448,421

PATENT SPECIFICATION

Application Date: Sept. 4, 1934. No. 25497/34.

Complete Specification Left: Aug. 20, 1935.

Complete Specification Accepted: June 4, 1936.

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PROVISIONAL SPECIFICATION

Improvements in and relating to Thermionic Valve Circuits

I, ALAN DOWER BLUMLEIN, a British Subject, of 7, Courtfield Gardens, Ealing, London, W.13, do hereby declare the nature of this invention to be as follows:-

The present invention relates to thermionic valve circuits.

In working many electrical devices into the input of an amplifier, it is known that the finite input impedance of the first valve (e.g. grid capacity), and the stray capacities and self capacities associated with the input circuit seriously limit the amount of signal that can be obtained. A typical instance of this occurs with a photo-cell as used for picking up the light for television transmission. In such a case the photo-cell may have a self capacity of say 10 m m Fds., and the input capacity of the valve may account for another 15 m m Fds., giving, with the addition of 5 m m Fds., of stray capacity, a total input capacity of 30 m m Fds.

If such a device is to be employed to handle frequencies up to 500 kc., a feed resistance to the photo-cell greater than 10000 ohms cannot be employed without serious loss of the upper frequencies. It is however advantageous to employ a resistance greater than this value to feed the photo-cell in order to obtain the best possible noise to signal ratio from the circuit, since by using a high resistance it is possible to obtain the maximum signal at the upper frequencies to overcome the "shot noise" of the valve, and at the same time the effect of the Johnson noise due to the resistance is reduced.

In such a system an increased response is obtained at the lower frequencies and it is necessary to correct for the relative top loss in later stages of the amplifier. This correction is dependent on the value of the capacity of the photo-cell, valve and wiring, and any change in these components usually involves a change in the amount of top correction applied. Similar troubles occur when devices other than photo-cells are used operating into the grid circuit of a valve, and the difficulty may be due not only to capacity, but also to leakage resistance. Thus if the leakage resistance of the grid-cathode path of the valve is not very much greater than the feed resistance, the low frequency and D.C. response will be affected by this leakage and any variation, either with time or due to changing the valve, will alter the amount of amplification and high frequency correction required in the later stages of the amplifier.

It is the object of this invention to provide means whereby the apparent input impedance of a valve, and in certain cases the shunt impedance of the device driving it, is considerably increased, thus enabling distortionless operating from input impedances which would otherwise be seriously shunted by the valve impedance or shunt impedance of the device under consideration.

According to this invention, a valve which is required to operate from an impedance comparable with or high compared to its own input impedance, has a loading impedance inserted in its cathode circuit so that the potential of the cathode varies in correspondence with the potential of the grid. The potential variations of the cathode are arranged to be comparable with those of the grid and in this way the effective input impedance of the valve can be substantially increased.

According to a feature of this invention, carious elements such as the shielding or other elements of the input circuit, are either connected to the cathode of the input valve, or are otherwise made to follow the potential of the grid or cathode of this valve in such a manner as to reduce effectively the shunting effect of any unwanted capacity or leakage associated with these screens or elements. The input valve may be provided with a screen or screens and/or guard rings, so arranged that the grid has only direct impedance to these screens or the cathode, so that by making these screens or guard rings follow the potential of the cathode, the total input impedance of the valve may be substantially increased. The output from the valve above described to a further valve may be taken from across the whole or a part of the impedance in the cathode circuit.

The invention will be described with reference to the accompanying drawings, in which three circuits embodying the invention are shown diagrammatically. It should be noted that in all the Figures points marked with an arrow head are assumed to be connected to a suitable source of potential having one terminal earthed.

Fig. 1 shows a simple circuit illustrating the operation of this invention. The source of voltages to be amplified is shown as being equivalent to the generator E in series with a high impedance G. A valve 1 has its screen conductively connected to the cathode by means of a floating battery B. The cathode is connected through an impedance L to a suitable source of potential which may be positive or negative, depending on the required bias of the valve. The anode is connected to a source of high tension. The signal output is taken from the cathode along the lead 2. An impedance z is shown dotted representing the input impedance of the valve 1 between grid and cathode. This impedance includes the impedance of the grid to the screen. It is assumed that the valve 1 is so arranged that there is no appreciable direct leakage or capacity between the grid and any elements other than the screen and cathode. If the impedance L is high compared to the inverse of the mutual conductance of the valve, the potential of the cathode will tend to follow the potential variations of the grid.

