GB951413A - Nonlinear amplifier - Google Patents

Abstract

951,413. Electric analogue computors. FERNSEH G. m.b. H. June 20, 1960 [June 20, 1959], No. 21532/60. Heading G4G [Also in Division H4] In a non-linear gamma correction circuit for television signals in which the minimum black level differs appreciably from the blanking level the mimimum black level in the signal is maintained at a substantially constant position on the nonlinear amplifier characteristic. Positively poled signals are applied to a nonlinear amplifier comprising a cathode follower 3, the cathode load of which comprises resistors 5-8 in series, the output being taken across resistor 8 and the junction points of the resistors being shunted to earth over diodes 9-11 which in conjunction with series resistors 13-15 give the desired non-linear characteristic. The input is applied over blocking condenser 2, the D.C. level at the grid of the valve 3 being determined by a clamp circuit 18, 19, 20 driven by pulses applied at terminals 21, 22 to be conductive during the blanking period. From the cathode of valve 3 the picture signal passes by capacitor 24 to diodes 27, 28 which are cut off during the blanking intervals by negative-going pulses applied to their anodes over condenser 39. The clamp circuit is returned to an adjustable negative potential derived from point 30 in a potentiometer 31-34. The circuit clamps the wave form at the cathode of the diode 28 so that the minimum black level is at the potential of the point 30. The blanking pulses in this waveform are negative going and are passed to condenser 36 by a gate circuit comprising diodes 25, 26 driven conductive during the blanking interval by positivegoing pulses applied to their anodes via condenser 37. The negative potential developed across condenser 36 is proportional to the distance between the blanking level and the minimum signal black level and is applied as a clamping potential to the grid of the valve 3 via the clamp circuit 18,19, 20. Switch 23 enables a fixed clamping potential from point 30 to be substituted.