PURPOSE: To keep the offset voltage at the zero level by feeding the fluctuation of the power voltage back to the base of the input after detection through the Zener diode.
CONSTITUTION: In the normal power circuit, the power voltage has fluctuation in the conditions under which the ratio becomes constant between the +B and -B power sources. Accordingly, the fluctuation factor is zero, and thus the offset voltage becomes constant to the fluctuation of the power voltage. However, in the actual case, the fixed offset voltage remains. In this connection, the voltage is extracted from the -B side of the power voltage and via Zener diode ZD in order to detect the fluctuation of the power voltage. In case the voltage fluctuation is detected, the voltage difference obtained by dividing Zener voltage VZ and the power source by resistances R 1 and R 3 is applied to the base of input transistor Q 1 in the direction where the offset voltage is negated.