Suppose for example, that a change of 1 volt in the positive direction on the grid causes a change in cathode potential of 0.99 volts, the potential difference across the input impedance z will only be 0.01 volt, so that the current drawn by the impedance z will only be 100th of what it would have been if the cathode had not followed the potential of the grid. The effective input impedance of the valve will then be 100 times greater than it would have been with fixed cathode and screen potentials.

If now the impedance G is large compared to the impedance z, but is smaller than the impedance 100 z, then a substantially faithful copy of the generator voltage is obtained on the grid whereas this would not have been the case if the cathode potential had been fixed. The voltage amplification obtained by the valve 1 may be in the neighbourhood of unity or less but the valve nevertheless enables the full E.M.F. to be obtained from the high impedance G.

The output potential difference developed across the impedance L may be taken by lead 2 to the grid circuit of a valve worked in a normal manner, since the impedance looking back along this lead will be approximately the inverse of the mutual conductance of the vale in parallel with L; this impedance will be much less than the normal input impedance of the next valve.

Alternatively, a resistance may be connected in the anode circuit of this valve and the output may be taken from across this anode resistance, but any attempt to obtain amplification in this manner will tend to decrease the effective input impedance of the valve due to the failure of the cathode to follow the grid potential satisfactorily.

It would at first sight appear that an improvement to the essential signal to noise ratio (due to the "shot noise" of the valve) is obtained. This however is not the case, since if the input impedance G is greater than z, the reduction of "shot" current due to the negative reaction of the high impedance cathode load and the apparently fixed grid potential, will not be realised, because the grid potential will tend to follow the cathode potential.

The method of operating an input valve above described provides a means by which the effect of the input impedance of the valve can be eliminated as regards its effect on the amplification characteristic, but the method does not eliminate the harmful effect of this input impedance on the noise introduced by the valve. If the high impedance input circuit G were associated with a valve of input impedance z, variable with frequency, having its cathode potential fixed, it would be necessary to follow this valve by suitable correction in the amplifier to allow for the shunting effect of the input impedance z. Any change in the constants G and z would involve a change in the correction. This invention provides a method by which this correction is in effect applied in the first valve by means of components which do not require to be adjusted to suit the particular correction obtained. In effect the valve constitutes a negative reaction circuit which automatically corrects for the loss due to the input impedance of the valve.

Fig. 2 shows a circuit by which not only is correction made for the input capacity of the valve, but also for the shunt capacity of the photo-cell from which it is fed. In this case, the photo-cell anode 3, is connected to the grid of the valve 1 and the photo-cell cathode 4 is connected through a condenser 5 to the cathode of the valve, this cathode being connected through a comparatively high resistance R to earth. The anode of the photo-cell is connected through a resistance 6 to suitable source of grid bias potential. The cathode 4 of the photo-cell is connected through a resistance 7 to a point of high negative potential for polarising the photo-cell. The screen of the input valve 1 is fed through a high resistance and is connected through a condenser 8 to the cathode.

The condensers 5 and 8 are sufficiently large to ensure that as regards the relevant A.C. frequencies, the cathode 4 of the photo-cell and the screen of the valve 1 follow the potential of the cathode of this valve. If desired, a screen 9 may encircle the lead joining the anode 3, and the grid of the valve 1. This screen is thus made to follow the A.C. potential of the cathode of the valve 1. The screen may be further extended so that there is very little direct capacity between the circuit of the anode 3 and earth.

By the means shown in Fig. 2, the effective input capacity of the valve 1, including the photo-electric cell capacity, stray wiring capacities and the grid capacity of the valve is reduced to say 1/10th of its normal value. For the case cited at the beginning of these notes, this would reduce the effective input capacity from 30 m m Fds. To 3 m m Fds., which would enable the anode resistance of the photo-cell to be increased from 10000 ohms to 100,000 ohms while maintaining the characteristic substantially flat up to 500 kc. This value of anode resistance for the photo-cell would probably be sufficient to ensure that the noise obtained is almost entirely due to the unavoidable "shot" noise of the valve 1. In effect an amplification of 10:1 would be obtained from this valve at the higher frequencies, since the use of this valve has enabled the resistance 6 to be increased 10 times. The reduction of the effective capacity of the photo-cell is only possible by virtue of the fact that the photo-cell is voltage saturated, that is to say the current through the cell for any given illumination does not vary with the voltage applied across the cell, and therefore the current received by the anode 3 is independent of the potential of the cathode 4, except for any capacity effect between these two elements.

