PURPOSE: To improve the S/N and to adjust independently the AC and DC voltage components by making the amplification factor of the DC voltage equal in the 1st amplification means and the 2nd amplification means.

CONSTITUTION: An input terminal IDC to an operational amplifier U1 is an input also to an operational amplifier U2 and connected to an inverting input of the operational amplifier U2 via a resistor R4. Thus, the DC voltage amplification factor of the 1st and 2nd operational amplifier circuits is equal to each other in this way and the DC voltage applied to terminals T, T' is equal. In order to make the voltage of an output 'O' in the circuit variable negatively, a resistor 34 is connected between the inverting input of the operational amplifier U2 and a positive voltage +V and a resistor 36 having an equal resistance to that of the resistor 34 is connected between the inverting input of the operational amplifier U1 and the +V. The effect of an AC voltage in the operational amplifier U1 is not given to the circuit comprising the operational amplifier U2.