PURPOSE: To prevent the reversal of an output voltage value even when an excessive input current and input level are supplied and to set an input current to a wide range by providing a selective amplifier to a current detector to limit an excessive current and always keeping negative feedback operation.

CONSTITUTION: The fundamental wave generated in a fixed oscillator 1 is supplied to the respective exciting windings 42 of two troidal cores 40a, 40b to hold a magnetic equilibrium state and, when DC flows to an input winding 41, the double wave in the output voltage of a detection winding 43 is selectively amplified by a selective amplifier 5 and the change of a phase to the double wave obtained from the fundamental wave is detected by a phase detector 6 and the detected error signal is set to output voltage and also applied to a feedback winding 44. In the current detector thus constituted, a gain limiter 53 to the vicinity of double wave frequency is provided to the twin T type band-pass filter and an input level limiter 54 is provided to the input terminal of the gain limiter 53. By this constitution, a negative feedback loop is kept even when an excessive current flows.