ELECTRONIC CHOPPER SYSTEM FOR USE IN FACSIMILE COMMUNICATION COMPRISING MEANS FOR ALTERNATELY GROUNDING AND UNGROUNDING INPUTS OF AMPLIFIER

An electronic chopper circuit for use in converting variable amplitude direct current signals to pulsed signals of similar characteristics. In conjunction with a facsimile scanning system, the modulated electrical signals generated by a photoelectric cell are applied to a differential amplifier after periodic interruption by the electronic chopping circuit in accordance with an input clock signal. The periodic interruption to the differential amplifier is provided by a dual emitter transistor which alternately grounds and ungrounds the input to the differential amplifier in response to an input clock signal. The gain of the differential amplifier is controlled by a field effect transistor which is operated as a variable resistor in response to a control voltage generated in response to the background level detected on a document. The output therefrom is a series of signal pulses, the heights of which are in direct relation to the level of the input signals.