GB536089A - Improvements in or relating to thermionic valve amplifiers - Google Patents

Abstract

536,089. Valve amplifying circuits. BLUMLEIN, A. D. July 28, 1939, No. 21912. [Class 40 (v)] The invention relates to a thermionic amplifier for providing an output representing a fractional power of the input so that the gamma value of a film in a television transmission may be reduced below unity. Fig. 1 shows an arrangement in which the signal is applied to the grid of a high impedance valve 2 which may include a resistance in its cathode lead to ensure, by negative feedback, that the current is proportional to the input. The valve 3 is maintained at a fixed bias and arranged to operate over a part of its characteristic curve exhibiting the necessary curvature in terms of grid voltage and anode current. A push-pull pair of valves may be fed from the grid and cathode of a valve working on the curved part of its characteristic and the push-pull valves may be provided with a common cathode resistance, Fig. 2 (not shown) so that differences of potential applied to the grids are amplified but not the absolute potential of the grid and cathode. Fig. 3 shows a valve 2 provided with a feedback resistance 17 to ensure linear current changes with direct coupling, as described in Specification 456,450, to valve 11 provided with negative feedback. With this arrangement non-linearity of the valve 11 produces the required fractional power output by feedback at its grid. The anode resistance 13 may be made infinite and the coupling 14 may be made zero, while the provision of resistance in the cathode lead of the valve 11 will control contrast.