A wireless charging receiver operates on a resonance principle and includes an impedance matching circuit coupled between an antenna and a rectifier circuit. The impedance matching circuit has both series-connected and parallel-connected capacitors. At least one of the capacitors is a tunable variable capacitor. A method is provided for automatically adjusting a capacitance value of the at least one variable capacitor based on an error voltage between a target rectifier voltage and a measured rectifier voltage. Automatically adjusting the antenna impedance of the receiver provides for improved power transfer efficiency for changing operating conditions. In one embodiment, one or more of the parallel-connected capacitors are variable capacitors. In another embodiment, one or more of the series-connected capacitors are variable capacitors.