Application Date: Nov. 6, 1937. No. 31368/38
(Divided out of Application No. 30464/37.)
Complete Specification Left: Oct. 29, 1938.
Complete Specification Accepted: May 6, 1939.
Improvements in or relating to Thermionic Valve Circuits
We, ALAN DOWER BLUMLEIN, of 37, The Ridings, Hanger Lane, Ealing, London, W.5, and ERIC LAWRENCE CASLING WHITE, of 7, Vine Lane, Hillingdon, Middlesex, both British subjects, do hereby declare the nature of this invention to be as follows:
The present invention relates to thermionic valve circuits.
The invention will be described with reference to the accompanying drawing in which the figure is a circuit diagram illustrating a particular embodiment of apparatus according to this invention.
For ease of description, the embodiment shown in the Figure will be described with particular reference to a well-known standard of television transmission in which the black level is maintained at approximately 30% of the peak signal representing peak white and the synchronising signals consist of pulses which reduce the amplitude of the signal from the black level in a ‘blacker than black’ direction to zero. The line synchronising pulses of 10 micro-seconds duration on the standard transmitted waveform are followed by black level intervals lasting 5 micro-seconds. The frame synchronising pulses are of 40 micro-seconds duration and are followed by black level intervals of 10 micros-seconds duration. Signals of this waveform are illustrated on page 373 of the issue of ‘The Wireless World’ of 4th October 1935.
In the Figure, 6 represents a pentode valve in which 1 is the control grid, 2 is the cathode, 3 is the screening grid, 4 the suppressor grid and 5 is the anode. The cathode 2 is connected by means of a high resistance 9 to earth and is also connected by a high resistance 7 to the suppressor grid 4 which is by-passed to earth by condenser 8. The screening grid 3 is connected to a source of positive potential which is not shown. The anode 5 of the valve 6 is connected to a positive source of potential through a time delay network comprising inductances 26, condensers 27 and the terminating resistance 25. The cathode 2 of valve 6 is connected via a condenser 10 to the grid 11 of a further valve not shown. Grid 11 is connected through a high resistance 12 to a tapping on the potentiometer 13 connected between the source of positive potential and earth. The condenser 14 between the tapping and earth is for decoupling purposes. The output from the delay network is passed through a condenser 17 to the grid 21 of a further pentode valve 19, the grid 21 and cathode 20 being joined by a leak resistance 18. The cathode 20 is by-passed to earth by condenser 16 and is biassed slightly positively by means of a variable tap on potentiometer 13. The screening grid 22 of valve 19 is connected to a suitable point on the potentiometer 13 and is by-passed to cathode 20 by condenser 15. The suppressor grid 23 is directly connected to the cathode 20. The anode 24 of pentode valve 19 is connected to the grid 11 of the further valve.
In operation, television signals of the standard waveform are applied to the grid 1 of the valve 6 with the synchronising signals in a negative sense, it being assumed in the first place that the D.C. component is present but that the signals vary in strength owing to a variable attenuation. It is thus apparent that the datum level will alter in absolute value and consequently the absolute black level will vary. The first valve 6 acts as a cathode follower to provide a low impedance coupling to the valve 19. A cathode follower valve is one in which the potential of the cathode thereof follows substantially the potential of the control grid. The suppressor grid 4 is held at mean cathode potential by the leak resistance 7. The potential on the suppressor grid does not vary with the cathode potential variations owing to the by-pass condenser 8. This device provides the correct suppressor potential while relieving the circuit of the extra cathode capacity which would be entailed by the use of a direct cathode-suppressor grid connection. The use of the condenser 8 and resistance 7 in this manner is a feature of the invention and can be employed other than in connection with the features of the present application. In the absence of signals, the grid 11 tends towards a positive potential on account of leak resistance 12, which is connected to the tap on the potentiometer 13, but is prevented from going very positive by the anode 24 of the pentode 19 which represents a low resistance to its cathode 20 which is held at a lower potential than 11 by the tapping on potentiometer 13. The grid 21 of this valve is connected to its cathode 20 by a leak resistance so that in the absence of signal the grid is at cathode potential, thus making the valve conducting.
