JPS5711582A - Video amplifying circuit - Google Patents

Abstract

PURPOSE:To achieve the video signal with a normal luminance level even with rapid variation of an average luminance level of objective, by holding the DC potential of pedestal of the video signal to a constant value even immediately before clamping, not to remove the DC component at demodulation and amplification. CONSTITUTION:A pulse generating circuit 10' transmits pulses corresponding to the blanking period of a pulse amplitude modulation PAM signal, and a speed-up circuit 23 make conductive a transistor TRQ1 of a sample hold circuit 22 in response to this pulse. Through the conduction of this TRQ1, the output of a DC voltage level conversion circuit 20 is sampled and stored in a capacitor C1. This stored voltage is applied to an error voltage detecting circuit 21 and compared with the reference voltage by a variable resistor VR. Thus, the circuit 21 gives the output difference to the circuit 20 to control the output at a terminal 31 of the circuit 20. Thus, the DC voltage level of a PAM signal applied to an LPF8 can be kept constant.