Tracking load to optimize power efficiency in RF to DC rectifier circuits

A tracking load circuit for RF rectifiers is described. The tracking load is designed to maximize RF to DC conversion efficiency as the RF input power to the rectifier changes. The circuit consists of two DC to DC converters with two control loops. The duty cycle of the first stage converter is adjusted by an analog control loop to have an input impedance that is optimum for maximizing the power efficiency of the RF rectifier. A second stage converter, either a classical boost or buck converter, converts the fluctuating voltage from the first stage to a regulated output voltage. The application of the tracking load is illustrated with experimental results for a CMOS class D rectifier and a 10 W class F GaN RF rectifier.