Light receiving circuit having fast response time and a stable output waveform

Claim 1

A light receiving dicruit, comprising: (a) an active device having an emitter, a base and a collector, a first constant voltage source continuously applied to said base, and a second constant voltage source continuously applied to said collector through a first resistance to thereby provide a continuous reverse bias at the junction of said collector and said base; (b) a light responsive switching device having a variable resistance and having a first lead and a second lead, said first lead connected to electrical ground and said second lead connected to said emitter of said active device; (c) whereby the bias voltage at the junction of said emitter and said base of said active device is a function of the value of said first resistance, said second voltage source and said variable resistance, and is forward biased when the level of light on said light responsive switching device reaches a predetermined level, said active device thereby conducting current through said first resistance, said active device and said light responsive device when said predetermined level is reached; (d) wherein said predetermined level produces a small change in said variable resistance of said light responsive switching device, and (e) whereby the magnitude of the voltage across said light responsive switching device need only drop from approximately the magnitude of said first constant voltage source to just below the magnitude of said first constant voltage source to cause said conduction of said active device.