US2407074A - Feed-back control arrangement for coupled tuned circuits - Google Patents

Description

3, 1946. GREEN I 2,407,074 4 FEEDBACK CONTROL ARRANGEMENT FOR COUPLED I UNE1 CIRCUITS Filed May 16, 1944 "5 Sheets-Sheet 1.

I INVENTOR.

Em M's r G ut/v Sept 3,1946. E EN R 2,407,074

FEEDBACK CONTROL ARRANGEMENT FOR COUPLED TUNED CIRCUITS Filed May 16, 1944 5 Sheets-Sheet 2 mmvron. I E mvnr Gum FEEDBACK CONTROL ARRANGEMENT FOR COUPLED TUNED CIRCUI TS Filed May 16, 1944 5 Sheets-Sheet 5 '+MTI Fig 12.

ma ENTOR. E Mes?- 6'05! 4 rran um Patented Sept. 3, 1946 FEED-BACK CONTROL GEMENT FOR COUPLED TUNED CIRCUITS Ernest Green, Chclmsford, England, assignor to Radio Corporation of America, a corporation of Delaware Application May 16, 1944, Serial No. 535,824 In Great Britain February 17, 1943 4 Claims. (Cl. 179-171) This invention relates to coupled tuned cira cuits, and provides means for controlling the effect of feed-back from one of the tuned circuits to the other.

The present invention is particularly suitable for controlling the effect of feed-back of energy from a tuned output circuit to a tuned input circuit, when these two circuits are coupled by means of one or more thermionic valves. The tuned output circuit may be tuned to the same frequency (fundamental frequency) as is the input circuit, or it may be tuned to a frequency which is, or to frequencies which are, in harmonic relationship to that to which the input circuit is tuned. It may be that in tuned circuits thus coupled there is a measure of feed-back from the output tuned circuit to the input circuit, and this feed-back may be regenerative or it may be degenerative. Moreove'r, feed-back may or may not be desirable, depending upon its nature, for example whether it is regenerative or degenerative, and also upon the purpose for which the coupled circuits are to be employed.

The present invention is mainly, though not exclusively, concerned with those cases in which in the output circuit there are components which it is desirable should not be fed back to the input circuit. These components which are not desired to be fed back may be the fundamental, or odd harmonics, or even harmonics. For the purposes of explanation the fundamental frequency may be regarded as an odd harmonic since it behaves, generally, in the same manner as do the odd harmonics. Moreover, since the anode-grid space and the'grid-cathode space of a valve may be regarded as capacitors in series, with a tapping point at the grid, the invention will be stated, in general, as being applied to tuned circuits coupled by a capacity-potentiometer.

According to the invention in one aspect, a cireuit arrangement including a tuned input circuit and a tuned output circuit coupled together by means of a capacity-potentiometer, wherein the input circuit is connected across part of the capacity-potentiometer while the tuned output circuit is connected across the wholeof the capacity-potentiometer, is provided with an impedance connected effectively across the said part of the capacity-potentiometer, said impedance being constituted as aseries-L/C circuit element resonant at a frequency near to the frequency of the component of frequency in the output circuit, the eftect of feed-back of which it is desired to con-'- trol, for example reduce or eliminate.

According to a first modification of the invention the capacity-potentiometer may consist of two capacity-potentiometers effectively in parallel, with the tuned input circuit connected from a tapping point on one capacity-potentiometer to a similarly placed tapping point on the other capacity-potentiometer, and with its mid-point connected to one end (conveniently referred to as the lower end) of the parallel-connected capacity-potentiometers so that each of the two electrical halves of the tuned input circuits is connected acrosspart of a different one capacitypotentiometer, while the tuned output circuit is connected across the whole of the parallel-connected capacity-potentiometers; the said impedance being connected from the electrical midpoint of the tuned input circuit to the lower end of the parallel-connected capacity-potentiometers. l

According to a second modification of the invention, one end (conveniently referred to as the upper end) of the capacity-potentiometer may be connected to one end of a neutralization capacitor, and the tuned input circuit may be connected from. a tapping point on the capacitypotentiometer to the other end of the neutralization capacitor and have its mid-point connected to the lower end of the capacity-potentiometer, while the tuned output circuit is connected across the whole of the capacity-potentiometer, the said impedance being connected from the electrical mid-point of the tuned input circuit to the lower end of the capacity-potentiometer.