This method of working is applicable to other devices where the D.C. potential of the electrode from which signals are drawn does not materially affect the current flowing to that electrode, i.e., where the device has a very high internal impedance apart for self capacity or conductive leakage.

Fig. 3 shows a photo-cell with three stages of a following amplifier. There is provided a photo-cell having anode 3 and cathode 4, the anode being fed from a resistance 6. The anode is connected directly to the grid of the input valve 1 and the connecting lead may be shielded by a screen 9 connected to the cathode of the valve 1. The cathode 4 of the photo-cell is polarised by a floating battery 10, which is connected between it and the cathode of the valve 1. This cathode is connected through a high cathode loading resistance R1 to a suitable source of biasing potential (not shown). The screen of the valve 1 is connected through a Neon tube, or similar constant potential device, to the cathode and is fed through a resistance 11. The polarising batter 10 and the Neo tube 12 have been substituted for the condensers 5 and 8 in Fig. 2, since it is required to handle all frequencies down to direct current.

In order to reduce the effects of the capacity to ground of the screen 9 and other equipment connected to the cathode of the valve 1, from loading the cathode too severely, a further screen 13 is arranged external to the cathode circuit of the valve 1 and this screen may if desired embrace the valve 1 itself although this is not shown. The screen 13 is caused to follow the cathode of the valve 1 by being connected to the cathode of a valve 14 whose grid is connected to the cathode of the valve 1 and whose cathode circuit is provided with a resistance R2. This valve is a "follower" valve operating in the same manner as valve 1, and serves to reduce the general capacity load on the cathode of the valve 1. The anode 3 of the photo-cell is supplied with current through the resistance 6, which, owing to the reduced input capacity from the photo-cell, may be higher than would otherwise be necessary for obtaining a flat characteristic. This resistance is fed from a further resistance 15 which is shunted by a condenser 16. Possible values may here be resistance 6-100,00 ohms, resistance 15-2 megohms and condenser 16-0.006 m Fds.

The resistance 15 and condenser 16 serve to give an enhanced response for the very low frequencies, so that the input to the valve 1 from the photo-cell at these frequencies is large compared to the effect of any random variation of battery potentials or valve characteristics. The cathode of the valve 1 is connected over the lead 2 to the grid of the valve 17. The anode of this valve is fed through a resistance 18 and feeds a potentiometer consisting of resistance 19 and 20 connected in series. The lower end of resistance 20 is taken to a point having such a value of negative potential that the correct grid bias is obtained for the following valve 21.

Resistances 18 and 20 in parallel may for example have a value of the order of 4200 ohms, and the valve 17 may have a mutual conductance of say 5.0 m. amps. per vol. The resistance 19 is chose so that for very low frequencies no amplification is obtained between the grids of the valves 17 and 21, the amplification being substantially that of valve 21. The condenser 22, which shunts the resistance 19, increases the amplification for the higher frequencies, so that at these frequencies amplification is obtained between the grids of the valves 17 and 21 also.

By suitably proportion the resistances 18, 19 and 20, and the condenser 22 with reference to 6, 15 and 16, the overall characteristic may be made flat and the output from the valve 21 will require no further correction. The small inductance 23 may be used to correct the high frequency loss produced by the capacities of the anode of valve 17, the grid of valve 21 and the associated coupling circuits.

The condenser 16 is arranged to be sufficiently large to prevent its value being substantially affected by any stray capacity bridged across it or by the self capacity of the resistance 15.

It will be noted that a comparatively high output is obtained from the photo-cell at very low frequencies. There is no amplification of this D.C. voltage previous to the grid of the valve 21, so that no instability will be obtained due to the slight variation of the biasing voltages etc. It should be noted that all sources supplying biasing potentials etc. must have either very low regulation, or a resistive regulation invariant with frequency.

The values given above are only given by way of example and, for instance, a much higher value may be found suitable for the photo-cell anode resistance 15. Also the correction for the loss due to condenser 16 can be effected in more than one stage by any of the well know methods.

It should be noted that, even if the leakage resistance between the grid and cathode of the valve 1 is comparable with the resistance 15, this leakage will not appreciably affect the output obtained from the valve 1, since there will be very little voltage operative across this leakage.

