GB801446A - Improvements in and relating to non-linear amplifiers for use in television - Google Patents

Abstract

801,446. Television. BRITISH BROADCASTING CORPORATION. July 19, 1956 [July 20, 1955], No. 21046/55. Class 40 (3). [Also in Group XL (c)] In a gamma correction circuit using a non- linear valve as the anode load of a constant current stage, the high-frequency response is improved by connecting an amplifier between the cathode and grid of the non-linear valve. Positive going signals are fed via terminal 10 to the constant current stage 11 which has an anode load comprising the non-linear valve 12, the normally conducting diode 18 and an external circuit 19. The anode load is shunted by a resistor 13 so that the valve 11 need only operate linearly down to about 1.5 mA. The signals at the cathode of valve 12 are amplified by a valve 14 and fed back to the grid of valve 12 to reduce the time constant of the anode circuit. The output, which is proportional to the input raised to the power 0.45, may be taken from terminal 15, but a larger inverted output is available at terminal 16. Black level clamp. The diode 18 is conducting during the line period, hence the diode 17, the anode of which is tapped down by about 3V, is insulating. Negative line suppression pulses, applied via terminals 19 cut off diode 18 and hence cause diode 17 to conduct. Samples of the anode current of valve 12 are thus available through resistor 20 during each line suppression period. The resulting voltage pulses are amplified by valves 21, 22 (gain about 1,000), rectified by the diode 23 and the resulting negative voltage fed via a cathode follower 24 as bias to the grid of valve 11 in order to maintain the current at the required small value to provide the correct black level. Spurious low frequency signals present at terminal 10 are said to be reduced by 60db.