US2714135A - Wide band high frequency thermionic valve circuits - Google Patents

Description

July 26, 1955 E. MOP. LEYTON WIDE BAND HIGH FREQUENCY THERMIONIC VALVE CIRCUITS Filed Nov. 25, 1949 5y cPd zcmm E BAND HIGH FREQUENCY THEONIC VALVE CIRCUITS Eric McPhail Layton, London, England, assignor to Electric & Musicai industries Limited, Hayes, England, a British company Appiication November 23, F249, Serial No. 128,943

Claims priority, application Great Britain November 25, 1948 3 Claims. (Cl. 179-l71) This invention relates to wide band high frequency thermionic valve circuits, and especially but not exclusively to wide band radio frequency amplifiers.

One difiiculty associated with wide band high frequency thermionic valve circuits is the limitation which is imposed on the band width by capacity in the circuit. For example, one well known form of high power radio frequency amplifier comprises two thermionic valves driven in push-pull, and having a tuned anode circuit from which the output is taken, the anode circuit comprising inductances tuned to parallel resonance by the stray and inherent capacity associated with the anode circuit. If the valves are driven at their cathodes, the most important contributions to the capacity of the anode circuit are made by the anode-to-control electrode the valves and inductances and this capacity is in general sufficient to limit appreciably the pass-band of the ampli- An object of the present invention is to provide an improved wide band high frequency thermionic valve circuit with a view to enabling the limitation of the band width imposed by capacity in the circuit to be reduced.

A further object of the present invention is to provide a Wide-band thermionic valve circuit comprising a thermionic valve having an anode, cathode and control electrode, said valve having an input circuit such that said valve is driven at its cathode, and having a tuned output circuit whereof a substantial part of the inductance is connected between the anode and ground and a substantial part is connected between the control electrode and ground said latter part of the inductance being in the form of a screen which effectively screens the output circuit of said valve from the input circuit whereby feedback from said output circuit to said input circuit is substantially prevented and an increase in the effective bandwidth of said output circuit is obtained due to the inductance of said output circuit being divided into two parts.

Where a thermionic valve has a tuned output circuit which is connected between the anode and control electrode, and comprising inductance which is tuned mainly by stray capacities, the principal capacities effective in the circuit comprise the stray capacity between the anode and ground, the stray capacity between the control electrode and ground and the interelectrode capacity between the anode and control electrode. By dividing the inductance of the output circuit into two parts, in accord ance with the invention, the ground point of the output circuit is located at an intermediate point in the inductance. Consequently, the first two of the capacities referred to are connected in series one with the other across the inductance and, if it is assumed that the first two capacities are of the same order of magnitude, their resultant is then about one half the magnitude of either of the capacities individually and is in parallel with the third capacity, namely the interelectrode capacity hetween the anode and control electrode. The efifective magnitude of the tuning capacity is thus reduced as compared with the case in which substantially all the inductance of the output circuit is connected between the anode and ground or between the control electrode and ground for, in this latter case, although the stray capacity to ground from the control electrode (or the anode as the case may be) is short-circuited, the magni tude of the capacity effective in parallel with the aforesaid interelectrode capacity is of the same order of magnitude as the stray capacity to ground from the other of said electrodes. By virtue of the present invention, there is thus obtained a substantial increase in the ratio of inductance to capacity in the output circuit with a corresponding improvement in the band width.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be more fully described with reference to the accompanying drawing wherein Figure 1 illustrates diagrammatically one example of a high power, wide band radio frequency amplifier in accordance with the present invention and Figure 2 is a representation of Figure 1 using the conventional symbols for lumped impedances.

Referring to the drawing, the amplifier comprises two thermionic valves 1 and 2, which are shown as triodes, and the oscillations which are to be amplified are fed in push pull to the cathodes of the valves 1 and 2 from a source indicated conventionally at 3, the amplifier being arranged for class B or class C operation. The control electrodes of the valves 1 and 2 are respectively connected to ground by inductances 4 and 5, while the anodes are also connected respectively to ground by inductances 6 and 7. The valves 1 and 2 are high power valves having the control electrode lead in the form of external conductive flanges 4a and 5a respectively, and inductances 4 and 5 are constituted by the outer surfaces of conductive tubes connected between the respective control electrode flanges of the valves and the ground plane 15 of the circuit which is indicated by the reference character 15. The leads from the source 3 to the cathodes of the valves 1 and 2 pass longitudinally through the tubes 4 and 5 along the axes thereof, the tubes thus shielding the input circuit from the output circuits of the valves. The inductances t5 and 7 are similarly constituted by the outer surface of a single conductive tube whose mid-point is grounded for radio frequency oscillations via a blocking condenser 6 and connected to a source of positive potential indicated by the arrow 9 via a high frequency choke i The conductive tubes 4, 5, 6 and 7 are shown in longitudinal cross section in the drawing. The capacity shown in dottedlines at 11 represents the total stray capacity between the control electrodes of the valves 1 and 2, that is the resultant of the total stray capacity to ground of the inductances 4 and 5 and of the control electrodes, and similarly the capacity 12 represents the total stray capacity between the anodes of the valves 1 and 2. The capacities 11 and 12, the inductances 4, 5, 6 and '7, and in addition the capacity between the anode and control electrode of each of the valves 1 and 2 serve to provide an appropriately tuned output circuit for the amplifier, and a push-pull output of suitable peak amplitude can be obtained for example at the taps 13, 14. By

