PURPOSE: To avoid the change in the frequency characteristics and the input/output impedance even with the gain changed, by making gain adjustment through the use of the inphase gain of an operational amplifier.

CONSTITUTION: An input terminal IN is connected to an inverse input terminal - of an operational amplifier A via an input resistor R1, and the terminal - is connected to an output terminal OUT of the amplifier A via a feedback resistor Rf. The terminal IN is connected to a noninverting input terminal + of the amplifier A via a variable resistor VR3. Where the signal attenuation rate of a variable resistor VR3 is (k), voltages of input and output signals are respectively as ei, eo, and Av is the gain and RfR1, Av=eo/eiRf/R1×(k-1) and the gain Av can be changed by changing the attenuation factor (k). The feedback amount is constant since the resistors Rf, R1 are constant and the frequency characteristics are not changed even with the gain changed. Since the input impedance of the terminal + of the amplifier A is sufficiently greater than the maximum value of the resistor VR3, the input impedance can not be changed even if the position of a slider of the VR3 is changed.