PURPOSE: To speed up a circuit operation by connecting the base of a 1st transistor(TR) to a 1st power supply, connecting its collector to a 2nd power supply, leading a signal input wire to the base of the 2nd TR, applying the reference voltage of a prescribed level to the base of a 4th TR and leading a signal output line to the collector of the 3rd TR.
CONSTITUTION: TRs Q1, Q2 form a differential pair and a common base TR Q3 is provided to make a collector.emitter voltage constant at the switching of the TR Q2. A reference voltage to the TR Q1 and a base voltage to the TR Q3 are imparted from an emitter follower TR Q4. A level shifting circuit 20 is provided between the bases of the TRs Q1, Q3 and the level shifting voltage determines the collector.emitter voltage constant at the switching of the TR Q2. Thus, the collector.emitter voltage at the switching of the switching TR is held to an optimum value independently of the reference voltage and the circuit operation is speeded up.