US1959165A - Electron discharge amplifier - Google Patents

Description

May 15, 1934. J. KAAR 1,959,165

ELECTRON DISCHARGE AMPLIFIER Filed Oct. 26, 1932 Fig. 1.

Inventor: Ira J. Kaar,

by Mu /L H is Attorney.

Patented May 15, 1934 UNITED ST ELECTRON DISCHARGE AMPLIFIER Ira J. Kaar, Schenectady, N. Y., assignor to General Electric Company York , a corporation of New Application October 26, 1932, Serial No. 639,630

8 Claims.

My invention has for one of its objects to provide an arrangement for preventing self-oscillation or reaction between the anode and grid circuits in an ultra high frequency amplifier by neu-- tralizing the eiiect of the inherent reactances of the cathode, the screen grid element and the leads connected thereto.

In accordance with my invention I provide means for maintaining the screen grid and the cathode at the radio frequency potentials under which condition maximum advantage is derived from the screening action of the screen grid.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My in- '20 vention itself, however, both as to its organization and method of operation together with further objects and advantages thereof, will be understood best by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 discloses a preferred embodiment of my invention, and Figs. 2 and 3 are explanatory illustrations of my invention.

Referring to Fig. 1 of the drawing, I have illustrated therein a portion of a high frequency amplifier including an electron discharge device of the type commonly used in high and ultra high frequency transmitters. It is of course to be understood that my invention is also applicable to high and ultra high frequency amplifiers of low power such as are commonly used in radio receiving apparatus. The electron discharge device 1 is provided with a filamentary cathode 2, a control grid 3, ascreen grid 4 and an anode 5. The screen grid 4 is provided with the usual lead thereto extending throughone terminal of the electron discharge envelope adjacent to the leads connected to the filamentary cathode 2. A second lead is connected to the screen grid at the other extremity thereof and this extends to a terminal 6 located at one side of the envelope. This terminal is usually connected through a large capacitor (not shown) to the cathode and ground. The amplifier is provided with a suitable input device or transformer 7, the secondary of which iscOnnected at one terminal to the control grid 3, and at the other terminal to a suitable source of grid biasing voltage which, in turn, is connected to the cathode and ground. The output circuit of the amplifier is provided with a suitable output device or transformer 8, the primary of which is connected at one terminal tothe anode 5 and the other terminal of which is connected to the positive terminal of a suitable source'of anode voltage. The filamentary cathode 2 may be energized by means of a transformer 9 which is connected to a suitable source of alternating current. The screen grid 4 is supplied with positive voltage from a convenient source. A plurality of capacitors 10 and 10a are connected in series across the connections to the cathode. The point between the capacitors is connected to the midpoint on the secondary of the transformer 9 and this is maintained at ground potential.

The amplifier as described is not suitable for amplification of frequencies above approximately 20 megacycles. This amplifier depends upon its freedom from self-oscillation or anode circuit to grid circuit reaction upon the screen grid 4 which is interposedbetween the control grid 3 and the anode 5, and which is supposedly maintained at cathode potential for all frequencies. It has been found, however, that sufficient reactance exists along the length of the filament, the leads to the filament and the screen grid, and the screen grid itself, to permit the screen grid to assume potentials during high frequency operation which will depart from the cathode potential. This results in ineffective shielding and in unstable operation of the electron discharge device, thereby causing self-oscillations or giving rise to degenerative or regenerative coupling between the anode and control grid circuits. This is caused bythe grid circuit to plate circuit coupling which exists when the screen grid is not maintained at the same po tential as the cathode at the operating frequencies. I

In accordance with my invention, the screen grid lead-in adjacent to the conductors is main-' tained at alternating current ground and cathode potential by means of a'capacitor 11 which is connected to the ground and to the cathode. A variable reactor 12 which may be either an inductor or a capacitor is connected between the ground and cathode, and the terminal 6 which is the connection to the other extremity of the screen grid 4. In order to compensate for the inherent reactance in the elements of the electron discharge device and the leads thereto, the reactor 12 is adjusted so that its reactance value is equal and opposite to the algebraic sum of the reactances of the elements and leads at the operating frequency. This, however, does not imply that the series circuit comprising the cathode to screen grid capacity, leads and external capacitor 12 are all tuned to series resonance, as

this adjustment results in maximum difference in potential between the cathode and screen grid, and this condition is to be avoided.