The circuits so far outlined are, insofar as they may involve two capacity-potentiometers, pushpush circuits; that is, both capacity-potentiometers look into the same end of the tuned output circuit. There is another group of circuits to which the invention can be applied, namely push-pull circuits; that is circuits in which each capacity-potentiometer looks into a different end of the tuned output circuit.

According to a third modification of the invention, a circuit arrangement including a tuned input circuit and a tuned output circuit coupled together by means of a pair of capacity-potentiometers wherein the tuned input circuit is connected from a tapping point on one capacitypotentiometer to a similarly placed tapping point on the other and has its mid-point connected to the common lower end of the two capacitypotentiometers, while the tuned output circuit is connected at one of its two ends to the upper end of. one 'of the two capacity-potentiometers, at the other of its two ends to the upper end of the other of the two capacity-potentiometers, and

at its mid-point to the common lower end through a connection including an inductor, is provided with an impedance connected effectively from the mid-point of the tuned input circuit to the common lower end of the two-capacity potentiometers, said variable impedance being constituted as a series-LC circuit element resonant at a frequency near to the frequency of the component of frequency in the output circuit,'the effect of feed-back of which it is desired to control, for example, reduce or eliminate.

According to a fourth modification of the invention, in a circuit arrangement organized as set out in the immediately preceding paragraph so far as the input and output circuits are concerned, the said impedance is connected effectively from the mid-point of the tuned output circuit to the common lower end of the two capacitypotentiometers, instead of from the mid-point of the tuned input circuit.

According to a fifth modification of the invention, a circuit arrangement organized as set out in the last but one paragraph so far as the input and output circuits are concerned, and including neutralizing condensers, one connected from the upper end of either capacity-potentiometer to the tapping point on the other, is provided with two impedances, one connected across each neutralizing condenser, said impedances being constituted as series-L/C circuit elements resonant at a frequency near to the frequency of the component of frequency in the output circuit, feedback of which it is desired to control, for example, to reduce.

If, because the operating frequencies are high, or because the capacities of th constituents of the capacity-potentiometers are large (large valves for example), the inductances in the various leads, particularly in the leads connecting the tuned input circuit to the said tapping points, cannot be neglected but are neutralized by the inclusion of condensers in series in these leads connecting the input circuit to the tapping points, the circuit according to the third modification of the invention may itself be modified, by being provided with impedances connected across the said condensers included in the said leads, the said impedances being constituted as series-L/C circuit elements resonant at a frequency near to thefrequency of the component of frequency in the output circuit, the effect of feed-back of which it is desired to reduce or eliminate. This arrangement is known as the sixth modification of the invention.

In the circuit arrangements according to the invention, and to the modifications thereof, the capacity-potentiometer or capacity-potentiometers, as the case may be, may include the anodegrid and grid-cathode capacities of a thermionic valve or. of thermionic Valves, in series, and in the circuit arrangement according to the inventionor its first and second modifications the said part of said capacity-potentiometer or of said capacity-potentiometers, as the case may be, includes the grid-cathode capacity of the said thermionic valve or thermionic valves. Similarly, in the circuit arrangements in which reference is made to tapping points on the capacity-potentiometers and the lower end thereof, the part between this point and lower end may be the gridcathode capacity of the said thermionic valve, or thermionic valves.

All the circuit arrangements above described may be constituted as frequency multipliers, the tuned output circuit in all such cases being resonant at a frequency higher than, but harmonicall related to, that at which the tuned input circuit is resonant. In all the circuit arrangements above described, the said impedance may include series connected inductors and capacitors, either or both of which may be pre-set or variable, tuned about the harmonic frequency at which the tuned output circuit is resonant, or

about a harmonic frequency present in the said tuned output circuit.