that despite the presence of the inductances 4 and 5 the circuit illustrated has the advantage of a so-called grounded grid amplifier in that the input circuit is subsence of such screening substantial positive feedback would occur from the output to the input circuit and this would tend to counteract the increase in band-width made possible by dividing the inductance of the tuned output circuit. This is illustrated in Figure 2 in which the inductances provided by the outer surfaces of the tubes 4 and 5 are denoted as lumped inductors and similarly the inductances constituted by the outer surfaces of the tubes 6, 7 are denoted as lumped inductors. The screening which exists between the output and input circuits has been represented by the dotted line 16. The screening is such that the potentials on the outer surfaces of the tubes 4 and 5, arising from the fact that these outer surfaces constitute inductances in the output circuit, exist also on the parts of the cathode leads of the valves 1 and 2 enclosed by the tubes 4 and 5 whereby the potential between cathode and control electrode is not altered by potential variations in the output circuit. The potential variations between cathode and control electrode in each of the valves 1 and 2 are therefore produced, for all practical purposes only by the oscillations fed directly from the source 3.

To illustrate the effect of connecting part of the inductance of the tuned output circuit to the control electrodes and part to the anode of each of the valves 1 and 2, it will be assumed that the inductances 4, 5, 6 and 7 are all equal, that the capacities 11 and 12 are equal to one another and to the total capacity which would exist between the anodes if all the inductance in the output circuit were connected to the anodes and that these capacities are large compared with the capacity between the anode and control electrode in each valve. The capacities 11 and 12 are each in effect composed of two equal capacities having a common electrode which is grounded. Consequently, the left hand component of the capacity 11 is in series with the left hand component of the capacity 12 between the high potential ends of the inductances 4 and 6. Therefore, on the basis of the assumptions specified at the beginning of this paragraph, the

magnitude of the resultant of these two component capacities is one half of the magnitude of either of the component capacities individually so that there is a substantial reduction in the capacity in parallel with the interelectrode capacity between the anode and the control electrode of the valve 1 as compared with what would be the case if, say, the control electrode of the valve were directly grounded and only the left hand component of the capacity 12 were in parallel with said interelectrode capacity. In the same way, the right hand components of the capacities 11 and 12 are connected in series between the high potential ends of the inductances 5 and 7 so that the magnitude of the capacity in parallel with the interelectrode capacity betwen the anode and control electrode of the valve 2 is reduced. The effective magnitude of the tuning capacity of the output circuit of the amplifier is therefore substantially reduced and the inductance-to-capacity ratio of the output circuit is increased with a corresponding increase in the band width of the circuit (assuming that the effective resistance of the output circuit is unaltered). A substantial improvement in the inductance-to-capacity ratio would of course be obtained even if the capacities 11 and 1?. are unequal provided that they are not too dissimilar.

The inductance in the output circuit need not of course be divided equally as assumed above and a substantial increase in the inductance-to-capacity ratio of the output circuit and in the band width of the circuit can be obtained even of the inductance is not equally divided, practically useful limits for the magnitude of the part of the inductance connected to the control electrode being between 20 to 80 percent of the whole inductance. It will be appreciated of course that in high power thermionic valves, the electrode construction is such that some incidental inductance is usually associated with the control electrode and it is to be understood that the present invention is restricted to cases in which such incidental inductance is artificially increased so that its magnitude is within the range specified. The present invention is thus distinguished from circuits embodying a conventional grounded grid amplifying valve in that in such circuits, the control electrode of the valve is connected to the ground plane in a manner to render the impedance between the control electrode and ground a minimum. Moreover, where reference is made herein and in the claims to ground, it is to be understood to mean a point at which there is substantially zero amplitude of high frequency oscillations, and does not necessarily imply a point of Zero direct-current potential.

I claim:

1. A wide-band high frequency thermionic valve circuit comprising a thermionic valve having at least an anode, a cathode, and a control electrode between said anode and cathode, an inductor in the form of a tubular conductor connected at one end to said control electrode, means grounding the other end of said tubular conductor for high frequency oscillations, a source of input signals, an input circuit including a lead extending longitudinally through said tubular conductor from said source to said cathode, and an inductor connected from said anode to ground, said first and second inductors being tuned to resonance by stray and distributed capacity to constitute a tuned output circuit screened from said input circuit by said tubular conductor.

2. A wide-band high frequency thermionic valve circuit comprising a thermionic valve having at least an anode, a cathode, and a control electrode between said anode and cathode, an inductor in the form of a tubular conductor connected at one end to said control electrode, means grounding the other end of said tubular conductor for high frequency oscillations, a source of input signals, the input circuit including a lead extending longitudinally through said tubular conductor from said source to said cathode, and an inductor connected from said anode to ground, said first and second-mentioned inductors being tuned to resonance by stray and distributed capacity to constitute a tuned output circuit screened by said tubular conductor from said input circuit, and said tubular conductor being dimensioned to comprise not less than 20 per cent and not more than 80 per cent of the total inductance of said output circuit.

3. A wide-band high frequency thermionic valve circuit comprising two valves each having an anode, a cathode, and a control electrode between said anode and said cathode, an inductor in the form of two tubular conductors connected respectively at one end to said control electrodes, means grounding the other end of each of said tubular conductors for high frequency oscillations, a source of input signals, an input circuit including leads extending longitudinally through said tubular conductors from said source to said cathodes for feeding input signals in push-pull to said cathodes, and an inductor connected from the anode of each valve to ground, the inductors connected to the anodes of said valves and the inductors connected to the control electrodes of said valves being tuned by stray and distributed capacity to constitute a tuned output circuit screened from said input circuit by said tubular conductors.

References Cited in the file of this patent UNIT ED STATES PATENTS Young Feb. 6,

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