US2230819A - Thermionic valve circuits - Google Patents

Description

Feb.4, 1941. ELC. WHITE 2,230,819

THERMIONIC VALVE CIRCUITS Filed Aug. 4, 19:59

' INVENTOR. ERIC LAWRENCE czva WHITE ATTORNEY.

Patented Feb. 4, 1941 3 UNITED STATES PATENT OFFICE THERMIONIC VALVE CIRCUITS Eric Lawrence Casling White, Hillingdon, Engpany of Great Britain Application August 4,

1939, Serial No. 288,340

In Great Britain August 23, 1938 5 Claims.

The present invention relates to thermionic valve circuits and more particularly to saw-tooth scanning generators as used for television and like purposes.

In cathode ray tube television systems it is common practice to employ magnetic deflection of the cathode beam and for this purpose deflection coils are placed on or near the tube and currents of saw-tooth shape are passed through the coils. In television transmitters the required waveform of the frame saw-tooth wave is substantially exponential if the tube screen is located obliquely to the gun axis, whilst in television receivers the required wave form is usually substantially linear. Should the output circuits, as, for example, the output valve, introduce any distortion, then the wave form in the deflecting coils will be distorted. It has been proposed in this connection to apply negative feedback to such a circuit in order to correct for this distortion.

It has been proposed to obtain the feedback voltage for correcting purposes from an impedance in series with the anode-cathode path of the output valve as is shown in British patent specification No. 424,221, but in some cases it is desired to employ an output transformer, for example, where the scanning coils are fed by a relatively long cable, the primary winding of the transformer being arranged in the anode-cathode circuit of said valve Whilst the secondary winding is connected to said cable. In this case the transformer and the cable may introduce distortion.

The object of the present invention is to provide an improved circuit in which an output transformer is used, and in which the negative feedback serves to correct for distortion introduced by the output transformer.

According to the invention there is provided a thermionic valve circuit for the generation of current of substantially saw-tooth waveform in, for example, the scanning coils of a television receiver or transmitter, which comprises a thermionic valve having an input and an output circuit, ,means for setting up electric oscillations of substantially saw-tooth waveform in said output circuit, said output circuit comprising the primary winding of a transformer, the secondary winding of which is arranged to supply current of substantially saw-tooth waveform to said coils, and a negative feedback path for feeding back to said input circuit potentials to cause said saw-tooth current to assume a desired waveform, said feedback potentials being derived from a suitable impedance arranged in circuit with the secondary winding of said transformer.

With such an arrangement it is also found that the output from the circuit is substantially independent of the resistance in series with the scanning coils. Consequently, it is possible to vary the resistance in series with the coils, for example by inserting different lengths of cable feeding the scanning coils, without substantially affecting the shape of Waveform or the amplitude thereof.

In order to avoid power losses in the output circuit it is preferable to employ an amplifier for amplifying the feedback voltage before application to the input circuit.

In order that the said invention may be clearly understood and readily carried into effect, alternative circuit arrangements embodying the invention will now be more fully described by way of example with reference to the accompanying drawing in which,

Figures 1 and 2 illustrate such alternative circuits.

Referring to the drawing, Figure 1 shows a saw-tooth current generator suitable for supplying frame deflecting currents for a cathode ray tube. The pentode valve 4, which acts as a discharge valve, has applied to its grid, via condenser 2 and leak resistance 3 from the terminals I, suitable positive pulses at the frame scanning frequency which serve to render the valve 4 conducting at intervals. The anode of Valve 4 is connected through the anode resistance 5 to a tapping on the potentiometer 6 which supplies a suitable positive potential from the H. T. source (shown as a battery 33) The anode of valve 4 is connected to the grid of the triode output valve 9 by means of a condenser and leak resistance la, the valve 9 being suitably biased by the oathode resistance lfl. A condenser 8 is also provided connected as shown and is arranged to be charged when the valve 4 is non-conducting through the resistance 5 and to be rapidly discharged through the valve 4 when the latter is rendered conducting. Saw-tooth potentials are thus applied to the grid of valve 9.

The condenser 8 is connected serially with resistance 5, the condenser l, and the resistances 30 and 35 and also a portion of the resistance 6 and with the batteries 33 and 34, thus making a complete charging circuit therefor.

In the anode circuit of valve 9 is an output transformer with a primary winding II and two secondary windings l2 and 13. The output circuit is decoupled by means of the choke It and condenser l5. In some'cases the choke I4 may be replaced by a resistance. The outer terminals of the secondary windings l2 and I3 are connected to the leads l8 which supply the currents to the deflecting coils I9. The inner ends of the secondary windings are joined together by means of two equal resistances l6 and I1 the centre being connected to the earth line. The high potential end of resistance I1 is connected through the condenser 20 to the grid of a triode valve 22, 2| being the grid leak and 23 the cathode biasing resistance, the latter also serving to linearise the amplification by means of negative feedback. The output from valve 22 is coupled to the grid of triode valve 29 by means of the coupling network comprising anode resistance 24, condenser 21 and leak resistance 28, the resistance 25 and condenser 26 servin for decoupling purposes. The valve 29 is connected as a cathode follower. Resistance 3| and condenser 32 decouple the anode circuit and resistance 30 is the cathode resistance across which the output is derived. The battery 34 supplies a suitable bias, preferably with decoupling by resistance 35 and condenser 36, for the valve 29 and the output of this valve is coupled back into the input circuit of valve 9 via the condenser 8. 1

