US2373184A - Ultra high frequency vacuum tube circuit - Google Patents

Description

April 10, 1945. E. w. HEROLD ULTRA HIGH-FREQUENCY VACUUM TUBE CIRCUIT Filed May 28, 194s Okra/7 INVENTOR ATTOHIMIY Patented Apr. 10, 1945 ULTRA HIGH FREQUENCY VACUUM TUBE CIRCUIT Edward W. Herold, Kingston, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 28, 1943, Serial No. 488,913

12 Claims.

My present invention relates generally to vacuum tube circuits adapted for operation at such high frequencies that the input conductance of the tube is an appreciable factorin determining the response characteristic of the circuit.

In vacuum tube circuits operated at high frequencies, such as are involved in the reception and transmission of television signals, there is a finite inherent input conductance between the input electrode terminals (signal grid and cathode terminals) by virtue of the transit time effect and also because of the self-inductance of the cathode lead. The input conductance has a loading effect on the input circuit connected to the tube input terminals, and in the case of a tuned input circuit the result is a damping of the circuit and a loss of selectivity. It is desirable therefore to operate the tubewith an input conductance as low as possible.

It is now generally known that the self-in ductance of the cathode lead of an amplifier or converter tube, which is common to both plate and control grid circuits, represents a degenerative coupling between these circuits. This degeneration exhibits itself, when conventional tubes are used, as a resistance which appears in shunt with the input terminals of the tube and which decreases as the frequency increases. The 1 common cathode lead inductance may be due to the length of the internal cathode lead, or to the length of the connecting wire, or to both.

The effective input conductance is directly proportional, approximately, to this inductance. It depends also, approximately, on the square of the operating frequency. This form of input loading, due to the cathode-lead inductance, probably accounts for a considerable portion, if not actually the major portion, of the input loading in many commercial tubes (Proc. I. R. E., August 1938, page 1011).

It is, therefore, the principal object of my invention to provide an electron discharge tube circuit in which loading effects, due to the cathode lead inductance, are eliminated or substantially reduced.

A method which has been proposed for reducing input loading effects is the use of two cathode leads, described in U. S. Patent No. 2,118,122 to Van der Pol et al. An investigation or the use of two cathode leads is described in the U. S. Patent No. 2,256,293 to Salzberg who proposed by-passing the input circuit to one of these leads. and bypassing the output circuit to the other cathode lead thereby substantially eliminating coupling between them. In making use of such. a. circuit,

the patentee found that the by-passing of the screen-grid of a pentode to one of the cathode leads (the one common to the input circuit) led to an increased positive input loading due tothe coupling between the input and the screen circuit through this cathode lead. On the other hand, by-passing the screen grid to the other cathode lead (the one common to the plate circuit) caused the screen grid to have an A.-C. potential which led to regeneration or negative input loading. The latter state of affairs led to oscillation and other difficulties and was considered undesirable. As a result, Salzberg proposed the use of internal by-passing of the screen grid to the cathode inside the tube where the leads could be made extremely short. An alternative cure has been proposed involving the use of three cathode leads, wherein two are used as above while the third is used for the screen-grid return (Wireless World, March 16, 1939, page 246). Both the internal by-passing method and the use of a third cathode lead are costly methods of solving the problem of how to by-pass the screen grid.

Accordingly, it is another important object of my invention to provide a cheap and simple circuit whereby the screen grid may be by-passed to an easily adjustabl connection intermediate between the end points of the two cathode leads. By the proper adjustment of this connection, the input loading may be adjusted either positive, negative, or it may be made negligibly small.

Still another object of my invention is to provide a mean whereby feedback effects due to the leads to any electrode of a vacuum tube may readily be controlled and varied.

A further object is to provide in a high frequency circuit utilizing an electron discharge device having a plurality of electrodes, a pair of externally available connection points for at least one of the electrodes, the connection of an impedrnce between these points, and a high frequency connection from at least one other electrode to an intermediate point on said impedance in order to control feedback effects caused by the leads to the electrode provided with the external connection points.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims but the invention itself together with further objects and advantages will best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 illustrates a circuit, in elementary form, embodying the features of my invention. F 8. 2 is a modification of the circuit shown in Fig. 1, and Fig, 3 shows a circuit in which my invention is applied to another form of tube.

