US2118122A - Amplifying system - Google Patents

Description

May 24, 1938. B. VAN DER POL ET AL 2,118,122

AMPLIFYING SYSTEM Filed Nov. 17, 1935 6 El 4 X INVENTOR BALTHASAH VAN DER POL f KLAA P STHUMUS lzrpuf BY Circa/f 7 ATTORNEY Patented May 24, 1938 UNITED STATES PATENT OFFICE AMPLIFYING SYSTEM Ware Application November 17, 1933, Serial No. 698,486 In the Netherlands January 26, 1933 1 Claim.

This invention relates to amplification circuit arrangements comprising thermionic amplifying valves having one or more grids, and particularly to the manner in which the various circuits associated with the amplifying valve such, for example, as the control grid and anode circuits of a triode or the control grid and screen grid circuits of a screen grid valve are connected to the cathode of the amplifying valve.

A well known phenomenon which appears with triodes and with screen grid valves having a great steepness of the characteristic, particularly when the anode circuit of the triode or the screen grid circuit of the screen grid valve includes an impedance which is low for high frequency currents, consists in that there is a certain conductivity between the control grid and the cathode which is dependent upon the negative grid voltage and which in the case of the control grid and the cathode having an oscillatory circuit included between them, brings about a supplementary damping in the circuit. This conductivity obviously has the same effect as a resistance in parallel with the input oscillatory circuit and would cause a damping thereof. Simultaneously with this conductivity a grid-cathode capacity which is also dependent upon the negative grid voltage and which is to be imputed essentially to variable space charge phenomena, occurs between the control grid and the cathode.

The invention has for its object partly or wholly to neutralize or to balance the said phencmena, via, the decrease in control grid-cathode resistance and the control grid-cathode capacity that is dependent upon the negative bias.

According to the invention, this object is obtained by so proportioning the impedance of the part of the anode-cathode connection in the case of a triode or the part of the screen grid-cathode connection in the case of a tetrode which is in common to the control grid-cathode connection that the conductivity between the control grid and the cathode and/or the control grid-cathode capacity that is dependent upon space charge phenomena are neutralized partly or wholly whereby the detuning of the resonant input circuit by these factors is substantially eliminated.

In order that the invention may be clearly understood and readily carried into eifect some embodiments thereof will now be described more fully with reference to the accompanying drawing in which:

Fig. 1 illustrates diagrammatically a triode and the various impedances associated with its ele ments when it is connected in a circuit;

Fig. 2 is a diagram of the impedances illustrated in Fig. 1

Figs. 3 and 4 show circuit diagrams for connecting a triode in accordance with the invention; and

Fig. 5 is a circuit diagram of another modification of the invention and Fig. 6 shows a circuit diagram of a screen grid amplifier embodying the 1 invention.

Fig. 1 shows by Way of example, an indirectly heated triode comprising an electrically heated filament F, a cathode K, a grid G and an anode A. These electrodes are connected by means of connecting wires to contact pins I, 2, 3, 4 and 5 I fixed in the base. The said connecting leads include impedances designated by 29. and Zr. In addition, the internal tube capacity Cat, Cgf and Cag are indicated in this figure. These impedances (for example the inductances of the connecting wires) and the capacities which are al 20 ways present bring about a mutual coupling between the anode and the control grid circuit. Owing to this mutual coupling a certain apparent admittance is set up between the terminals 3 and 4. The impedance Zr, which may be the selfinductance of the lead in wire to the cathode is also in the anode circuit and the anode current being in phase with the grid voltage, causes a voltage drop over the self-inductance of the cathode .lead which lags behind the applied input grid voltage by an angle of substantially degrees. Between the input terminals of the valve, there is connected the series circuit of the cathode-grid capacity and the self-inductance of the cathode lead. As the cathode-grid capacity is relatively small, this series circuit has a substantially capacitive character. The voltage drop over the cathode lead will therefore cause a current to flow through this series circuit which is in phase with the input voltage, so that the effect produced on the input circuit is that of a shunt resistance, i. e. of an additional damping. In order to establish this it is supposed that in regard to high frequency currents the terminals 3 and 5 are perfectly short-circuited with each other and that a high frequency alternating voltage having an angular velocity 01 is supplied between the terminals 3 and 4. From the replacement diagram shown in Figure 2 of the con nections of Figure l the apparent admittance present between the terminals 3 and 4, can be computed. Subject to the condition that the 55 impedances Z5, and Zr are small relatively to the impedance of the internal electrode capacities of V the tube it is found that wherein S, or the mutual conductance, equals the 'and Li it is found that the apparent admittance between the terminals 3 and 4 is represented by from which it is clear that this admittance behaves as if an ohmic resistance dependent upon the frequency is interconnected between the terminals 3 and 4.

