This thesis presents the design of a Power Management Unit (PMU) integrated circuit, intended for implantable medical devices (IMDs) powered by ultrasonic wireless power transfer. The design takes into account the specific constraints of IMDs, such as limited available power and the need for high efficiency to avoid thermal damage to surrounding tissue. The circuit was designed in a standard 0.18 μm CMOS technology from TSMC, and simulations were performed using Cadence software. The PMU design is based on an active rectifier composed of a core, two comparators, two buffers, a temperature-compensated current reference, and a voltage limiter. The core rectifier was designed using two PMOS transistors in a cross-coupled configuration and two NMOS transistors as active switches for AC to DC voltage conversion. The comparators were designed with a common-gate topology to generate the contro...