| 摘要 |
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. |