In the circuit arrangements in which the impedance is stated to be connected to a mid-point of the tuned input circuit, and in the circuit arrangements in which the impedance is stated to be connected to a mid-point of the tuned output circuit, this mid-point may be found on the inductive element of these tuned circuits, or it may be found on the capacitative element thereof. In the former cases, the impedance may be constituted by the choke in the grid bias or anode current leads as the case may be.

In all the circuit arrangements, there may be provided more than on impedance in all of the places where one only is indicated, the impedances being connected in parallel with each other and being constituted as series-L/C circuit elements resonant at a frequency near to the frequencies of different components of frequency in the output circuit, the effect of feed-back of which different components it is desired to control, for example to reduce or eliminate.

In the drawings:

Fig. 1 shows one embodiment and Fig. 2 shows the equivalent circuit thereof,

Fig. 3 shows a modification and Fig. 4 is the equivalent circuit,

Figs. 5 and 6 show two further embodiments,

Fig. 7 shows another modification, while Figs. 8, 9 and 10 are equivalent circuits thereof,

Figs. 11, 1'2 and 13 show different modifications.

In 'the circuit illustrated in Fig. l, which illustrates the aspect of the invention as first set out, a triode V has a tuned input circuit Ll, Cl con- .nected across its grid-cathode space G-F, a blocking condenser C5 being included in the connection between the cathode F and the end of the tuned .input circuit to which the cathode is connected. The end of this tuned input circuit remote from the grid of the triode is connected to the cathode of the mode through a connection which includes a source of grid bias (not shown, but indicated at GB), the connection being made at the side of the blocking condenser C5 remote from the cathode F. The tuned input circuit is resonant at an applied fundamental frequency which it is desired to multiply. A tuned output circuit L2, C2, is connected across the anode-cathode space A-F of the triode V, a blocking condenser C5 being included in the con nection between the cathode F and the end of the tuned output circuit to which the cathode is connected. The end of the tuned output circuit remote from the anode A of the triode is connected to the cathode of the triode through a connection which includes a source (no-t shown, but indicated at +HT) of anode current, the connection being made at the side of the blocking condenser C6 remote from the cathode F. The output circuit is tuned to, or the oscillatory energy therein includes, a harmonic frequency.

The equivalent circuit is illustrated in Fig. 2, wherein like references are used to those used in Fig. 1, together with a few additional references, The source of the harmonic frequency in the tuned output circuit is represented by generator B and resistor Rp. To the harmonic frequency the triode will appear astwo capacitors C3 and C4 in series, respectively constituted by the capacity between the anode A and grid G and that between the grid G and cathode F, with the tuned input circuit connected acros the latter of these two capacitors. The tuned input circuit being tuned to the fundamental frequency, the impedance between the gridand cathode will be capacitative at theharmonic frequency. These two apparent capacitors C3 and C4 constitute a allel) and the potential fed back to the grid isin the same phase as that on the anode and is therefore degenerative. For the purpose of the invention, a. variable impedance Z! is connected capacity-potentiometer, and the potential fed back to thepoint G (the grid of the triode) is in the same phase as that on the point A (the anode) and is therefore degenerative. For the purpose of the invention variable impedance Z! is con-q trates the first modification of the invention, two

triodes V and V have their anodes A and A" connected together directly, and their cathodes F and F likewise are connected together directly. A tuned input circuit Ll, Cl is connected in push-pull to the grids G and G of the two triodes. The mid-point of the inductor Ll in the tuned input circuit is connected to the cathodes F, F" through a connection which includes a source of grid bias (not shown, but indicated at GB) and a resistor 0r"(as shown) inductor L3. The tuned input circuit is resonant at an applied fundamental frequency which it is' desired to multiply. A tuned output circuit L2, C2 is connected across the parallel related anodecathode spaces A-F' and A"-F". of the two triodes, a blocking condenser C6 being included in the connection between the oathodes and that'end of the' tuned output circuit to which they are connected. The end of the tuned output circuit remote from the anodes of the triodes is connected to the cathodes of the triodes through a connection which includes a source (not shown but indicated at +HT) of anode current, the connection being made at the side of the blocking condenserCG remote fromthe cathodes F, F". The output circuit is tuned to, or the oscillatory energy therein includes, an even harmonic frequency.

