PURPOSE: To equalize the operation level between input and output and to obtain an amplification degree by equalizing the potential of input terminal and output terminal of a differential amplifier.

CONSTITUTION: The first DC power source E1 and a signal source eS are connected to a noninversion input terminal Pa of a differential amplifier A. Resistances Ra, Rb are connected in series between an output terminal Pc of the differential amplifier A and the second DC power source E2. The connecting point of resistances Ra, Rb is connected to an inversion input terminal Pb. The voltage levels of DC power sources E1 and E2 are substantially equal, and the resistances Ra, Rb form a feedback loop. The valves of resistances Ra, Rb are set to make the voltage applied to the inversion input terminal Pb to E1+eS. The amplified output voltage of E1+(Ra+Rb)eS/Rb can be taken out from the output terminal.