Application Date: July 8, 1935. No. 18754/36.
(Divided out of Application No. 19480/35 (462,530).
Complete Specification Left: July 6, 1936.
Complete Specification Accepted: March 8,1937.
Improvements in and relating to the Reduction of the Effects of Unwanted Reactance introduced in Electric Circuit Arrangements by certain Circuit Elements
I, ALAN DOWER BLUMLEIN, of 32, Audley Road, Ealing, London, W.5, a British Subject, do hereby declare the nature of this invention to be as follows:-
The present invention relates to the reduction of the effects of unwanted reactance introduced in electric circuit arrangements by certain conduit elements, and relates more particularly but not exclusively to circuits which are required to handle signals covering a wide range of frequencies.
In many resistance coupled devices, especially amplifiers used in television, trouble is experiences owing to the stray capacity introduced by certain circuit elements. For example, if a D.C. coupling is required between two valves and it is effected by introducing a source of D.C. between the output of one valve and the input of the next, the stray capacity to ground of this source may be too great to allow the circuit to be operated in a normal manner. Similarly, the capacity of a cathode heating battery to ground may be excessive when the load is put in the cathode circuit of a valve. Finally, in valves where the coupling resistance is required to dissipate much energy, it may be found difficult to provide a resistance which will dissipate the power and at the same time have reasonably low self capacity. These stray capacities may cause considerable attenuation of high frequencies with respect to low frequencies and may introduce other harmful effects.
This invention provides means by which the effects of certain stray capacities can be eliminated or substantially reduced.
The invention will now be described by way of example with reference to the accompanying diagrammatic drawings wherein
Fig. 1 shows a circuit for supplying potential to the anode which collects photo-electrons in a cathode ray television transmitting tube,
Fig. 2 shows a modification of a part of Fig. 1,
Fig. 3 shows a modification of a part of Fig. 2,
Fig. 4 shows a circuit of a direct coupled amplifier,
Fig. 5 shows a modification of Fig. 4,
Fig. 6 shows a circuit according to the invention in which the effect of shunt capacity across a resistance which is required to dissipate a considerable power is eliminated or substantially reduced,
Fig. 7 shows a circuit according to the invention for feeding heating current to the cathode of a valve of the cathode follower type and
Fig. 8 shows a modification of Fig. 7 in which provision is made for the supply of bias potential to the control electrode of a subsequent amplifying valve.
Referring to the drawings, Fig. 1 shows a circuit for maintaining the potential of a collected electrode 1 positive with respect to the signal plate 2 of a double ended cathode ray transmitting tube 3. The signal plate 2 carries a large number of conducting particles insulated therefrom, and from one another, each particle having an exposed conducting surface on each side of the plate 2. The exposed surfaces of he particles on the left of the plate 2 are rendered photo-electrically active, an image of the object to be transmitted is cast upon the photo-electric mosaic surface thus formed and photo electrons are emitted by the mosaic surface and collected by the electrode 1 when this is maintained at a suitable positive potential with respect to the signal plate 2. The surfaces of the particles on the right of the signal plate are scanned by an electron beam from an electron gun (not shown). The particles are thereby consecutively caused to return to a datum potential and currents representative of the light and shade of the picture are set up in a load resistance 4 which is connected between the signal plate 2 and earth. Potential differences are thus set up between the point 5 and earth and may be fed by means of lead 6 to the input of an amplifier. It is desired to apply a difference of potential between electrode 1 and the point 5. If battery 7 were directly connected between electrode 1 and point 5, its capacity to earth indicated by the dotted condenser 8 would appear as shunt capacity across load resistance 4. This shunt capacity would reduce the effective impedance of load 4 at high frequencies and would therefore attenuate the output of these frequencies which is obtained from lead 6.
In accordance with the present invention, the battery 7 is connected between electrode 1 and point 5 through two series resistance 9, 10 of high value, a by-pass condenser 11 of capacity large compared with the capacity 8 being connected between point 5 and electrode 1. The effective shunt impedance across load resistance 4 is then the resultant of capacity 8 in series with the two resistances 9, 10 in parallel. By making resistances 9, 10 of sufficiently high value, for example one or two megohms each, it is usually possible to arrange that this effective shunt impedance has a negligible effect on the frequency characteristic of the system over the range of frequencies which it is required to handle. In some cases the shunt impedance may cause a small amount of attenuation of the highest frequencies which may be corrected in a subsequent amplifier.