The valve 1 is conveniently constructed so that there can be no direct leakage from the anode or screen of the valve to the grid, which leakage might otherwise bias the grid very positive. Such leakage may be prevented by suitable guard rings connected either directly or through a small source of voltage to the cathode. If the heating battery or (in the case of an indirectly heated valve) the heating winding of the cathode of valve 1 introduces a large loading capacity across the cathode load of this valve, the heating battery or winding may be caused to follow the cathode potential by means of screens or connections to a "follower" valve such as 14.

The invention is described above with reference to a photo-cell which is typical of a high impedance device which requires an amplifying valve of very high input impedance. The invention is readily applicable to any other device where it is desired to obtain an effective high input impedance. In the examples given, it is advantageous to choose for the valve 1 9as is know for the first stages of amplifiers) one having a high slope at low anode current and low capacities between grid and cathode and between grid and screen.

The present invention may be applied advantageously to the first amplifying valve following a television signal producing device of the type where a plurality of elements are exposed to light and are switched, for example by means of a cathode ray, successively to produce the required signal. In such an application it may be arranged that electrodes or elements of the transmitting device which have large capacity to the signal circuit may be made to follow the potential of the cathode of the firs amplifying valve. Similarly, if a large D.C. output is require, it may be arranged that the potentials of certain electrodes "follow" in order to maintain equilibrium potential conditions in the device.

Dated this 4th day of September, 1934.

REDDIE & GROSE,

Agents for the Applicant,

6, Breamís Buildings, London, E.C.4.

COMPLETE SPECIFICATION

Improvements in and relating to Thermionic Valve Circuits

I, ALAN DOWER BLUMLEIN, a British Subject, of 7, Courtfield Gardens, Ealing, London, W.13, 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 thermionic valve circuits.

In working many electrical devices into the input of an amplifier, it is know that the finite input impedance of the first valve (e.g. grid capacity), and the stray capacities and self capacities associated with the input circuit seriously limit the amount of signal that can be obtained. A typical instance of this occurs with a photo-cell as used for picking up the light for television transmission. In such a case the photo-cell may have a self capacity of say 10 micro-microfarads, and the input capacity of the valve may account for another 15 micro-microfarads, giving, with the addition of 5 micro-microfarads of stray capacity, a total input capacity of 30 micro-microfarads.

If such a device is to be employed to handle frequencies up to 500 kc., at which frequency the capacitative reactance of the source and valve is approximately 10000 ohms, a feed resistance to the photo-cell greater than 10000 ohms cannot be employed without serious loss of the upper frequencies. It is however advantageous to employ a resistance greater than this value to feed the photo-cell in order to obtain the best possible noise to signal ratio from the circuit, since by using a high resistance it is possible to obtain the maximum signal at the upper frequencies to over come the "shot noise" of the valve, and at the same time the effect of the Johnson noise due to the resistance is reduced.

In such a system an increased response is obtained at the lower frequencies and it is necessary to correct for the relative top loss in later stages of the amplifier. This correction is dependent on the value of the capacity of the photo-cell, valve and wiring, and any change in these components usually involves a change in the amount of top correction applied. Similar troubles occur when devices other than photo-cells are used operating into the grid circuit of a valve, and the difficulty may be due not only to capacity, but also to leakage resistance. Thus if the leakage resistance of the grid-cathode path of the valve is not very much greater than the feed resistance, the low frequency and D.C. response will be affected by this leakage and any variation, either with time or due to changing the valve, will alter the amount of amplification and high frequency correction required in the later stages of the amplifier.

It is an object of this invention to provide means whereby the apparent input impedance of a valve, and in certain cases the shunt impedance of the device driving it, is considerably increased, thus enabling distortionless operation from input impedances which would otherwise be seriously shunted by the valve impedance of shunt impedance of the device under consideration.

It has been found that so long as the capacitative reactance of the resultant shunt capacity of the source and the valve coupled thereto is not less than twice the effective shunt resistance of the source at any frequency within the range to be handled, the shunting effect of this reactance is not serious.

According to the present invention there is provided a thermionic valve circuit comprising a thermionic valve having a cathode, an anode and a control electrode, and an input circuit connected between said control electrode and earth and containing a source of electrical variations, wherein the capacitative reactance of the resultant shunt capacity of said source and said valve is less than twice the effective shunt resistance of said source at least some part of the range of frequencies to be handled, characterised in that, for the purpose of reducing the effective value of said resultant shunt capacity, there is provided a negative feed-back path between the anode-cathode circuit and said input circuit, said feed-back path comprising an impedance element connected between said cathode and earth.

