Theoretical energy-conversion efficiency for multi-stage RF energy harvesters

A model has been developed through closed-form equations which simply determines the theoretical output power and energy-conversion efficiency for an RF-to-dc energy harvesting circuit with N rectifying elements. This model improves on previous works which are limited to evaluating circuits with only a single rectifying element or a single frequency excitation. Validation has been obtained through the simulation of circuits with ideal components which show good agreement for harvesters with up to 6 diodes along with measurements of a 5.8 GHz voltage doubler which show agreement to within 5.6%. The relationships between the number of stages and output loads is discussed, and the theoretical maximum efficiency for a harvester with N ideal rectifying elements is presented.