As an alternative to correcting the residual attenuation, the load resistance 4 the magnitude of which will be denoted by R may be so modified that the resultant load (including the shunt impedance mentioned above) behaves as a pure resistance equal to R. if each of the resistances 9, 10 has a magnitude 2r and if the capacity 8 has a value C, then the effective shunt impedance is the resultant of a resistance r in series with a capacity C.
Now if an inductance shunted by a resistance is inserted in series with load resistance 4, it is possible to correct the effective impedance of the load for the effect of C. The shunt resistance r may be considered as made up of two parts, on equal to R, and the other equal to (r-R). The required inductance shunted by resistance R will then be the inverse of C and (r-R) about the resistance R. The resistance 4 is therefore replaced by a circuit comprising an inductance 12 of magnitude L shunted by a resistance 13 of magnitude r1 in series with load resistance 4 of magnitude R as shown in Fig. 2, where r1 is equal to EQUAT. HERE and L is equal to CR2. In this example, the impedance between the point 5 in Fig. 2 and earth is still not a pure resistance owing to the large shunt capacity of the signal plate 2 of Fig. 1, and the input capacity of an amplifier electrode connected to lead. However, the load portion of the circuit (that is the elements forming the constitute for the simple resistance 4 on the battery feed) is resistive, thus enabling a simple correction to be made later for the shunt capacity of the signal plate. The load circuit 4, 12, 13 which is shown in Fig. 2 may be replaced by the equivalent circuit shown in Fig. * without departing from the spirit of the invention. This alternative arrangement can, by suitable choice of components, be made an exact counterpart of the given in Fig. 2.
Figure 4 shows a circuit similar to that of Fig. 2 used for a direct coupling between two valves 14, 15 of a direct coupled amplifier. As in Fig. 1, the battery 7 (or some source of potential such as a rectifier) which is necessary to maintain the potential of the grid 16 and valve 15 at a suitable value with respect to the anode 17 of valve 14 is built up by resistances 9, 10 to hold off its *** capacity, represented by condenser 8 to earth, and the main anode resistance has in series with it an inductance 12 *** resistance 13 in parallel, to made a total effective resistance in the circuit a constant value. Alternatively, the arrangement of Fig. 5 may be employed wherein a resistance 13 and inductance ** I series are connected in shunt with resistance 4 in order to correct the impedance change produced by the battery feed resistance and stray capacity of *** battery.
Figure 6 shows a large valve 18 which is to be resistance coupled to the *** stage. In order that this valve *** operate up to high modulation frequencies, such as one or two megacycles per second, the self capacity of the *** resistance must be small. As, however, we are considering a large valve, *** anode resistance must dissipate *** power, the greater part of which is due to the steady feed current of the valve. In place of a simple anode reactance of magnitude R two resistances 20 each of magnitude R two are connected in series. Resistance 19 is shunted by inductance 21 of magnitude L and reactance 20 is shunted by a condenser ** such magnitude that the total *** capacity effectively across resistance *** may be denoted by C where EQUAT. HERE resistance 20 must take the steady feed of the valve 18, and is therefore required to dissipate the greater part of the power. As, however, this resistance is to be shunted by a condenser, it can have a high self capacity provided this does not exceed the value C as defined above. The resistance 20 in practice is shunted by a condenser 22 which is the difference between the required value C and the self capacity. The resistances 19 can be made of small self-capacity since this resistance is only required to dissipate the alternating current power for high frequencies handled by the valve; at low frequencies this resistance will be shunted by the comparatively low impedance of the inductance 21, and will not be required to pass nay appreciable current. the inductance 19 must be formed with as lows self capacity as possible, as no advantage will be obtained if the self capacity of this inductance approaches the self capacity of a simple anode resistance which could have been used. Small corrections in the values of the resistances 19, 20 can be made to allow for any series resistances in the inductance 21, and similarly if inductance 21 is given an iron core, the effective losses may constitute some of the resistance shunted the inductance, so that the actual value of resistance 19 will differ from that calculated as described above by the mount of these losses. For use at very high frequencies, an inductance such as that shown in very conveniently made with a core of iron laminations including an air gap, the laminations being very thin, say of the order of 0.05 mm, or less.
