Adjustable Load With Tracking Loop to Improve RF Rectifier Efficiency Under Variable RF Input Power Conditions

Terminating an RF rectifier with an optimum load resistance is important to maximize the efficiency of converting RF power to dc power. The optimum load resistance can also vary significantly as a function of RF input power. Therefore, an adjustable load circuit that tracks changes in RF power is required to maximize efficiency. As a way of implementing an adjustable load circuit that can provide a regulated output voltage to power a host system, a two-stage power-conversion circuit is proposed. The first stage is a dc-to-dc discontinuous mode converter that is controlled by an input loop to regulate the load impedance presented to the RF rectifier. The analysis, design, and experimental results of the first stage adjustable load circuit with a tracking loop are described. The load is connected to a 985-MHz 10-W GaN Class-F RF rectifier and results with the load circuit are compared to the optimum load.

I. Introduction

There are many applications of RF rectifiers including energy harvesting for low-power sensors [1]–[4], wireless power transfer [5]–[13], and rectification in outphasing and switch-mode power amplifiers that employ energy recycling as a way to improve power efficiency [14]–[18]. The RF power ranges in these applications are equally diverse and vary from mW levels in energy harvesting to high power in wireless power and amplifier systems. Device technology and circuit topologies for RF rectifiers are also diverse. Examples include CMOS voltage multiplier designs [1], CMOS synchronous switching rectifiers designs [3], Schottky diode rectifiers designs [5]–[7], and discrete high-power switching designs using GaAs [8] or GaN devices [5], [10].