An active rectifier in 6.78 MHz resonant-coupling wireless power receivers for space-limited portable and wearable applications is presented. To improve the power efficiency and reduce the die area, a fully NMOS transistor-based active rectifier with a unified delay-locked loop (DLL) controller is proposed. The proposed rectifier has been implemented in a 0.35 µm CMOS technology with a die area of 0.612 mm². The measured peak and minimum power efficiency within the power range of 80 mw to 440 mW are 91.8% and 69.6%, respectively.