of America, a corporation of Delaware Application April 18, 1946, Serial No. 662,978

3 Claims.

This invention relates to low frequency power amplifiers, and more particularly to improvements which would stabilize and improve the operation of such amplifiers. a

In carrying out my invention I have utilized the inherently high plate resistance of a pentode discharge tube as an impedance to couple to a load the output energy from a second discharge tube operating as a low frequency amplifier. A combination of several resistors and a capacitor is used to stabilize and improve the operation of the amplifier circuit including the coupling impedance.

From the foregoing it will be understood that the principal object of my invention is to stabilize and improve the operation of a low frequency power amplifier. Other objects and advantages of my invention will be made apparent in the description to follow. This description is accompanied by a drawing the sole figure of which represents a preferred circuit arrangement.

Referring to the drawing, I show therein two pentode discharge tubes V1 and V2. In the tube V1 is an anode I connected to a main direct current source the positive terminal of which is The cathode 2 and the suppressor grid 5 are connected through a resistor R1 to an anode 6 in tube V2. This tube also possesses a cathode l, a control grid 8, a screen grid 9 and a suppressor grid Ill. The cathode 'l and suppressor grid II] are connected through a resistor R2 to ground which may be considered th negative terminalof the main direct current source.

Suitable screen grid potential may be obtained from taps on the main direct current source or from a voltage divider which is connected across the terminals of this source. For convenience, however, the screen grid 4 is shown connected to the positive terminal of an independent source II the negative terminal of which is connected to the anode 6 of tube V2. The screen grid 9 is indicated as being connected to the positive source terminal B1 which is of lower voltage than that of the terminal B2.

Input signals are applied to terminals l2, one of which is grounded and the other of which is coupled across a capacitor [3 to the control grid in tube V2. The potentials of the input signals :are designated em. Output signals are designated em and are impressed across any suitable load .or utilization device indicated by the component marked Z. This load is coupled to the output .circuit of tube V2 across a capacitor [4.

The control grid 8 in tube V2 is connected to ground through a resistor I5. The control grid 3 in tube V1 is suitably biased by means of a voltage divider consisting of resistors R3, R4 and R5, where a tap on resistor R4 constitutes this element as a potentiometer and the tap itself is connected to the grid 3 through a resistor R6. The grid 3 is also coupled through capacitor C1 to the anode 6 in tube V2.

Cathode resistors R1 and R2 are included to increase the linearity of the characteristics of tubes V1 and V2, respectively. In some cases, however, these resistors may be omitted. The choice of a pentode type tube V1 as the coupling impedance may be attributed to the inherently high plate resistance of such a tube and to its relatively low impedance to direct current. In this way a minimum of direct current power is wasted in the coupling impedance and a maximum of alternating power is dissipated in the load impedance Z. On the other hand, if the plate resistance of a triode could be made sufiiciently high with respect to the load impedance Z, say, by a factor of 10, then equally satisfactory operation might be expected.

Since the distribution of the plate supply voltage between tubes V1 and V2 is all important when operating tubes in series, and since minor variations in tube characteristics are unavoidable, particularly in the case of pentodes, wide variations in this distribution may be compensated by the use of stabilizing resistors R3. R4, R5 and Re and capacitor C1. The resistors maintain the grid of the coupling impedance tube at a constant D.-C. potential. Thus, if any variation in the plate voltage distribution between the tubes occurs, it automatically produces a change in the grid-to-cathode voltage of the coupling tube which tends to compensate for the original change in the voltage distribution. Condenser C1 causes the grid of the coupling tube to follow the signal excursions of its cathode and therefore to behave in its normal fashion.

I claim:

1. An electronic amplifier comprising two discharge devices each having a cathode, an anode and three grids, namely a control grid, a screen grid and a suppressor grid, means including a direct current source and two circuits connected across its terminals for activating said devices, one of said circuits being arranged to traverse the space paths of said devices in series, the other of said circuits constituting a voltage divider, said divider having an adjustable tap thereon connected to the control grid of the device at the positive end of the series circuit, means appropriate to each device for positively biasing its screen grid with respect to its cathode, the cathode and suppressor rid in each device being interconnected, means for applying input signals to the control grid of said device at the negative end of the series circuit, and means coupling a load to the anode and cathode of the last said device.

2. An amplifier according to claim 1 and including a capacitor in circuit between the grid of the device at the positive end of the aforesaid series circuit and the anode of the other device.

3. In a low frequency power amplifier, two electron discharge devices each having electrodes including at least an anode, a cathode, a control grid,a screen grid and a suppressor grid connected to the cathode, input terminals on which the low frequency voltage to be amplified may be impressed, a coupling between said input terminals and the control grid and cathode of one device, a source of direct potential having a negative terminalconnected to the cathode of said one device and a positive terminal connected to the anode of the other device, a resistor connecting the anode of said one device to the cathode of the said other device, an output circuit coupled across the anode to cathode impedance of said one device, a biasing circuit including a resistor connected between the control grid and cathode of said one device, means for. maintaining the screen grids of said devices electropositive relative to their cathodes, apparatus for applying a substantially constant biasing potential to the control grid of the other device comprising a voltage divider in parallel with said source of direct potential, said voltage divider including a potentiometer resistor with a point thereon connected through a voltage dropping resistor to the control grid of said other device, and means ap plying to the control grid of said other device a potential which follows the potential set up on the cathode of said other device by virtue of its connections to the anode of the one device, comprising a capacitor coupling the anode of said one device to the control grid of the other device.

WARREN A. ANDERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,305,919 Eaton Dec. 22, 1942 2,326,614 Bowman Aug. 10, 1943. 2,428,295 Scantlebury Sept. 30, 1947 FOREIGN PATENTS Number Country Date 540,834 Great Britain Oct. 31,1941

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