The source of variations is taken to include the source itself (such as a photo-electric cell) in which the variations are set up, together with any load resistance which may be provided. Thus the effective shunt resistance of the source is the resultant of the load resistance and the resistance of the cell in parallel. The resultant shunt capacity of the source and the valve includes all capacity which is effectively in shunt with the load resistance and which is therefore tending to reduce the output of the system at the upper end of the band of frequencies which is to be handled. In general this shunt capacity is the resultant of the input capacity of the valve between grid and cathode as normally measured, the capacity of the source and the capacity of the wiring and any other components associated with the input circuit.

In order to reduce the shunting effect of stray capacity of the source it must be arranged that this stray capacity appears between the control electrode and cathode of the valve. In many cases the stray capacity of the source tend to appear between the control electrode of the valve and earth. In these cases it is necessary to keep the direct capacity to earth of the source itself as small as possible and to provide decoupling circuits between the source and any components such as batteries which may e employed and which would otherwise increase the capacity to earth of the source. In another arrangement the source and batteries or the like associated therewith, are provided with screens, which are either coupled to the cathode of the valve through a low impedance path, or are otherwise caused to follow the potential of the cathode.

The various elements such as the shielding or other elements of the input circuit, are either connected to the cathode of the input valve, or are otherwise made to follow the potential of the control electrode or cathode of this valve in such a manner as to reduce effectively the shunting effect of any unwanted capacity associated with these screens or elements. The input valve may be provided with a screen or screens so arranged that the control electrode has mainly direct capacity to these screens or the cathode, so that by making these screens follow the potential of the cathode, the total input impedance of the valve may be substantially increased. The output from the valve above described to a further valve may be taken from across he whole or a part of the impedance in the cathode circuit.

The invention will be described by way of example with reference to the drawings filed with the Provisional Specification, in which three circuits embodying the invention are shown diagrammatically. It should be noted that in all the Figures points marked with an arrow head are assumed to be connected to a suitable source of potential having one terminal earthed.

Throughout this specification the term "earth" is used for convenience and must be given a wide interpretation.

Where it is impossible to make a direct connected, or a connection through a path of relatively low impedance over the range of frequencies to be handled, between the apparatus and physical earth, or where for some other reason such a connection is not provided, then the term "earth" must be taken to mean some object such as the chassis of the apparatus which is of substantial extent and is such that points therein have substantially no alternating potential difference between them. Thus if a potential source of good regulation has one of its poles connected to the chassis, its other pole is, for the purpose of this specification, an earth point. If a potential source is connected between two points in a circuit and has no connection with earth (as defined above) except through a high impedance path, then the source will not tend to reduce the magnitude of any potential difference (either alternating or steady) between either of the two points to which it is connected and earth, and the source may then be said to be isolated from earth.

Referring to the drawing, Fig. 1 shows a simple circuit illustrating the operation of this invention. The source of voltages to be amplified is shown as being equivalent to a generator E in series with a high impedance G. a valve 1, which in this and other examples illustrated is a screen grid valve, has its screen conductively connected to the cathode by means of a floating battery B. The cathode is connected through an impedance L to a suitable source of potential which may be positive or negative, depending on the required bias of the valve. The anode is connected to a source of high tension. The signal output is taken from the cathode along the lead 2. An impedance z is shown dotted representing the input impedance of the valve 1 between grid and cathode. This impedance includes the impedance of the grid to the screen. It is assumed that the valve 1 is so arranged that there is no appreciable direct leakage or capacity between the grid and any elements other than the screen and cathode. If the impedance L is high compared to the inverse of the mutual conductance of the valve, the potential of the cathode will tend to follow the potential variations of the grid.

Suppose for example, that a change of 1 volt in the positive direction on the grid causes a change in cathode potential of 0.99 volts in the same direction, the potential difference across the input impedance z will only be 0.01 volt so that the current drawn by the impedance z will only be one hundredth of what it would have been if the cathode had not followed the potential of the grid. The effective input impedance of the valve will then be 100 times as great as it would have been with fixed cathode and screen potentials.

If now the impedance G is large compared to the impedance z, but is smaller than the impedance 100 z, then a substantially faithful copy of the generator voltage is obtained on the grid whereas this would not have been the case if the cathode potential had been fixed. The voltage amplification obtained by the valve 1 may be in the neighbourhood of unit or less but the valve nevertheless enables the full E.M.F. to be obtained from the high impedance G.