When signals are applied to grid 1, delayed signals will appear on the grid of pentode 19 with synchronising signals in a positive sense. As explained in British Patent Specification 422,906 these synchronising signals will just cause grid current to flow and charge the grid condenser so as to re-establish the direct component on the grid of valve 19 with peak synchronising signals just passing grid current. The input at 1 must be sufficient to cause the synchronising amplitude on the grid of 19 to cover the grid base, that is valve 19 must be cut-off for all values of signal (black and picture signals) other than synchronising signals.
The line synchronising signals are approximately 10 micro-seconds long and are followed by 5 micro-seconds of black. The delay network in the anode circuit of valve 6 delays the signals to valve 19 by4.5 micro-seconds. The valve 19 is therefore switched on during the last 5.5 micro-seconds of the line synchronising period and the first 4.5 microseconds of black. When operating, the current passed by the anode of pentode 19 is sufficient to charge condenser 10 so that the black signals are slightly above the cathode (20) potential, thus passing a small anode current in each black interval to make up for the charge leaking away through leak resistance 12. Although the valve is switched on for 10 micro-seconds of which 5.5 are outside the black period, this does not upset the operation since the synchronising signals drive the anode of valve 19 negative and so are ineffective to pass anode current in 19, which is the black level ‘stabilising’ valve.
The leak resistance 12 is taken to a slight positive potential to ensure that condenser 10 is discharged rapidly enough to follow any negative wander of black due to variable attenuation of the signals and to ensure that the black signals pass anode current through valve 19. The effective time constant of condenser 10 and leak resistance 12 must be short enough to follow any wander of black level on grid 1.
The delay network in the anode circuit of valve 6 may comprise, for example, nine inductances and ten condensers terminated by a suitable resistance, each section giving a delay of half a micro-second. Suitable values are 1 milli-henry for each inductance a 2000 ohm terminating resistance and each condenser having a capacity of 250 micro-microfarads, except for the end condenser which has a capacity of 125 micro-microfarads. A value of 4.5 micro-seconds delay is used instead of the more obvious 5 micro-seconds in order to give an allowance of half a micro-second for the fall of signal on the grid of valve 19, due to loss of high frequencies in the delay network. In the arrangement shown, the black level is re-established at a value which is positive with respect to earth owing to the cathode of valve 19 being positive. The black level may be given a negative value by arranging that the cathode of valve 19 goes to a negative source of supply.
It should be noted that in connection with the frame synchronising signals which are approximately 40 micro-seconds long and are followed by 10 micro-seconds of black, the valve 19 is on for 35.5 micro-seconds during the frame signal and for 4.5 micro-seconds during the succeeding black interval. The device therefore continues to operate during the frame signals.
It can be assumed that the black level at the grid 1 is subject to variation due to changes of field strength varying the signals supplied by a radio receiver to grid 1, but that the D.C. component is present in the signals applied to grid 1. When the black level is stabilised as shown the resulting synchronising signal amplitude is dependent on signal amplitude and it is possible by rectifying the signal in a subsequent stage to provide an automatic volume control potential for the radio receiver.
In the above description of the Figure it has been assumed that the signal applied to valve 6 contains the D.C. component but that the signal is subject to variations in effective amplitude. It will be appreciated that even if the D.C. component is lacking from the signal applied to the control grid 1 that the valve 19 will insert a D.C. component into the signal on grid 11 at the correct black level. In this case it is necessary that the condenser 17 and resistance 18 in the grid circuit of valve 19 should have a shorter time constant than those in the circuit preceding grid 1, which cause the loss of the D.C. component (the object in doing this is explained in the previously mentioned Patent Specification No. 422,906)
Although the use of the stabilising device shown in the Figure has been described with particular reference to its use in a radio receiver, it will be obvious that such a device will be equally suitable for use in a television transmitter, cable link or the like for the stabilisation of the datum level.
When the D.C. component is present in an electrical signal and the signal is subject to variations in effective amplitude, it is possible according to the invention to derive a corrective potential from the variations in datum level.
Dated this 28th day of October 1938
Chartered Patent Agent
Improvements in or relating to Thermionic Valve Circuits
We, ALAN DOWER BLUMLEIN, of 37, The Ridings, Hanger Lane, Ealing, London, W.5, and ERIC LAWRENCE CASLING WHITE, of 7, Vine Lane, Hillingdon, Middlesex, both British subjects, 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 of the type designed to handle signals of high frequency, i.e. such frequencies that the inter-electrode capacities of a thermionic valve employed in the circuit are effective to modify the functioning of the circuit.
