A precise positive-peak detector with adaptive micro-current compensator is proposed to trace the tank amplitude V T of wideband VCOs. The compensator includes three micro-current I micro branches controlled by automatic frequency calibration (AFC). A decoding circuit is connected between the AFC and the compensator. If oscillation frequency of VCO increases, by use of the decoding circuit, the branch with smaller I micro would be adaptively chosen for precisely tracing the V T . In TSMC 0.18µm CMOS technology, when the proposed positive-peak detector is used for a VCO with output frequency from 1.52GHz to 3.2GHz, the maximum detection error is 80mV under different corners and temperature from -40℃ to 80℃.