The output potential difference developed across the impedance L may be taken by lead 2 to the grid circuit of a valve worked in a normal manner. If z is very high compared with G the impedance measured between lead 2 and earth (the output impedance) approximates to the reciprocal of the mutual conductance of the valve in parallel with L. If z were very small compared with G the output impedance would approximate to the reciprocal of the mutual conductance of the valve multiplied by its amplification factor in parallel with L. This output impedance would in general be high and would be unsuitable for feeding to the input of another valve in normal manner. In practice, z is not very small compared with G, the effective impedance in parallel with L is then not many times the reciprocal of the mutual conductance of the valve, and the output impedance is in general suitable for feeding another valve in normal manner.

Alternatively, a resistance may be connected in the anode circuit of this valve and the output may be taken from across this anode resistance, but any attempt to obtain amplification in this manner will tend to decrease the effective input impedance of the valve due to the failure of the cathode to follow the grid potential satisfactorily.

It would at first sight appear that an improvement to the essential signal to noise ratio (due to the "shot noise" of the valve) is obtained. This however is not the case, since if the impedance G is greater than z, the reduction of "shot" current due to the negative reaction of the high impedance cathode load and the apparently fixed grid potential, will not be realised, because the grid potential will tend to follow any variations of cathode potential produced by valve noise.

The method of operating an input valve above described provides a means by which the effect of the input impedance of the valve can be substantially eliminated as regards its effect on the amplification characteristic, but the method does not eliminate the harmful effect of this input impedance on the noise introduced by the valve. If the high impedance input circuit G were associated with a valve of input impedance z, variable with frequency, having its cathode potential fixed, it would be necessary to provide suitable correction in the amplifier subsequent to this valve to allow for the shunting effect of the input impedance z. Any change in the constants G and z would involve a change in the correction. This invention provides a method by which this correction is in effect applied in the first valve by means of components which do not require to be adjusted to suit the particular correction obtained. In effect the valve constitutes a negative reaction circuit which greatly reduces the effect of the lowering of the input impedance of the valve at high frequencies.

Fig. 2 shows a circuit by which not only is correction made for the input capacity of the valve, but also for the shunt capacity of the photo-cell from which it is fed. In this case, the photo-cell anode 3 is connected to the grid of the valve 1 and the photo-cell cathode 4 is connected through a condenser 5 to the cathode of the valve, this cathode being connected through a comparatively high resistance R to earth. The anode of the photo-cell is connected through a resistance 6 to a suitable source of grid bias potential for the valve 1. The cathode 4 of the photo-cell is connected through a resistance 7 to a point of high negative potential for polarising the photo-cell. The screen of the input valve 1 is fed through a high resistance and is connected through a condenser 8 to the cathode.

The condensers 5 and 8 are sufficiently large to ensure that as regards the relevant A.C. frequencies, the cathode 4 of the photo-cell and the screen of the valve 1 follow at least approximately the potential of the cathode of this valve. If desired, screen 9 may encircle the lead joining the anode 3, and the grid of the valve 1. This screen is thus made to follow the A.C. potential of the cathode of the valve 1. The screen may be further extended so that there is very little direct capacity between the circuit of the anode 3 and earth.

In order to prevent leakage across the surface of the insulators supporting the cathode 4 of the photo-cell and the control electrode of valve 1, it is desirable to provide guard rings on these insulators. The guard rings may take the form of bands of metal forming closed paths around the insulators. In the case of the photo-cell the guard ring must be earthed and the control grid guard ring of the valve 1 must be connected to the cathode of this valve.

By the means shown in Fig. 2, the effective input capacity of the valve 1, including the photo-electric cell capacity, stray wiring capacities and the grid capacity of the valve is reduced to say 1/10th of its normal value. For the case cited at the beginning of this specification, this would reduce the effective input capacity for 30 micro-microfarads to 3 micro-microfarads, which would enable the anode resistance of the photo-cell to be increased from 10000 ohms to 100,000 ohms wile maintaining the characteristic substantially flat up to 500 kc. This value of anode resistance for the photo-cell would probably be sufficient to ensure that the noise obtained is almost entirely due to the unavoidable "shot" noise of the valve 1. In effect an amplification of 10:1 would be obtained from this valve at the higher frequencies, since the use of this valve has enabled the resistance 6 to be increased 10 times. The reduction of the effective capacity of the photo-cell is only possible by virtue of the fact that the photo-cell is voltage saturated, that is to say the current through the cell for any given illumination does not vary with the voltage applied across the cell, and therefore the current received by the anode 3 is independent of the potential of the cathode 4, except for any capacity effect between these two elements.

