In this paper we have reviewed several types of integrated AC-to-DC converters which have been widely used for inductively-powered applications. The limitations for achieving high power conversion efficiency (PCE) in each AC-to-DC converter have been considered in order to design highly power-efficient converters for applications that are in need of higher power levels with very low heat dissipation, such as implantable microelectronic devices (IMD). In this paper, we also propose a fully integrated active voltage doubler with offset-controlled high speed comparators, which provides much lower dropout voltage and far better PCE at high frequency (HF) range compared to its passive counterparts. Our active voltage doubler was fabricated in a 0.5-μm 3M2P standard CMOS process, occupying 0.144 mm2 of chip area. With 1.72 V peak AC input at 13.56 MHz, the active voltage doubler provides 2.4 V DC output across the 1 kΩ load, achieving high PCE of 78%.