In a number of such circuits it is necessary to connect an impedance between the cathode of a thermionic valve and the negative terminal of the source of anode negative terminal of the source of anode current for the valve. For example, with the so-called ‘cathode follower valve’ it is necessary to connect an impedance between the cathode and the negative terminal of the source and to apply the input between the control electrode of the valve and the negative terminal so that the potential of the cathode of the valve follows substantially the potential of the grid of the valve, the output in this case being usually taken from the cathode impedance. Furthermore, it is usual in order to reduce distortion in thermionic amplifiers and the like to insert an impedance in the cathode circuit in order to provide negative feedback for the purpose of straightening the valve characteristic and thus reducing distortion, the output in this case being usually taken from the anode circuit. In a particular case when a valve of the tetode type, which may contain electrostatic shielding plates, or of the pentode type, which has a suppressor grid, is employed in such circuits it is usual to connect the shielding plates, or the suppressor grid, directly to the cathode in order that the bias on the said plates or suppressor grid is equal to the potential of the cathode. It has been found that when such circuits are operating at high frequencies the effect of the additional capacity of the shielding plates, or the suppressor grid, to ground is detrimental to the satisfactory operation of the circuits. For example, in the case of the cathode follower valve the extra capacity added between cathode and ground will tend to stop the potential of the cathode following that of the grid at high frequencies, whilst in the case of the negative feedback circuit a ready mentioned the feedback is reduced at high frequencies and thus the circuits are not satisfactory.
Similar difficulties arise in thermionic valve circuits in which impedances are connected in both anode and cathode circuits and outputs are derived from each impedance.
It is the object of the present invention to provide improved means whereby these and other disadvantages are substantially overcome or reduced.
According to the invention a thermionic valve circuit of the type referred to is provided which, in operation, causes the potential between the cathode of said valve and earth to fluctuate, and wherein the effective cathode to earth capacity of said valve would be increased by the direct connection of a further electrode of said valve to said cathode, wherein said further electrode is maintained at substantially the mean or low frequency potential of the cathode whilst it is maintained at a substantially fixed potential relative to ground for high frequency signals.
In order that the said invention may be clearly understood and readily carried into effect the same will now be more fully described with reference to the accompanying drawings in which:-
Figure 1 represents an embodiment of the invention as applied to a cathode follower circuit.
Figure 2 represents a further embodiment of the invention as applied to a cathode follower type of circuit, and
Figure 3 represents a further embodiment of the invention as applied to a negative feedback amplifier.
Referring to Figure 1, the reference numerals 8 and 9 represent the input terminals to a pentode thermionic valve 13 which is employed as a cathode follower valve. The cathode 14 of the valve 13 is connected to earth by means of resistances 11 and 12 which are arranged in series. The junction of the two resistances is joined to the grid 15 of the valve by means of a leak resistance 10, the resistance 11 serving as a bias resistor in order to give the grid the correct bias potential. The screening grid 16 of the valve is taken to a suitable positive potential by means of a resistance 7 and is bypassed to ground by condenser 6 and the anode 18 is suitably decoupled from a source of positive potential by means of the anode decoupling resistance 19 and the decoupling condenser 20. The output of the valve is taken from line 24, which is connected to the cathode 14.
According to normal practice the suppressor grid 17 of the valve 13 would be connected directly to the cathode 14 of the valve. This may be desirable from the operating valve conditions, since even if the anode does not supply a load, it enables the amount of screen current to be more definite than is the case where the suppressor joined to the screen. In such a case it will be seen that the capacity of the suppressor grid to ground is added to the capacity of the cathode 14 to ground and thus the effective impedance between cathode and the negative source of supply, that is, the impedance due to the resistances 11 and 12 is reduced at high frequencies and thus the potential of the cathode 14 does not follow the potential of the grid 15 so efficiently as at low frequencies. In order to avoid this additional capacity due to the suppressor grid, the suppressor grid 17 is, according to one embodiment of the invention, connected to a point on a potential divider 21 and 22 in such a way as to give the suppressor grid 17 a potential equivalent to that of the mean potential of the cathode 14. Should the resistance 22 be of a high value it may be necessary to by-pass this resistance by means of a condenser 23. It will be seen that the suppressor grid capacity is now removed from the cathode circuit and does not affect the operation of the circuit whilst the suppressor grid is maintained at the potential necessary for it to perform its suppressor action.
