GB798549A - Improvements in and relating to electric amplifiers - Google Patents

Abstract

798,549. Television. FERNSEH GES. Jan. 21, 1955 [Jan. 22, 1954], No. 1890/5. Class 40 (3). [Also in Group XL (c)] An amplifier with protection against overload and which is more particularly intended as a video modulation amplifier in a television transmitter comprises resistance-capacity coupled stages 16, 17 and a cathode-follower output stage 25. A further cathode-follower 22 fed in parallel with the output valve 25 feeds a gain-control circuit comprising a rectifier 27 provided with a delay bias, by means of resistors 28, 29, an amplifier 30, and a voltage-doubling rectifier circuit 1, 2, 3. The delay is such that the gain control circuit only becomes operative when the video amplitude exceeds 95 per cent of the maximum allowable peak-white level. Beyond this level a gain-reducing voltage is developed across circuit 3 and is applied to the grid of the stage 16 via a filter 5, 6 of time-constant of the order of the period of one line, a cathode-follower 7 and a gate circuit 9. The gate circuit 9 comprises two pairs of oppositely-poled diodes, the pairs also being oppositely poled: the gate is normally closed, but is opened during the line flyback period by line pulses applied in appropriate sign to the commoned cathodes and also to the commoned anodes. The D.C. level is restored at the grids of both the stages 17, 25 by means of similar gated clamping circuits 18, 24 respectively. To prevent overload by a signal increasing too rapidly to be dealt with by the gain control circuit, a limiter is provided, operating at 25 per cent above maximum allowable peak white level: it comprises a series diode 19 with delay bias derived from resistors 20, 21. In a modification, Fig. 2 (not shown), the gate circuit 9 is omitted, the gain control voltage being applied instead to the cathode of the valve 16 : the diodes 1, 2 must then be reversed. The gain control amplifier 30 may be of narrower band width than the main amplifier.