work presents a novel RF energy harvesting system at 2.45 GHz, operating over a wide dynamic RF range (44 dB), starting from ultra-low power. It consists of three rectification branches, each one optimized for a selected power interval, between -20 divided by +24 dBm. Through an automatic distribution network, the incident RF power is routed to the most efficient rectification branch, with no need for external control circuits. The first branch is optimized for low-power conditions, in the range -20 divided by 0 dBm, and the second and third branches for 0 divided by 9 dBm and 9 divided by 24 dBm, respectively. For branch decoupling, an original strategy, based on the concurrent nonlinear design of power-dependent matching networks and self-powered switches, is used. To maximize each rectifier performance, Enhancement mode High Electron Mobility transistors (HEMT) are adopted by means of the self-biasing mechanism. In this way, for any possible incoming RF power, only the most suitable rectifier path is active and the maximum RF-to-dc conversion efficiency is preserved. The system has been fabricated and tested demonstrating a conversion efficiency higher than 30% from -13 dBm to 24 dBm, with the lowest and highest measured dc output voltage of 0.1 and 13 V, respectively.