A further method of avoiding the capacity effect will now be described with reference to Figure 2 of the accompanying drawings. In this figure like components to those in Figure 1 have been given line numerals and it is assumed that except for the suppressor grid circuit that the circuit behaves in a similar manner. In this case the suppressor grid 17 is connected to the cathode 14 by means of a high resistance 26, and, in addition, the suppressor grid is by-passed to ground by means of a large condenser 25. Due to the direct current connection of the suppressor grid 17 to the cathode 14 by means of the resistance 26, the suppressor grid 17 is maintained at the mean potential of the cathode 14, the condenser 25 serving to prevent the potential of the suppressor grid fluctuating at high frequencies. Alternatively, the resistance may be replaced by/or in combination with, an inductance of suitable value.
Figure 3 illustrates an embodiment of the invention as applied to an amplifier employing negative feedback in the cathode circuit. In this case the valve 13 has an anode resistance 28 across which output potentials are derived and passed out via lead 27. The resistance 11 serves a dual purpose of providing negative feedback for the purpose of reducing distortion in the amplifier and for providing a suitable negative grid bias for the grid of the valve 15. The normal connection of the suppressor grid directly to the cathode 14 has the effect at high frequencies of reducing the effective cathode impedance which determines the amount of negative feedback, and thus the feedback decreases at high frequencies and for these frequencies the output from the amplifier is increased with consequent distortion. For the purposes of eliminating this effect, the suppressor grid is connected to the cathode 14 by means of a resistance 26 and is also connected to ground by means of a large condenser 25. As in the case of Figure 2, suppressor grid is biassed at mean cathode potential and the effect of the suppressor grid to earth capacity is rendered ineffective on the feedback circuit.
Alternatively, the bias for the suppressor grid 17 of Figure 3 could be obtained, according to the invention, in a similar manner to that indicated with reference to Figure 1, namely, by connecting the suppressor grid to a tapping from a potential divider so as to provide the suppressor grid with a bias potential equivalent to the mean potential of the cathode of the valve.
In the negative feedback amplifier case not only is there the effect of the direct suppressor grid to ground capacity, but there is also the effect of the suppressor grid to anode capacity, thus when the grid of the valve is positive, the potential of the anode is made negative and thus the anode to suppressor grid capacity tends to stop the cathode feedback circuit operating correctly at high frequencies in a similar manner to the well known ‘Miller effect’. By adopting the circuits according to the invention this effect is reduced.
A further circuit to which the present invention is applicable is that in which a thermionic valve circuit is provided giving outputs from both the cathode and anode circuits for example for obtaining a push-pull amplifier. This case is similar to that described with reference to Figure 3, only differing in what the cathode impedance is normally larger than that necessary for the correction of distortion. Similar ill effects due to the suppressor grid to ground capacity occur, and this causes distortion in that added capacity lowers the high frequency output from the cathode circuit and increases the high frequency output from the anode circuit. By adopting the methods already outlined it is possible to overcome or reduce these ill effects.
An example of a further case in which both cathode and anode impedances are employed is in television apparatus. In one particular application described in detail in our copending Application of even date a television signal is applied to the grid of such a valve circuit, signals from the cathode impedance being fed to the anode of a further screen grid valve used for D.C. re-insertion whilst signals in an inverted sense are passed from the anode circuit to the control grid of the further screen grid valve in an inverted sense for the purpose of switching on the screen grid valve during black level portions of the television signal which succeed the synchronising pulses, the signals in the anode circuit having passed through a delay network.
Since, in the embodiment of the invention described, the suppressor grid is connected effectively for high frequencies to the end of the cathode impedance remote from the cathode of the valve a certain amount of negative feedback will be applied to the suppressor grid of the valve and the gain will be reduced slightly from that obtainable with the normal arrangement. In this connection, the arrangement of Figure 1 is preferred in that the feedback to the supressor grid is constant for all frequencies handled, whereas with the circuit of Figure 2 the feedback decreases for low frequencies.
Although the invention has been described as applied to pentode valves it will be understood that the invention can also be applied to tetrode valves having shielding plates (see Patent Specification No. 423,932), the shielding plates being connected as described above.
Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:-
Dated this 28th day of October, 1938
F. W. Cackett
Chartered Patent Agent