Figure 7 illustrates an arrangement for feeding current to a valve 37 required to have a floating cathode, as for example, when the valve is employed with the load in the cathode circuit. Such a valve is known as a cathode follower since the potential of its cathode can be arranged substantially to follow that of its control grid. In this case the filament 38 is fed with alternating current from a transformer 39 which has capacity denoted by condenser 31 of value C between the live filament windings 40 and the earthy primary or core, or both primary and core. This capacity is eliminated by feeding the filament 38 through tightly-coupled chokes 28, 29. These chokes are shunted by a resistance 30 and a further resistance 32 shunts the stray capacity C. resistance 30 is bridged off from the centre tap 41 on a low resistance 42 across the filament 38, and the two halves of resistance 42 in parallel constitute in effect a portion of resistance 30. As the choke comprising inductances 28, 29 requires low self-capacity and has to carry heavy currents, requiring thick wire, it is advantageous to wind this choke on a metal core resembling a transformer core, but having a large gap (preferably in the middle of the coil) and formed of very thin laminations. As the filament current passes through the coils in series opposition, the core is not magnetised by the filament heating current, but is magnetised by the space current of the valve 37. The transformer capacity 31 also comprises the self-capacity of the resistance 32. The same circuit may be employed to allow for the capacity to earth of a battery, machine, or rectifier feeding the filament with direct current.
Figure 8 shows a direct coupling for a valve having its load placed between its cathode and earth, it being required not only to supply cathode current to the valve, but also to supply a bias potential for a subsequent valve, which may require instead of the positive potential of the cathode 38 of the valve 37, a negative potential. The valve 37 is working into a cathode load R and it is required to eliminate, as far as possible, the capacities to ground of the load resistance (required to dissipate the heat produced in the standing feed of the valve), the capacity to ground of the cathode rectifier, (and associated transformer capacities) of the rectifier supplying the bias potential. In Figure 8, 43 represents a low voltage heavy current rectifier providing the cathode current, 44 represents a comparatively high voltage rectifier supplying the bias to a subsequent valve, which it is assumed is liable to run into grid current on extreme amplitudes. The cathode load is formed by two resistances 30, 32 in series, of which resistance 32 is required to dissipate the heat produced by the steady and low frequency space current. resistance 30 is only required to dissipate the heat produced by the high frequency alternating currents. The unwanted capacities are held off by means f a triply-wound choke coil, the three windings 28, 29 and 45 being coupled as tightly as possible. The two windings 28, 29 correspond to windings 28, 29 of Fig. 7 and are of thick copper, so as to carry the cathode current, whereas the winding 45 is of thinner wire but well insulated from the other two windings on account of the higher voltage of rectifier 44. The insulation of the choke end to end, must stand the peak amplitude of the valve output. The cathode rectifier 43 and the cathode are shunted by resistance 46, 42 from which cathode centre points 47, 41 are obtained. The resistances 46, 42 being of small value, do not materially affect the design, but can be allowed for if desired. The centre point 41 of the cathode and the upper end of the choke winding 45 are closed by a resistance 48 and condenser 49 in series, designed to build out to a pure resistance, the effective impedance of rectifier 44 as seen though the leakage inductance of the choke. Any choke capacity shunting this leakage inductance can be allowed for by a modification of the circuit building it out to a pure resistance. By this means the effective shunting capacity across the cathode due to the heat-dissipating resistance 32 and the two rectifiers 43 and 44 is effectively removed and replaced by the self-capacity of the choke coil. If this coil is well designed and if it can be given an iron core to reduce its size, it is in general possible to effect a reduction of stray capacity of as much as 10:1 for a frequency range extended from 0 to 2 megacycles per second.
Dated this 6th day of July, 1936.
REDDIE & GROSE,
Agents for the Applicant,
6, Bream’s Buildings, London, E.C.4.