From" this latter equation it is apparent that the resistance between the terminals 3 and 4 is composed of a positive and a negative part. If the former partis larger than the latter, Rg will be a positive magnitude so that in the case of a tuned circuit being connected between the terminals 3 andA, as shown in Fig. 1, said circuit is clamped by the resistance Rg, which denotes the apparent shunt resistance across the input circuit. In addition it is apparent from the equation that the greater the steepness S of the amplifying valve characteristic and the frequency of the alternating voltage impressed on'the grid, the larger is the said damping. For this reason it was hitherto impossible to use amplifying valves with very great steepness of characteristic in high frequency amplifiers, since what could be won in amplification by the greater steepness was invalidated by the greater damping. It is thus desirable that the apparent resistance'between the grid and the cathode should be rendered a maximum which according to the invention is ensured by minimizing the inductance L: which, as is shown in Fig. 2 when the impedance Zr is replaced therein by aninductance Lf, couples the anode circuit to the grid circuit, in which case, as is immediately apparent from the above equation, the resistance R becomes very large or may even assume a negative value so that a tuned circuit connected between the grid and the cathode is but little damped or even negatively damped. 7

According to the invention, the inductance can be made very low byvconnecting the anode circuit and the grid circuit each separately to the oathode and by so arranging the connecting wires necessary for this purpose that the coupling between both is small. The arrangement of such an amplifying valve with two connectingwires from the base to the indirectly heated cathode is diagrammatically shown in Fig. 3. This figure shows an indirectly heated triode comprising 7 an anode A, a grid G and an indirectly heated cathode K. For the sake of clearness the heater is omitted. Two wires I and 2 are fixed to a point P of the cathode and are connected to separate pins in the base as diagrammatically shown. The two connecting wires are spaced as far apart as possible in order to minimize the magneticcoupling between them. The grid circuit is connected to one of the connecting wires, say I, and the anode circuit to the other connecting wire 2. Thus it is ensured that the inductance designated by Li becomes very small and consequently the apparent conductivity also becomes very small or even negative. It is not necessary that the two connecting wires l and 2 should be connected to the same point of the cathode but as an alternative they may be connected to different points of the cathode.

The method represented in Figure 3 has a limitation in the fact that in connection with the second connecting wire to the cathode the base has to be provided with a supplementary connection or pin. This disadvantage can be obviated by using one of the current leads for the filament that heats the cathode K, partly as a connecting wire to the cathode. This circuit arrangement is illustrated in Figure 4 in which the indirectly heated cathode K is taken out of the envelope of the valve not only by a conductor l but also by way of a conductor 2 provided between a point of the cathode and the heater and of one of the cathode supply leads 3 and 4. If the heater is supplied with alternating current it is desirable that the grid circuit should be connected to the conductor I, the anode circuit to one of the cathode supply leads 3 and 4 in order that the disturbances by the fluctuating voltage of the filament may be avoided as far as possible.

If the common connecting conductor to the cathode includes in addition to an inductance 111 an ohmic resistance Rf it is found that the admittance between the terminals 3 and 4 of Fig. 1 can be designated approximately by the expression =jwC,,;+jwC f-S{jwC,,-R+wC,,;L;w L,,

from which it is apparent that the coupling introduces an apparent grid-cathode impedance which can be represented in part by a negative capacity, that is to say, by an impedance with the same frequency dependency as a capacity but with opposite phase. Such a coupling of the anode and the grid circuit permits of partly or wholly neutralizing the increase of the grid-cathode capacity with decreasing negative grid voltage which is due to space charge effects, since this capacity and the negative capacity brought about by the coupling due to the resistance R: are dependent in the same sense upon the slope S. Such a coupling by means of an ohmic resistance included in' the connecting conductor to the cathode, is particularly useful when a plurality of amplifying stages are interconnected by oscillatory circuits, the tuning of the circuits being effected by means of condensers which are mechanically coupled to each other. If in such an amplifier the gridbiases of the amplifying valves are varied differently, for example for volume control, the circuits will thus be caused to be out of tune relatively to each other. As described hereinbefore this can be obviated by means of ohmic resistances included in the oathode leads, as by a correct choice of these resistances the capacity, brought about by the amplifying valves, in parallel with the oscillatory circuits is made independent of the negative grid voltage.

When the amplifying valve shown in Figure 1 serves as a detector, the impedance Za inthe anode circuit behaves, insofar as high frequency currents are contemplated, essentially in a capacitative manner. In this case Za can be represented by and for the admitance between the terminals 3 and 4 in Figure 1 is found from which it is apparent that the latter term viz.,

brings about a very high damping. This damp ing can be neutralized by also choosing for Z: a capacity C: so that 1 i a z R c, c,, This case in which the anode and the grid circuits are coupled by a capacity Cf in the common cathode lead is shown in Figure 5. The operation of this circuit arrangement will be clear from the above considerations so that further consideration can be dispensed with.

In the specification as well as in the drawing the invention is only mentioned as applied to indirectly heated triodes. It will, however, be obvious that the invention can also be applied with advantage to multigrid valves, for example screen grid valves, in which case the anode circuit of the triode has to be replaced by the screen grid circuit of the tetrode. Fig. 6 shows such an embodiment of the invention as applied to a tetrode having a screen grid SG having an impedance ZSG in the screen circuit and which corresponds to the impedance Za of Fig. l. The usual useful load circuit impedance ZL is connected to the anode and the cathode and heater connections I 3 and 4 are similar to those shown in Fig. 4.

What we claim is:

In combination, an electron discharge tube having a cathode, a heater for said cathode, a control grid, a screen grid and an anode, a connection within said tube between said heater and one end of said cathode, a circuit including an impedance connecting said screen grid and said heater, an input circuit connecting said control grid and being directly connected to the other end of said cathode and an output circuit connected to said anode and directly to said one end of said cathode.

BALTI-IASAR VAN DER POL. KLAAS POSTI-IUMUS.

Images