I. Introduction
Passive radio-frequency identification (RFID) tags and certain implantable microelectronic devices (IMDs) such as cochlear implants do not include batteries due to cost, size, weight, safety, and lifetime limitations. Inductive power transmission provides these devices with the required power on a continuous basis [1], [2]. In these systems, wireless power transmission across a pair of loosely coupled coils is preceded by a power amplifier (PA) in the external unit and followed by an efficient rectifier in the implanted unit to provide the IMD with an unregulated supply [3].