Referring first to Fig, 1 which discloses the basic features involved in the present invention, the circuit is seen to comprise a tube T provided with an indirectlyheated cathode K having the two leads Li, L: within the tube envelope G which in this instance is'shown to be of glass. Thetube is further provided with a signal or input control grid G1, a screen grid G2, suppressor grid G3, and an output or anode electrode P. An input circuit I is connected to the control grid G1 and, by means of the high-frequency by-pass C1, to the external connection point or terminal A of the cathode lead L]. which represents the selfinductance of this lead. The output circuit is by-passed by way of the condenser C2 to the second cathode lead La, whose external connection point or terminal is indicated at B. Terminals A and B are conductively connected externally by a wire L and the screen-grid by-pass condenser C3 is returned to a point on this external connection between A and B. Voltages appropriate to the operation of the tube may be applied through resistors i, 2 and 3 and lead 4 without afiecting appreciably the flow of high-frequency currents. The plate, screen and signal control grid voltages are represented respectively by Eb, Es and Fig. The cathode return or lead 4 may be connected directly to ground or through a self-biasing resistor-condenser network as well known'in the art.

It will readily be understood that the external connection L between A and B comprises an inductance and. by adjustment of the connection of condenser C3 to an adjustable point on this inductance, the screen grid alternating potential may be adjusted between that of point A and that of point B. The inductance of the connection between points A and B is preferably made larger than the inductance of Lror L2 in order that the output circuit return current passes mainly through L2. In practice, the connection from A to B may be simply a wire of somewhat greater length than the cathode lead length. Alternatively, it may consist of one or two turns of wire. The adjustment of the connection of the by-pass condenser C: to the conductor L between points A and B may be made experimentally until the input loading is made the value desired. Once this position has been determined, a permanent connection may be made. It is, of course, understood that after such an initial design isadopted, duplicates may be made, as in mass production, without experimental adjustment by duplicating the position of the connection. 7

It should be pointed out that the connections and by-passlng of other electrodes in the tube are frequently of great importance. For example, as shown in Fig. 1, the suppressor grid Go is connected to the output circuit return at point B. Inasmuch as the plate-to-suppressor grid capacitance may form an appreciable part of the output circuit, this may be of advantage. In the same way, the plate-to-shell capacitance of a metal tube or the plate-to-shield capacitance of a shielded tube may be an important part of the output circuit so that the shell M (Fig. 2) or shield S (Fig. 1) might also be connected to point B. This connection will not be present it the tube is an unshielded glass tube or an internally shielded tube whose shield lead is not ava'lable externally.- In some cases. however. it will be advisable to have the shell and suppressor grids at intermediate potentials between A and B. In this event, they may be connected to points along the external connection L between A and B as shown in Fig. 2 which discloses a circuit having complete flexibility in this regard for obtaining desired loading or feedback effects. In the circuit of Fig. 2 the tube T i of the type having a metal shell or envelope M and difiers from Fig. 1 only in the respect that the various connection to the impedance L are adjustably set at intermediate points between A and B in order to provide complete flexibility.

Although the circuits have been applied to a pentode, they are equally applicable to tetrodes, hexodes, heptodes or tubes of st.ll other constructions. It will be noted also that no reference has been made to the use of the tube to which the circuit is to be connected because it may be applied to amplifiers, detectors or oscillators. Furthermore, although the connection L between points A and B has been shown so that it behaves effectively as an inductance, it is clear that the principles of the invention will also be carried out if this connection is an impedance of any sort. For example, it may be in the nature of two capacitances in series whose intervening connection is used for the high-frequency return of grid G2, or it may be any other su table combination of inductance, capacitance and resistance.

In some tubes, as for example tubes in which the signal control grid is not directly adjacent to the cathode, the lead effects are more serious in the electrodes adjacent to the control grid than they are in the cathode lead. In such tubes it is, therefore, desirable to bring out the two connections to such an adjacent electrode rather than to the cathode. For example, Fig. 3 shows the application of the invention to a tube T" having a space-charge grid Go between the cathode K and the signal grid G1. The grid Go is usually of a sufliciently fine mesh that it substantially shields the cathode from effects due to potential variations of the other grids. Thus, as far as the signal grid G1 is concerned, the source of electrons appears to be the space-charge grid Go. Whereas in the tubes of Figs. 1 and 2, the cathode had two leads, in the tube of Fig. 3 it is the grid G0 which has two leads. The application of the invention i otherwise similar to the previous figures except that, wherever R. F. returns were made in Figs. 1 and 2 to the cathode circuit L joining points A and B, they are now made to the space-charge grid circuit L joining points A and B of Fig. 3. The cathode K of Fig. 3 may be connected by means of the high-frequency by-pass C4 to a point on L which will not usually be critical as to position since the cathode in this system will ordinarily play a relatively minor role;

