The present invention provides a transistor circuit suited for a synchronizing signal separator. More particularly, the transistor circuit can separate the noise-free synchronizing signal from a composite video signal.The transistor circuit comprises a transistor having an emitter coupled to an input terminal through a capacitor and to a constant current source, a base applied with a bias voltage and a collector connected to an output terminal, a comparator comparing the emitter voltage with a reference voltage, and means for turning the transistor off in response to the comparator output. A current charging circuit is preferably added.