This method of working is applicable to other devices where the D.C. potential of the electrode from which signals are drawn does not materially affect the current flowing to that electrode, i.e., where the device has a very high internal impedance apart from self capacity or conductive leakage.

Fig. 3 shows a photo-cell with three stages of a subsequent amplifier. There is provided a photo-cell having anode 3 and cathode 4, the anode being fed from a resistance 6. The anode is connected directly to the grid of the input valve 1 and the connecting lead may be shielded by a screen 9 connected to the cathode of the valve 1. The cathode 4 of the photo-cell is polarised by a floating battery 10, which is connected between it and the cathode of the valve 1. This cathode is connected through a high cathode loading resistance R1 to a suitable source of biasing potential (not shown). The screen of the valve 1 is connected through a neon tube, or similar potential stabilising device, to the cathode and is fed through a resistance 11. The polarising battery 10 and the neon tube 12 have been substituted for the condensers 5 and 8 in Fig. 2, since it is required to handle all frequencies down to direct current.

In order to reduce the effects of the capacity to earth of the screen 9 and other equipment connected to the cathode of the valve 1, from loading the cathode too severely, a further screen 13 is arranged external to the cathode circuit of the valve 1 and this screen may if desired embrace the valve 1 itself although this is not shown. The screen 13 is caused to follow the cathode of the valve 1 by being connected to the cathode of a valve 14 whose grid is connected to the cathode of the valve 1 and whose cathode circuit is provided with a resistance R2. This valve is a "follower" valve operating in the same manner as valve 1, and serves to reduce the general capacity load on the cathode of the valve 1. The anode 3 of the photo-cell is supplied with current through the resistance 6, which owing to the reduced input capacity from the photo-cell, may be higher than would otherwise be necessary for obtaining a flat characteristic. This resistance is fed from a further resistance 15 which is shunted by a condenser 16. Possible values may here be resistance 6-100,000 ohms, resistance 15-2 megohms and condenser 16-0.006 microfarads.

The resistance 15 and condenser 16 serve to give an enhanced response for the very low frequencies, so that the input to the valve 1 from the photo-cell at these frequencies is large compared to the effect of any random variation of battery potential or valve characteristics. The cathode of the valve 1 is connected over the lead 2 to the grid of the valve 17. The anode of this valve is fed through a resistance 18 and feeds a potentiometer consisting of resistances 19 and 20 connected in series. The lower end of resistance 20 is taken to a point having such a value of negative potential that the correct grid bias is obtained for the subsequent valve 21.

Resistances 18 and 20 in parallel may for example have a value of the order of 4200 ohms, and the valve 17 may have a mutual conductance of say 5.0 milliamps per volt. The resistance 19 is chose so that for very low frequencies substantially no amplification is obtained between the grids of the valves 17 and 21, the amplification being substantially that of valve 21. The condenser 22, which shunts the resistance 19, increases the amplification for the higher frequencies, so that at these frequencies amplification is obtained from both valves 17 and 21.

By suitably proportioning the resistances 18, 19 and 20, and the condenser 22 with reference to 6, 15 and 16, the overall characteristic may be made flat and the output from the valve 21 will require no further correction. The small inductance 21 may be arranged to constitute with the anode-to-earth capacity of the valve 17, grid-to-earth capacity of the valve 21, and the shunt capacities of the associated coupling circuits a section of a low-pass filter which, although it provides a cut-off at a high frequency, nevertheless improves the response below the cut-off frequency. In this way the inductance may serve to correct the high frequency loss which would otherwise be introduced by these shunt capacities.

The condenser 16 is arranged to be sufficiently large to prevent its value being substantially affected by any stray capacity bridged across it or by the self capacity of the resistance 15.

It will be noted that a comparatively high output is obtained from the photo-cell at very low frequencies. There is no amplification of this D.C. voltage previous to the grid of the valve 21, so that no instability will be obtained due to the slight variation of the biasing voltages etc. It should further be noted that all sources supplying the biasing potentials etc. must have either very low regulation, or a resistive regulation invariant with frequency.

The values given above are only given by way of example and, for instance, a much higher value may be found suitable for the photo-cell anode resistance 15. Also the correction for the loss due to condenser 16 can be effected in more than one stage by any of the well known methods.

