This paper presents a transmitter architecture for wireless power transfer with automatic resonance frequency tracking to maintain high efficiency over wide variations of antenna distance. By injection-locking the source oscillator to the output resonance in a positive feedback loop, the closed-loop transmitter functions as a power oscillator with the oscillation frequency determined by the most dominant resonance in the coupled antennas. We show that this frequency tracking minimizes the change in input impedance presented to the power amplifier (PA), and hence mitigates mismatch that can cause a sharp non-linear drop in PA efficiency. The power oscillator was tested well above and below critical coupling, and maintained PA efficiency above 60 % even when highly over-coupled at 25 % spacing (10mm/40mm) below critical coupling. Compared to an open-loop system, the charging range with efficiency over 50 % is doubled.