In operation, assuming that it is necessary to produce a linear saw-tooth current in the coils l9, the time constant of resistance 5 and condenser I is made at least twenty times the frame period and short positive pulses at the frame frequency are applied to the grid of valve 4. A sawtooth potential is generated in the manner described and is supplied to the grid of valve 9, consequently producing a saw-tooth current in the coils 19. However, should the resistance of the leads I8, frequency distortion in the output transformer and amplitude distortion in transformer and valve 9 alter the waveform of the current flowing through the coils l9 the potentials produced across the series resistance II will differ from-a linear saw-tooth. The potentials across resistance I! are amplified by the linear amplifier 22 and are applied to the cathode follower valve 29 which feeds back the potentials in a negative sense to the condenser 8.

The negative feedback is preferably of magnitude about ten times the required grid swing of valve 9, and acts in the well known manner to reduce the distortion due to the valve 9, the output transformer and the leads. In order to reduce power loss in the output circuit the resistances l6 and I! are naturally made small in value and as the potentials developed across these resistances are consequently small, it is necessary to employ the amplifier valve 22 in order to increase the feedback potentials.

The efiect of any low frequency loss due to the transformer may be even further reduced by introducing a suitably low frequency loss in the feedback circuit, e. g., by making the time constant of the grid coupling circuits of either valve 22 or valve 29 sufficiently short.

In the case where the coils l9 are used for de fiecting the cathode ray in a tube using a sloping screen or mosaic, it is necessary to distort the saw-tooth potential into a portion of an exponential curve as described in the specification of British Patent No. 443,952. This may be achieved in the present instance by reducing the time constant of resistance 5 and condenser 1. A suitable time constant for a frame period of 0.02 second is 0.031 second when the tube is of the iconoscope type. The variable tapping in the potentiometer 6 serves to control the amplitude of the saw-tooth potential.

In a modification of the circuit described, the cathode follower valve 29 may be omitted and the lower end of condenser 8 may be connected directly to the anode of valve 22. The arrangement employing the cathode follower valve has the advantage'that' a quickerreturn stroke is obtained for the saw-tooth waveform. If desired the resistance I'I may be part of a potentiometer and the tapping point is then connected. to the grid of valve 22, alteration of the tapping point varying the amount of feedback and hence the amplitude of the saw-tooth oscillation, such an arrangement forming a pre-set amplitude control. The tapping point may in some cases be 'taken from a high resistance shunting the resistance II.

A simplified form of circuit arrangement isshown in Figure 2 from which the cathode follower valve 29 of Figure 1 is omitted, and in which an amplifier is interposed between the valves. 4 and 9. The valve 4 in Figure 2 has a re-' sistance 31 connected in its cathode lead, this resistance being connected across the battery 34 and a condenser 38. At the cathode of the valve 4 a saw-tooth waveform is produced of opposite sense to that produced at the anode of the valve 4 in Figure 1, the condenser 38 charging negatively in the absence of a pulse at the grid of the valve 4 and being brought to a positive potential when the valve is rendered conductive on the application of a pulse. The saw-tooth waveform is applied to the grid of an amplifying valve 39 through a condenser 49, the valve 39 being used instead of the valve 22 in Figure 1. A resistance 41 is connected between the grid of the valve 39 and a point in thebattery 34.

The feedback voltages from the resistance I! are fed to the cathode of the valve 39 which therefore amplifies the difference between the input and the feedback voltage, the amplified difference being taken from the anode of the valve 39 by the usual condenser l and grid leak resistance To to the grid of the output valve 9. As the valve 39 does not amplify the whole feedback voltage but merely the difference between this feedback voltage and the input saw-tooth voltage, the latter is made approximately smaller in amplitude. The accuracy of the final waveform produced does not depend directly upon the linear operation of the amplifier 39 as it does in the case of the valve 22 in Figure 1.

I claim:

1. A thermionic valve circuit for the generation of currents of a substantially saw-tooth waveform in, for example, the scanning coils of a television receiver or transmitter which comprises a thermionic valve having an input and an output circuit, means for setting up electric oscillations of substantially saw-tooth wave form in said output circuit, said output circuit comprising the primary winding of a transformer, the secondary winding of which is arranged to supply currents of substantially saw-tooth waveform to said coils, and a negative feedback path for feeding back to said input circuit potentials to cause saidsawtooth current to assume a desired Waveform, said feedback potentials being derived from .a suitable impedance arranged in circuit with the secondary winding of said transformer. g 2. A thermionic valve circuit according to claim 1, wherein an amplifier is provided in the feed back circuit for amplifying the feedback potentials.

10 ply the scanning coils.

5. Apparatus in accordance with claim 1 wherein said feedback potentials are applied to the input circuit of an amplifying tube, said amplifying tube having anode-cathode and at least one control electrode, an impedance element connected in the anode-cathode circuit of the amplitying tube and connected directly tothe cathode thereof, and means for applying the potential of a point on said impedance means to input circuit of said apparatus.

ERIC LAWRENCE CASLING WHITE.

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