To generate a step voltage at a high speed with high quality by connecting a resistor and a tunnel diode leading to reference voltage sources a, b to a connecting point between Schottky diodes A, B and delivering a logic voltage to a load.

A resistor Ra and a resonance tunneling diode(Da) connecting to constant voltage sources Va, Vka are connected to a connecting point (a) between Schottky diodes (De, Ds). Furthermore, output voltages Vol, Voh of a logic power supply Vo are applied to an anode of the De and a cathode of the diode Ds connects to a load resistor RL via a resistor R1. Before the power supply Vo is changed, the Ds is nonconductive in this circuit and when the power supply voltage is changed, the Da transits from a low level to a high level, the diodes De, Ds become conductive and nonconductive, respectively. Thus, a step voltage generated in a resistor RL is set to a voltage changing at a high speed of nearly several ps with high quality not causing pre-shoot/ after-shot.