Improvements in and relating to the Reduction of the Effects of Unwanted Reactance introduced in Electric Circuit Arrangements by certain Circuit Elements
I, ALAN DOWER BLUMLEIN, a British Subject, of 32, Audley Road, Ealing, London, W.5, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:-
The present invention relates to electric circuit arrangements, and is particularly concerned with the provision of means for reducing the effects of the unwanted reactance introduced by certain circuit elements; for example, the invention provides means for reducing the effects of the inherent capacity and inductance of electric resistances, and the inherent capacity of inductance coils. The invention relates more particularly but no exclusively to circuit arrangements which are required to deal with signals covering a wide range of frequency.
The effects of stray capacity, for example, are observed in resistance-coupled circuits such as amplifiers, in which a thermionic valve has in its anode circuit a resistance which constitutes its load; if the valve is one which is required to dissipate considerable power, the load resistance must be of a robust construction so as to be capable of passing substantial current without overheating, and its self-capacity may be so high that relative attenuation of the higher frequencies of the signals to be amplified takes place. A similar effect may be present when the anode circuit load is constituted by a choke coil, the stray capacity of the coil serving as a relatively low impedance shunt at the higher frequencies. The inherent inductance of a load resistance in a thermionic valve amplifier may give rise to undesirable resonances with the stray shunt capacity, or, if the latter has been substantially eliminated by a method such as hat of this invention, may produce a relative accentuation of the higher frequencies. Furthermore, such stray reactances may produce other harmful effects, such as undesired phase shifts.
It is an object of the present invention to provide novel or improved means for reducing the effects of unwanted reactance introduced in electric circuit arrangements by circuit elements such, for example, as resistances and inductance coils.
The invention accordingly provides a circuit arrangement comprising a circuit element which, in operation, is required to carry relatively large currents, wherein, for the purpose of reducing the effects of the inherent shunt capacity of said element, there are provided in series with said element with respect to a source of power, building-out means comprising a resistance and an inductance coil in shunt with on another, said resistance being one having less self capacity and rated to dissipate less power than that dissipated, in operation, in said element, and wherein said building-out means are so constituted and arranged that, together with said circuit element, they simulate over a wide range of frequencies substantially a pure resistance.
The invention further provides a circuit arrangement comprising a resistance element which in operation is required to carry relatively large currents, and hence has substantial inherent inductance effectively in series with said element, there are provided in shunt with said element with respect to a source of power, building-out means comprising a resistance and a condenser in series with one another, said resistance being one having less self capacity and rated to dissipate less power than that dissipated, in operation, in said element, and wherein said building-out means are so constituted and arranged that, together with said circuit element, they simulate over a wide range of frequencies substantially a pure resistance.
The circuit element may be a resistance, in which case the resistance selected to form part of the building-out means is one having a relatively small power-dissipation rating; the power-dissipation rating of a resistance decreases as its power to carry current without overheating decreases. In this way, the inherent capacity and inductance of the building-out resistance can be made negligibly small compared to that of the resistance constituting the circuit element.
Further features and applications of the invention will be apparent from the following description, in which reference will be made to the accompanying diagrammatic drawings. In the drawings,
Fig. 1 illustrates the application of the invention to reducing the effects of the self-capacity of the anode load resistance of a thermionic valve,
Fig. 2 shows a modification of the arrangement of Fig. 1 in which the anode load comprises a choke coil,
Fig. 3 illustrates a further embodiment of the invention, a s applied to the reduction of the self-capacity of a resistance, and
Fig. 4 illustrates the application of the invention to the reduction of the inherent series inductance of a resistance.
Referring to Fig. 1, the valve 1 is fed with oscillations to be amplified from terminals 2 and is coupled to a further valve (not shown) by means of a condenser 3. The anode resistance 4 of the valve 1 has to pass substantial current, the greater part of which is accounted for by the direct component of the anode current of the valve, and resistance 4 accordingly has a considerable self-capacity; particularly is this so if the valve 1 is a so-called power valve, and if resistance 4 is of the wire-wound type. The self-capacity of the resistance 4 is indicated in Fig. 1 by the dotted condenser 5.