The equivalent circuit is illustrated in Fig. 4, wherein the source of harmonic frequency is represented by generator B and resistor R1). To the harmonic frequency the two triodes and input circuit will appear as two capacitors C3 and Cd in series, respectively constituted by the capacities C3 and C3 between anodes Aand A" and grids G and G" and that between the said grids and cathodes F and F" together with the bulk capacitance of the tuned input circuit all in parallel, with the two halves of the inductor Ll of the input circuit in parallel with each other and in series with the resistor or inductor L3 connected across the latter of these two capacitors C4. Again, the impedance between the grids and cathodes will be capacitative at the harmonic irequency. The two apparent capacitors constitute a capacity-potentiometer (or two such in pareffectively across the apparent capacitor C4 constituted by the grid-cathode capacitances. Physically this variable impedance is connected from the mid-point of the tuned input circuit to .the cathodes of the triodes. The connection may be effected to the mid-point of the inductor Ll of the tuned input circuit, or as shown in Fig.- 3, to the point of junction of two series-connected tuning capacitors Cl. By means'of this impedance Z1, the potential fed back can be reduced to zero or controlled to any desired extent according to the nature and value of the said impedance. Thus, if it take the form of an acceptor circuit at the harmonic frequency, as illustrated in Fig.

3, consisting of an inductor L1 in series with a capacitor C7, no potential will be fed back to the grid at that frequency.

In the circuit illustrated in Fig. 5, which illustrates the second modification of the invention, a triode V has one end of a tuned input circuit Ll, C'l connected to its grid G, the other end of the tuned input circuit being connected to one side of a neutralizing capacitor C8, the other side of which is connected to the anode A of the triode. The mid-point of the inductor LI in the tuned input circuit is connected to the cathode through a connection which includes a source of grid bias (not shown, but indicated at -GB) and a resistor or (as shown) inductor L3. The tuned input circuit is resonant at an applied fundamental frequency which it is desired to multiply. A tuned output circuit L2, C2 is connected across the anode-cathode spac A-F of thetriode, a blocking condenser C6 being included in the connection between the cathode F and that end of the tuned output circuit to which it is connected. The end of the tuned output circuit remote from the anode of the triode is connected to the cathode of the triod through a connection which includes a source (not shown, but indicated at +I-IT) of anode current, the connection being made at the side of the blocking condenser C6 remote from the cathode F. The output circuit is tuned to, or the oscillatory energy therein includes, a harmonic frequency. Such a circuit can be used for either odd harmonics, including the fundamental frequency, or even harmonics, and it resembles that of the first modification illustrated in Fig. 3, with one of the triodesof that circuit replaced by the capacitor C8. The degree of feed-back can be controlled in a similar way, by connecting a variable impedance, which may take, and as illustrated takes, the form of a series-L/C' circuit Ll, C1, between the mid-point of the tuned input circuit and the cathode F of the triode V. I

The arrangement illustrated in Fig. 1 may be difierently modified, as illustrated in Fig. 6. The fundamental circuit is exactly the same as the fundamental circuit of Fig. 1, that is as Fig. 1 without the series-L/C circuit L1, C'l. It has, however, a series-L/C circuit L9, C9 connected across the capacitor C3 between the anode A and grid G. This series-LC circuitis tuned to provide neutralization of feed-back at the funda-. mental frequency, and is a known device. In ad-- dition, a further series-L C circuit L'l C1, is connected across capacitor C3 to control the degree of feed-back at the harmonic frequency. In this arrangement, the feed-back will be a minimum when the resultant impedance is a maximum.

