AUTO GAIN CONTROL CIRCUIT

摘要

The automatic gain control circuit comprises a variable gain amplifying unit for amplifying a composite image signal to a variable gain by a predetermined control voltage; a low pass filter for low pass filtering the output signal of the variable gain amplifying unit and removing a color image signal therein; a synchronization signal dividing unit for dividing a horizontal synchronization signal from the output signal of the low pass filter; a delay unit for delaying the horizontal synchronization signal to be placed at a back porch portion of the composite image signal for a predetermined time; a current source for generating a reference clamp voltage and a reference voltage; a clamp unit for clamping the synchronization tip of the output signal of the low pass filter to the reference clamp voltage; a sample/hold unit for sampling and holding the output signal of the clamp unit by the delayed horizontal synchronization signal; a transconductance amplifying unit for comparing the voltage output from the sample/hold unit with the reference voltage and outputting a current signal according to the voltage difference; an integrating unit for integrating the current signal and outputting an error voltage of direct current; and an amplifying unit for amplifying the error voltage and applying it to the variable gain amplifying unit as the control voltage.