In order to reduce the effects of the self-capacity 5, a resistance 6 and an inductance coil 7 in shunt with one another are connected in series with resistance 4 between the anode of valve 1 and the positive terminal of the anode current course, and it is arranged that the relationship L=CR2 is substantially satisfied, where L is the inductance of coil 7, C is the magnitude of self-capacity 5 and R is the value of each of the resistance 4 and 6. In these circumstances, the anode circuit load of valve 1 is effectively a pure resistance of magnitude R. it is sometimes difficult to determine accurately the magnitude of stray capacity 5, and in this case, coil 7 may be given a value slightly larger than is likely to be necessary, and a condenser 8 is then connected in shunt with stray-capacity 5, the condenser 8 being of such a capacity that the total effective shunt capacity due to 5 and 8 is equal to EQUAT. HERE . The smaller the effective total shunt capacity, the easier it is to construct the inductance 7.
It will be seen that resistance 4 can be allowed to have a relatively high self-capacity, provided that the value thereof does not exceed EQUAT. HERE . Resistance 6, however, since it is shunted by coil 7 which provides a path of low impedance to direct and low frequency currents, only has to pass current at high frequencies, and there is accordingly chosen for resistance 6 one rated to dissipate less power than that dissipated in the anode resistance 4, and so constructed that its self-capacity is as low as possible, and is at least much smaller than that of resistance 4. The coil 7 is also made to have as low a self-capacity as possible, since, as will be apparent, no advantage is obtained unless the self-capacity of the coil 7 and resistance 6 is much smaller than the self-capacity of the resistance 4.
In certain cases, it may be desirable to swamp the stray capacity 5 of resistance 4 by making the capacity of condenser 8 relatively large compared to the stray capacity 5, so that at high frequencies, resistance 4 if effectively short-circuited: this is conveniently done if resistance 4 is not correctly represented at high frequencies by a resistance shunted by a condenser – for example, if resistance 4 has appreciable inherent series inductance. In practice, this arrangement may have the disadvantage that the inductance which coil 7 is required to have may be so large that the coil 7 itself has substantial stray shunt capacity. In this case, the whole circuit 4, 5, 6, 7, 8 is shunted by a further condenser, and the resultant circuit, which is effectively a resistance shunted by a condenser, is built out in the manner already described by means of a further inductance and resistance to simulate a pure resistance. The stray capacity of the further inductance can, in generally, be made relatively small. A similar arrangement may be adopted when the stray capacity of resistance 4 is large, or cannot be accurately determined.
If coil 7 is required to have a high inductance, and is air cored, it may be found that the length of wire necessary is so long that the coil has a resonance within the working frequency range at which its impedance from end to end is low. Such a resonance usually occurs at the frequency at which the length of wire in the coil is approximately half a wavelength. In this case coil 7 is conveniently wound on an iron ore, since an iron-cored coil can be given a greater inductance for a given length of wire. Thus coil 7 may be wound on a core of iron laminations, the core having an air gap and the laminations being very thin – say of the order of 0.05 mm. or less in thickness; such a coil is particularly suitable for circuits in which a high inductance is necessary and which are designed to work up to frequencies of the order of megacycles.
The value of resistance 4 my, in practice, require to be made slightly less than the value R in order to compensate for the series resistance of coil 7; approximate compensation is obtained if resistance 4 is less than the value R by the series resistance of coil 7, but more accurately, if r is the series resistance of coil 7, resistance 4 should be given a value equal to that of a resistance of value R and a resistance of value EQUAT. HERE in shunt. Similarly, if coil 7 has an iron core, the losses in the core are effective as a resistance in shunt with the coil, and the magnitude of resistance 6 is modified to take account of these losses.
In the arrangement of Fig. 2, in which pats which are common to Fig. 1 have the same references, the anode circuit of valve 1 is required to have a low resistance to direct current, but to have a substantially constant and relatively high impedance above a predetermined frequency. In order to obtain the desired low resistance to D.C., the valve 1 is coupled to the succeeding valve by means of a large iron-cored choke coil 9; the choke has, however, a high self-capacity (indicated by the dotted condenser 5) and this self-capacity has the effect of reducing the anode circuit impedance at high frequencies.