While I have shown and described certain preferred embodiments of my invention, it will be understood that various modifications and changes will occur to those skilled in the art without departing from the spirit and scope of this invention. I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I'claim is:

1. A high-frequency circuit comprising an electron discharge device having a plurality of electrodes at least one of which has two externally available connection points, an impedance connected between said points, and a high-frequency connection from at least one other electrode to an caused by the leads to the first-mentioned electrode.

3. A circuit comprising an electron discharge device having a cathode, a signal grid, an output anode, a grid interposed between cathode and signal grid, said interposed grid-having a pair of leads each terminating in an external terminal, and at least one other electrode, an impedance connected between said terminals, and means whereby the high-frequency currents flowing in the various electrodes may be by-passed to intermediate points on said impedance.

4. In conjunction with a high-frequency ampliher which includes an electron discharge tube having a plurality of electrodes, at least one of which has two externally available coimection points between which there is connected an impedance, the method of reducing high-frequency potential differences between another electrode and the one having two connection points, which consists in by-passing high-frequency currents in the circuit of said other electrode to an intermediate point on said impedance.

5. In conjunction with a high-frequency ampliiier tube which includes a signal grid, a cathode which has at least two externally available connection points between which there is connected an impedance, at least one other grid and an output anode, the method of reducing feedback which consists in by-passing high-frequency currents in the circuit of said other grid to an intermediate point on said impedance.

6. A high-frequency circuit which includes an electron discharge device and which permits the control of feedback efiects caused by the inductance of the leads to the electrodes of said device, wherein two leads from the two ends of an impedance are connected to an electrode and wherein the high-frequency currents flowing in the circuits of other electrodes are by-passed to points along this impedance.

7. A high-frequency circuit comprising an electron discharge tube provided with a cathode, a

signal control grid, at least one other grid and 8. A high-frequency circuit comprising an electron discharge tube provided with an indirectly heated cathode, a plurality of grids and an anode, a pair of leads having appreciable reactance at the operating frequency connected at one end to the cathode, a conductive connection externally of the tube between the other ends of said cathode leads, an input circuit connected between one of the grids and one end of said conductive connection, an output circuit connected between the anode and the other end of said conductive connection, and means for adjustably connecting as far as high frequency currents are concerned the remaining tube electrodes to said conductive connection.

9. A high-frequency circuit comprising an electron discharge tube provided with an irrdirectly heated cathode, a signal control grid, a suppressor grid and an anode, said cathode having a pair of leads connected thereto atone end, the opposite ends of said leads providing external terminals, a conductor connected between said external cathode terminals, an input circuit connected between the signal grid and one of the external terminals, an output circuit connected between the, anode and the other external terminal, a connection between the suppressor grid and the latter terminal, and means for variably connecting for high frequencies the screen grid to an intermediate point on said conductor connecting the cathode terminals.

10. A high-frequency circuit comprising an electron discharge tube having a metal envelope and provided with an indirectly heated cathode, a plurality of grids and an anode, said cathode having a pair of leads connected thereto at one end thereof, the opposite ends of said leads providing external terminals, a conductor connected between said external cathode terminals, an input circuit connected between one of the grids and one of the external terminals, an output circuit connected between the anode and the other external terminal, and variable connections from the remaining tube electrodes and the metal tube envelope to points along said conductor connecting the cathode terminals.

11. A high-frequency circuit as defined in claim 10 wherein the input and output return circuits have variable connections on the conductor which is connected between the external cathode terminals.

12. A high-frequency circuit comprising an electron discharge tube provided with a cathode, a space-charge grid. a signal control grid, at least one other grid and an anode, said spacecharge grid having a pair of leads, a conductive connection between the ends of said leads, an input circuit connected between the signal control grid and one end of said conductive connection, an output circuit connected between the anode and the other end of said conductive connection, and means for providing a high-frequency connection between said other grid and an intermediate point on said conductive connection,

EDWARD W. HEROLD.

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