It should be noted that, even if the leakage resistance between the grid and cathode of the valve 1 is comparable with the resistance 15, this leakage will not appreciably affect the output obtained from the valve 1, since there will be very little variable voltage operative across this leakage.

The valve 1 is conveniently constructed so that there can be no direct leakage from the anode or screen of the valve to the grid, which leakage might otherwise bias the grid very positively. Such leakage may be prevented as already described by suitable guard rings connected either directly or through a small source of voltage to the cathode. If the heating battery or (in the case of an indirectly heated valve) the heating winding of the cathode of valve 1 introduces a large loading capacity across the cathode load of this valve, the heating battery or winding may be caused to follow the cathode potential by means of screens or connections to a "follower" valve such as 14.

The invention is described above with reference to a photo-cell which is typical of a high impedance device which requires an amplifying valve of very high input impedance. The invention is readily applicable to any other device where similar considerations arise.

In the examples given, it is advantageous to choose for the valve 1 (as is known for the first stages of amplifiers) one having a high slope at low anode current and low capacities between grid and cathode and between grid and screen.

The present invention may be applied advantageously to the first amplifying valve following a television signal producing device of the type where a plurality of elements are exposed to light and are switched, for example by means of a cathode ray, successively to produce the required signal. In such an application it may be arranged that electrodes or elements of the transmitting device which have large capacity to the signal circuit may be made to follow the potential of the cathode of the first amplifying valve. Similarly, if a large D.C. output is required, it may be arranged that the potentials of certain electrodes "follow" in order to maintain equilibrium potential conditions in the device.

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:-

  1. A thermionic valve circuit comprising a thermionic valve having a cathode, an anode and a control electrode, and an input circuit connected between said control electrode and earth and containing a source of electrical variations, wherein the capacitative reactance of the resultant shunt capacity of said source and said valve is less than twice the effective shunt resistance of said source at least some part of the range of frequencies to be handled, characterised in that, for the purpose of reducing the effective value of said resultant shunt capacity, there is provided a negative feed-back path between the anode-cathode circuit and said input circuit, said feed-back path comprising an impedance element connected between said cathode and earth.
  2. A thermionic valve circuit according to claim 1, having at least one element such as a screen, the presence of which tends to increase the shunt capacity of said input circuit, wherein, for the purpose of reducing the effective value of said shunt capacity, said element is connected o said cathode either directly or through a path having a low impedance over the range of frequencies to be handled.
  3. A thermionic valve circuit according to claim 1 or 2, wherein said valve is of the so-called screen-grid type.
  4. A thermionic valve circuit according to claim 1 or 2 having at least one element such as a screen, the presence of which tends to increase the shunt capacity of said input circuit, wherein, for the purpose of reducing the effective value of said shunt capacity, said element is connected to the cathode of a second valve, the control electrode of said second valve being connected to the cathode of said first valve and said second valve being provided with a negative feed-back path between the anode-cathode circuit and the input circuit thereof, said feed-back path comprising a resistive impedance element connected between the cathode of said second valve and earth.
  5. A thermionic valve circuit according to claim 4, wherein said second valve is of the so-called screen-grid type.
  6. A thermionic valve circuit according to claim 3 or 5, wherein the screening rid of said valve is maintained at a suitable positive potential with respect to the cathode thereof by means of a source of potential difference having suitable points herein connected to said cathode and screening grid and being otherwise electrically isolated from earth at all frequencies within the range of frequencies to be handled.
  7. A thermionic valve circuit according to claim 3 or 5, wherein the screening grid of said valve, or, as the case may be, either of said valves, is fed, through an impedance element having a large impedance over the range of frequencies to be transmitted, from the positive terminal of a suitable source of potential difference, the negative terminal of which is earthed, and wherein a suitable potential-stabilising device is connected between said screening grid and said cathode for the purpose of maintaining these screen grid at a substantially constant potential with respect to the cathode.
  8. A thermionic valve circuit according to claim 7, wherein said potential-stabilising device comprises a gaseous discharge tube.
  9. A thermionic valve circuit substantially as described with reference to the drawing filed with the Provisional Specification.

Dated this 20th day of August, 1935.

REDDIE & GROSE,

Agents for the Applicant,

6, Breamís Buildings, London, E.C.4.

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Leamington Spa: Printed for His Majestyís Stationery Office, by the Courier Press. - 1936.