PURPOSE: To obtain an amplifier which deals with optional impedance of a load and has an optional amplification degree by comparing part of an output voltage with an input voltage through a low-frequency amplifier which does not use any amplifying element and adding electric power from a positive or negative power source to the output when the difference exceed a prescribed value.

CONSTITUTION: Two positive and negative power sources are used to turn on-off a high-speed switch part S according to an input. A detection part D outputs an on-off command to the switch part S when E2 is a prescribed value greater or less than E1. Namely, the positive or negative power source is connected and the connection is maintained until the difference between E1 and E2 is eliminated. In a figure, F is a low-pass filter.