In the circuit illustrated in Fig. '7, which illustrates the third modification of the invention, two triodes V and V have their cathodes F and F connected together directly, a tuned input circuit LI, Cl being connected on one of its sides to the grid G of triod V and on the other of its sides to the grid G" of the other triode V". The mid-point of the inductor LI in the tuned input circuit is connected to the cathodes F, F" through a connection which includes a source (not shown, but indicated at GB) of grid bias and a resistor or (as shown) inductor L3. The tuned input circuit is resonant at an applied fundamental frequency which it is desired to amplify or to multiply to an odd harmonic frequency. A tuned output circuit L2, C2 is connected at its one side to the anode A of triode V and at its other side to the anode A of the other triode V". The mid-point of the tuned output circuit is connected to the cathodes F, F of the triodes through a connection which includes a source (not shown, but indicated at +HT) of anode current and an inductor L4. The tuned output circuit is in the push-pull mode, resonant to odd (including the fundamental) frequencies, which are those desired. The anode A of triode V is coupled through a neutralizing capacitor CID to a pont where the tuned input circuit is connected to the grid G of triode V, and the anode A" of this triode V is connected to a point where the tuned input circuit is coupled through a neutralizing capacitor CH1 to the grid G of triode V.

The capacitors CIO and Clli neutralize the push-pull oscillations fed back from the anodes to the grids through the capacities C3 and C3 respectively. However, the push-pull excitation of the grids tends to excite the output circuit, in push-push, at the undesired even harmonic frequencies. An equivalent circuit for Fig. '7 is illustrated in Fig. 8, like references are used to those used in Fig. '7, together with a few added references. Thus, capacitors CH and CIZ-represent the general capacitances from the anode and grid, respectively to ground. For the even harmonic mode of oscillation, capacitors C3, C3", C Ill, and CH! (the latter two being represented in Fig. 8 as CHI) are efiectively in parallel, equipotential points for even harmonics being shown connected together. The response of the output circuit to even harmonic oscillations depends upon the impedance which this circuit offers to such oscillations, but it is clear that the neutralizing condensers (CHO in Fig. 8) will not neutralize even harmonics fed back to the grids.

The circuit of Fig. 8, that is ultimately of Fig. 7, can be further simplified to the equivalent circuit illustrated in Fig. 9, wherein again, so far as may be, like references are used to those used in Figs. 7 and 8. It will be seen to have resolved itself substantially to the circuit of Fig. 2. The source of even harmonic frequencies in the tuned output circuit is represented by generator B and resistor Rp. To the even harmonic the triodes will appear as the two capacitors C3! and CH2 in series, respectively constituted by the parallel capacitors C3, C3, CH1 and CH! (Clo of Fig. 8), and the parallel capacitors C4 Cd, and CH2, with the two halves of inductor L! of the input circuit in parallel with each other and in series with inductor L3 connected across capacitor CH2, and with the two halves of inductor L2 in parallel with each other and in series with in-' ductor L4 connected across the capacity potentiometer constituted by capacitors C3) and C412. Inductors L3 and L4 offer a high impedance so that generator B produces an even harmonic potential across the capacity potentiometer and a fraction of the anode potential will be fed back to the point G, essentially the grids of triodes V and V. The potential fed back to point G will be degenerative.

For the purpose of the invention, a variable impedance Z"! is connected effectively across the apparent capacitor C4l2 constituted by the gridcathode capacities, including the capacity between the grids and earth, in Fig. 8 designated C I 2. The connection may be effected to the midpoint of the inductor LI of the tuned input circuit, or as shown in Fig. 7 to the point of junction of two series-connected tuning capacitors Cl. In Fig. 7 the variable impedance Z1 is represented by the two halves of capacitor CI in parallel, with the variable capacitor C1 and inductor L7 connected in series to ground. If th connection is made to the mid-point of inductor Ll it consists of the two halves of LI in parallel, and these parallel halves in series with L! and C1 to ground. It will be noted that these additions are made at neutral points of the circuit as regards push-pull operation, and so will not interfere with normal working on the fundamental or old harmonics. If it is necessary to provide control on more than one even harmonic, say 2nd, 4th and 6th, then other variable impedances may be connected in parallel. It is not thought to be necessary to reproduce these arrangements in drawings.

