PURPOSE: To improve the follow-up property of an output signal to the frequency variation and amplitude variation of an input signal by adjusting the response time of the input signal by a fine current supplying means and extracting a fine current from a capacitor at a specific rate from an output current of the minute current supplying means by a minute current extracting means.

CONSTITUTION: A differential amplifier 11 outputs a signal amplified in accordance with a difference of voltage IN impressed to input terminals 10a, 10b and a full-wave rectified signal is outputted through a full-wave rectifying circuit 12 and a resistor 12c. An electronic switch circuit 13 turns on/off the full-wave rectified signal by an ON/OFF signal applied from the external. When a switch 13e is off, a capacitor 15 is charged with the full-wave rectified signal through a diode 13c, a resistor 13d and a TR 13b. When the switch 13e is turned on, the capacitor 15 is discharged. The minute current supplying circuit 14 adjusts the response time of the circuit 12 to the input signal IN. When the value of a resistor 14b is changed, the follow-up property of the capacitor 15 to the signal IN can be improved. The minute current extracting circuit 16 extracts a minute current from the capacitor 15 at a specific rate to an output current from the circuit 14.