GB454511A - Improvements in or relating to electron discharge device amplifiers - Google Patents

Abstract

454,511. Valve amplifying circuits; television. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Electra House, Victoria Embankment, London.-(Vance, A. W. ; 324, South 11th Street, Philadelphia, U.S.A.) April 1, 1935, No. 10110. [Classes 40 (iii) and 40 (v)] In a direct-current amplifier for amplifying intermittent signals, such as television picture signals separated by other signals such as synchronizing signals, the latter are employed to control the bias on the amplifier in such a manner as to prevent slow drifting of the output current. As shown in Fig. 4 applied to a television system, the picture signals are generated in a cell 19 and amplified in a preliminary stage 23 which is direct-coupled to a two-stage direct-coupled amplifier 29. The synchronizing signals generated in a cell 39 are amplified in the valve 149 and are conveyed to the grid circuit of the amplifier 23 through a condenser 47 and resistance 49. The synchronizing signals occur as negative pulses in the interval between sets of positive picture signals. From the output of the amplifier 29, the synchronizing pulses, which are positive at this point, are applied to the control grid of a balancer valve 123 biassed slightly beyond cut off. The plate resistance 137 of this valve is shunted by a condenser 31 and is in the grid-biassing circuit of the first valve 95 of the amplifier 29. The pulses are thus integrated in the condenser 31 and cause the bias to vary gradually in accordance with any drifting of the amplifier. In the modification shown in Fig. 6, the synchronizing-pulses are amplified at 43 and may, or may not be applied to the outgoing line 1, 3 by a potentiometer 215. They are applied to the grid of the balancer valve 123, in common with the output of a blocking amplifier 219, fed from the outgoing line. The synchronizing pulses may be produced by a series of apertures near the rim of the scanning-disc, which is provided with masks arranged to give the pulses during intervals in the picture signals. In a modification shown in Fig. 8, only a single cell 19 is provided, and a single set of apertures 11, Fig. 10, in the scanning-disc 7. A half-silvered mirror 223 reflects part of the light to a mirror 225 and thence to the cell 19 without being affected by the picture film 15. The remainder of the light passes direct to the cell 19 through the mirror 223 and the film 15 to provide the picture signals. In this way the picture never reduces the cell illumination below a certain level, whilst a mask 227 cuts off the illumination completely to provide the synchronizing pulse. In another modification the synchronizing pulses are of the same polarity as the picture signals, but of greater amplitude.