When large valves, or high frequencies are involved, the inductances in the connections within the valves, and those outside the valves by which the various components are connected together and the bulk capacitances to various parts of the framework, cannot be ignored. To the even harmonics, in this case, the circuit of Fig. 9, omitting the impedance Z1, appears as indicated in Fig. 10, in which the various symbols have the following significations: C9 is the bulk capacity between the top A of the potentiometer (the anode of the valves) L9 is the inductance of the connection, including that of the frame, between A and the point on the frame which for highfrequencies is a ground potential; CIU is the bulk capacity between point G (grid of the valves) and frame; LID is the inductance of the connection, including that of the frame, between G and the point on the frame which for high frequency is at ground potential; CH and LH are, respectively, the effective capacity and effective inductance between points A and G (anodes and grids of the valves including anode-cathode capacities) and neutralizing capacitors; C12 is the eifective capacity between points G and F (grids and filaments of the valves); and LIZ and LI3, which apply only in the case of valves, are the eiTective inductance between the filament and the point on the frame which, for high frequency, is at ground potential; LIZ being the effective in ductance of the filament leads, and LI3 the inductance of the frame between the point at which ,the filaments are connected thereto through blocking condensers, and the said ground point. For simplicity, the grid-feed chokes, and anodefeed chokes have been disregarded, and are not represented. In such cases, the harmonics fed back to the grid from the anode may have any phase. 'I-Iowevenby connection of a suitable impedance (Z1 not shown) from point G or G to pointE, or from point A or A to point E, control of the feed-back may be effected.

In the circuit illustrated in Fig. 11, which relates to the above case and illustrates the fourth modification of the invention, the input and output tuned circuits, respectively Ll, Cl and L2, C2 and triodes V and V are connected as just described, but instead of the variable impedance 21, or variable impedances if more than one are em- .ployed, being connected from the mid-point of the input circuit to. the common cathode connection, it is connected from the mid-point of the tuned output circuit to the said common cathode connection, so that voltage fed back to the grids of the triodes at even harmonic frequencies can be controlled. In Fig. 11, the variable impedance Z! is shown as consisting of series connected inductor L1 and'capacitor CT, connected from the mid-point of capacitor C2 to the common cathode'connection, that is to ground. It could, of course, be connected from the mid-point of in-' ductor L2.

In a circuit arrangement according to the fifth modification of the invention, the input and output circuits and the triodes are connected as in Fig. 7, omitting the variable impedance L1, 01. These circuits, so far as odd harmonics and the fundamental are concerned, resolve themselves into a four-armed bridge, as illustrated in Fig, 12 (omitting variable impedances Z1 and Z1") with the anode-grid capacities C3 and C3" in two opposite arms, and with the neutralizing capacitors CH1 and C in the other two opposite arms. The tuned input circuit LI, Cl are connected to the corners g and g of the bridge to which the grids G and G are connected. The tuned output circuit L2, 02 are connected to the corners a and a of the bridge to which the anodes A and A" are connected. I

Both the fundamental and odd harmonic electromotive forces operate across the diagonal to whichthe output circuit is connected. Neglecting the inductance in the arms of the bridge, balance will be maintained across the diagonal to which the input circuit is connected, both for the fundamental and the odd harmonics. Hence, there is no feedback from the anode to grid on the fundamental or odd harmonics. Control of feedback on any particular odd harmonic can be provided by connecting variable impedances Z1 and Z1 across the capacitors CID and CH1 respectively. An alternative to this would be to connect variable impedances between grid and anode of each valve, that is across capacitances C3 and C3". When large valves,.or high frequencies are involved, the inductances in the connections within the valves, and those outside the valves by which the various components are connected together and the bulk capacitances to various parts of the framework; cannot be ignored.

In the circuit illustrated in Fig. 13, which takes care of these considerations and is in accordance with the sixth modification of the invention, two triodes V and V have their cathodes F and F" connected together directly, a tuned input circuit Ll, C'l being coupled on one of its sides by a first grid-coupling capacitor C to the grid G of the triode V and on the other of its sides by a second grid-coupling capacitor C25" to the grid of tri, ode V". Grid bias potential is provided through a connection which includes a source of grid bias (not shown, but indicated at -GB) and resistors or; as shown inductors L and L" one for each valve. Thetuned input circuit LI, Cl is resonant at'an applied fundamental frequency which it is desired to multiply. A tuned output circuit L2,

C2 is connected at its one side to the anode A of triode V and at its other side to the anode A of triode V. The mid-point of the tuned output circuit is connected to the cathodes of the triodes through. a connection which includes a source of anode current and inductor L4. The tuned output circuit isresonant to, or has a component of its oscillations, an odd harmonic frequency. The anode A of triode V is coupled through a capacitor C10 to a point g where the tuned input circuit is capacitatively coupled to the grid of triode V, and the anode A" of triode V is coupled through capacitorCIO, to point 9" where the tuned input circuit is capacitatively coupled to the grid. of triode V.