It will be assumed that the inductance of the choke coil 9, divided by the self-capacity 5, is large compared with the square of the impedance (which will be referred to as R) which the anode circuit is required to have above the predetermined frequency referred to. The effect of the stray capacity 5 (the magnitude of which will be referred to as C) are thus reduced by shunting coil 9 by a resistance 10 of magnitude R, and connecting in series with the coil between the anode of valve 1 and the positive terminal of the anode current source (not shown) in further resistance 6 of the magnitude R shunted by an inductance coil 7 of magnitude L given by L=CR2. Since, above the predetermined frequency, the impedance of coil 9 is large compared with the resistance R, the arrangement is then such that, above this predetermined frequency, which can be relatively low, the anode circuit impedance is substantially constant and equal to R.
The resistance 6 and the coil 7 are arranged to have self-capacities which are small compared with that of coil 9; the self-capacity of resistance 10 may be regarded as forming a part of condenser 5. As in the arrangement of Fig. 1, the magnitudes of resistances 6 and 10 may be modified slightly to take into account the losses of coils 7 and 9, while resistance 10 may be reduced slightly in value to take account of the series resistance of coil 7. Coil 7 may be air-cored, or may have a core of comminuted magnetic material.
Fig. 3 illustrates an arrangement of the kind forming the subject matter of co-pending cognate Applications Nos. 19480/35, 25070/35 and 33805/35. The valve 11 operates with its load impedance connected between its cathode and the negative terminal of an associated source of anode current (not shown), and its filament is fed with alternating current from the secondary winding 12 of a transformer, the iron core of which is at an earthy potential. The stray capacity 13 of the secondary winding 12 to the core, and thus to earth or chassis, is "held off" (or prevented from having an undesirable effect in the manner described in the specification of the cognate applications referred to by means of the coupled choke coils 14 and 15; a resistance 16 is connected between the centre point of a relatively small resistance 17 shunted across the cathode and the centre point of a secondary winding 12, and a further resistance ** is connected from this point to earth. The parallel-aiding inductance, L, of the coils 14, 15 is made equal to CR2, where C is the magnitude of stray-capacity 13, and R is the value of each of resistances 16 and 18, and the arrangement is then such that the effective impedance between the cathode of valve 11 and earth is substantially a pure resistance of magnitude R.
Now the resistance 18 has to carry the direct and low-frequency currents passing through the valve 11, and it is accordingly necessary in practice to employ here a resistance having substantial stray capacity. For purposes of design, however, this stray capacity is treated as forming part of capacity 13, and if the stray capacities of coils 14, 15 and of resistance 16 are made relatively small, and if the parallel-aiding inductance of the coils 14, 15 has the value postulated, the effects of the stray capacity of resistance 18, as well as the stray capacity of winding 12 to earth, or to chassis or other conductor in the apparatus are eliminated or much reduced. It will be noted that resistance 16 has only to carry high-frequency current, and can therefore be made of low current-carrying capacity.
In Fig. 4, in which parts are common to Fig. 1 have the same references, the anode load resistance 4 of valve 1 has an inherent series inductance (shown dotted at 20) of magnitude L1 as well as inherent shunt capacity (shown dotted at 5). To reduce the tendency of the effective anode load to increase with increase in frequency due to inductance 20, resistance 4 is shunted by a resistance 21 in series with a condenser 22 of magnitude C1. Resistances 4 and 21 are given the same magnitude, R, and it is arranged that C1 is such that the relationship EQUAT. HERE is substantially satisfied. Then, if resistance 21 has not substantial effective shunt capacity, and if resistance 21 and condenser 22 are substantially non-inductive, the anode circuit load of valve 1 is effectively a pure resistance equal to R shunted by the self capacity 5 of resistance 4.
The self-capacity 5 of resistance 4 can be compensated for in the manner described with reference to Fig. 1 by the use of elements 6 and 7 connected as shown.
The invention is not limited to the arrangements described above by way of example, and many modifications within the scope of the appended claims will occur to those versed in the art.
Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:-
Dated this 6th day of July, 1936.
REDDIE & GROSE,
Agents for the Applicant,
6, Bream’s Buildings, London, E.C.4.
Leamington Spa: Printed for His Majesty’s Stationery Office, by the Courier Press. – 1937.