PURPOSE: To reduce the capacitance of the coupling capacitor and to make easy circuit integration, by holding the output potential of the coupling capacitor to a value dependent on the bias power supply through periodical switching of electronic switch.

CONSTITUTION: When the first circuit metwork 21 and the second circuit network 22 are coupled with the AC coupling capacitor C, the signal source e and the output impedance Z1 are connected in series with the output terminal P1 of the circuit network 21, the first electronic switch S1 is connected in parallel with the series circuit, and the bias power supply E1 is connected between the connection point of the switch S1 and the signal source e and ground. Further, the series circuit of the second electronic switch S2 and the bias power supply E2 and the input impedance Z2 are connected between the input terminal P2 connection point of the circuit network 22 and ground, and the switches S1 and S2 are synchronized to switch them with a given period. Further, the DC potential at the output and input of the capacitor C is held at a potential depending on the bias power supplies E1 and E2, to reduce the capacitance of the capacitor C and to make easy circuit integration of the circuit networks 21 and 22.