As' is apparent, this circuit also resolves itself into a balanced bridge. The tuned input circuit is connected across one diagonal from points g and y "i and the tuned output circuit is connected fromthe points a and (1" across the other diagonal. Provided'the arms of the bridge 'all contain equal effective values of inductance and capacitance the bridge will remain balanced as regards the points at which the tuned input and output circuits are connected, both for the fundamental and for the odd harmonic frequencies for which the circuit would be used. As indicated above, at high frequencies and with large valves the inductive reactance of the grid lead are apprcciable, and it is the function of the grid-coupling capacitors to neutralize this inductive reactance at the fundamental frequency. At the harmonic frequencies a regenerative voltage will be fed back to the grids. This can be controlled at any desired odd harmonic by variable series- L/C impedances Z1 and Z1" connected across the grid-coupling capacitors.

It has not been thought necessary to redra Figs. 12 and 13 since, unlike the equivalent circuits of other arrangements, the various elements have retained their identities, whereas in the equivalent circuits of the other arrangements some of the elements, as, for example, the two halves of capacitor CI in Fig. 13 which has lost its identity in capacitance C4 of Fig. 4, have apparently ceased to exist as separate entities.

What is claimed is:

1. A circuit arrangement including a tuned input circuit, a tuned output circuit, a pair of capacity-potentiometers coupling said circuits, the tuned input circuit being connected from a tapping point on one capacity-potentiometer to a similarly placed tapping point on the other and having its mid-point connected to the common lower end of the two capacity-potentiometers, the tuned output circuit being connected at one of its two ends to the upper end of one of the two capacity-potentiometers, at the other of its two ends to the upper end of the other of the two capacity-potentiometem, and at its mid-point to the common lower end through a connection including an inductor, wherein neutralizing capacitors are connected from the upper end of either capacity-potentiometer to the tapping point'on the other and two impedances are provided, one connected across each neutralizing condenser, said impedances being constituted as series-L/C circuit elements resonant at a frequency near to the frequency of the component of frequency in the output circuit, feed-bag}: of which it is desired to control.

2. A circuit arrangement including a tuned input circuit coupled to a tuned output circuit by means of a pair of thermionic valves having their cathodes connected together, and from the grid of one of which to the grid of the other of which said input circuit is connected, and from the anode of one of which to the anode of the other of which said output circuit is connected, wherein for the control of feed-back from said output circuit to said input circuit, at least one series L/C impedance network is connected from the mid-point of said input circuit to said cathodes, said impedance network being resonant at a frequency near to the frequency of a component frequency which it is desired to control.

3. A circuit arrangement including a tuned input circuit coupled to a tuned output circuit by means of a pair of thermionic valves having their cathodes connected together, the anode of either being coupled through a capacitor to the grid of the other, and from the grid of one of which to the grid of the other of which said input circuit is connected, and from the anode of one of which to the anode of the other of which said output circuit is connected, wherein for the control of 12 feed-back from said output circuit to said input circuit, at least one series L/C impedance network is connected across-each of said capacitors, said impedance network being resonant at a frequency near to the frequency of a component frequency which it is desired to control.

4. A circuit arrangement including a tuned input circuit coupled to a tuned output circuit by means of a pair of thermionic valves having their cathodes connected together, and from the grid of one of which to the grid of the other of which said input circuit is connected, and from the anode of one of which to the anode of the other of which said output circuit is connected, where'- in, in the connection between said input circuit and either of said grids a capacitor is included, and wherein for the control of feed-back from said output circuit to said input circuit, at least one series L/C impedance network is connected across each of said capacitors, said impedance network being resonant at a frequency near to the frequency of a component frequency which it is desired to control.

ERNEST GREEN.

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