Reference may now be had to Fig. 2 wherein I have diagrammatically shown the cathode 2, the screen grid element 4 and the capacitance A existing between these elements. The reactance of the cathode structure itself is indicated by the rectangle B connected in series with the cathode and the reactance of the leads to the cathode which is indicated by the rectangle 0. Usually these reactances are inductive in nature, but they may be capacitive. The reactance of the screen grid structure and the reactance of the screen grid leads are shown as rectangles D and E connected in series with the screen grid 4. These two systems are connected together by the capacitor 11. It will be obvious that if the sum of reactances of the circuit reactances B, C, 11, D and E is not zero, current will pass through the capacitance A, and a difference of potential will exist between the cathode and screen grid. If the loop circuit composed of the reactances A, B, C, 11, D and E is zero, then a maximum current will pass through the capacitance A and a maximum difference of potential will exist between the cathode and the screen grid. These conditions must be avoided if stable operation of the amplifier is to be obtained.

In accordance with my invention, a variable reactor 12 is inserted at any point in the loop circuit external to the discharge device, as for example a point between the capacitor 11 and the screen grid reactances D and E as shown in Fig. 3. When properly adjusted, this reactor 12 serves to cancel the effect of the reactances existing in the circuit between the cathode 2 and the screen grid element 4 by way of circuit B, C, 11, D and E. The reactor 12 is adjusted so that the algebraic sum of the reactances B, C, 11, 12, D and E is zero; thus no current will flow through the capacitance between the cathode and screen grid. Under this condition the screen grid is maintained at the same radio frequency potential as the cathode and the desired stable operation of the amplifier will be obtained.

This arrangement permits the amplifier to operate at ultra high frequencies and it is equally applicable to transmitting and receiving circuits.

While I have shown and described my invention in connection with certain specific embodiments, it will of course be understood that I do not wish to be'limited thereto since it is apparent that the principles herein disclosed are susceptible of numerous other applications, and modifications may be made in the circuit arrangements or in the instrumentalities employed without departing from the scope and spirit of my invention as set forth in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. The combination with a high frequency amplifier including an electron discharge device having a cathode, a screen grid element, and leads connected thereto, of means connected between said cathode and said screen grid element for introducing a reactance having a value equal and opposite to the algebraic sum of the reactances of said leads, said cathode and said screen grid at the operating frequency of the amplifier whereby the maximum effect of said screen grid element is obtained.

2. The combination, in a high frequency amplifier, of an electron discharge device having a cathode, a screen grid element, and leads. connected thereto, and a reactor connected between said screen grid element and said cathode having such a value of reactance that the inherent reactance of said cathode, said screen grid and the leads connected thereto is neutralized at the operating frequency of the amplifier.

3. The combination, in a high frequency amplifier including an electron discharge device having a cathode, a screen grid element, and leads connected thereto, said screen grid element being provided with connections at either extremity, of means connected between one extremity of said screen grid element and said cathode for maintaining said extremity at radio frequency cathode potential, and means connected between said cathode and said other extremity of said screen grid element having reactance opposed to the inherent reactances of said cathode, said screen grid and the leads connected thereto, and of value sufficient to neutralize said inherent reactances.

4. The combination, in a high and ultra high frequency amplifier including an electron discharge device having a cathode, a screen grid element, and leads connected thereto, said screen grid element being provided with connections at either extremity, of means connected between one extremity of said screen grid element and said cathode for maintaining said extremity at the same operating voltage as said cathode, and means connected between said cathode and said other extremity of said screen grid element for maintaining said screen grid at the same operating potential as the cathode at the operating frequency, said last means having a reactance opposite to, and sufiicient for neutralizing the algebraic sum of, the reactances of said cathode, said screen grid element and said leads connected thereto.

5. The combination, in a high and ultra high frequency amplifier including an electron discharge device having a cathode, a screen grid element, and leads connected thereto, said screen grid element being provided with connections at either extremity, of a capacitor connected between one extremity of said screen grid element and said cathode, and a reactor connected between the other extremity of said screen grid element and said cathode, said reactor being arranged to provide at the operating frequency of said amplifier a reactance having a value sufficient to neutralize the algebraic sum of the reactances of said cathode, said screen grid element and said lead connected thereto whereby said screen grid element is maintained at the same radio frequency potential as the cathode at the operating frequency of the amplifier.

6. The combination, in a high frequency amplifier including an electron discharge device having a screen grid element, a filamentary cathode, and leads connected thereto, said screen grid element being provided with leads at either extremity, of a by-pass capacitor connected between one extremity of said screen grid element and said cathode, and a variable reactor connected between the other extremity of said screen grid element and said cathode, said variable reactor being arranged to provide at the operating frequency of said amplifier a reactance having a value equal and opposite to the algebraic sum of the reactances of said cathode, screen grid element and said leads connected thereto whereby the maximum effect of said screen grid element is obtained.

7, In a high frequency amplifier having an tion of an electron discharge amplifier having a cathode, a screen grid element, and leads connected thereto, which includes maintaining one end of said screen grid element at substantially radio frequency cathode potential, and maintaining the algebraic sums of the reactances of said leads, said cathode and screen grid elements at the operating frequency equal to zero.

IRA J. KAAR.

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