An amplifier circuit comprising a driver (204, 304) configured to provide a switched mode input signal, a switching mode power amplifier (206, 306) configured to receive the switched mode input signal and provide an output signal for an external load (210, 310); and a sensor (208, 308) configured to sense the impedance of the external load (210, 310) The driver is configured to set the duty cycle of the switched mode input signal in accordance with the sensed impedance of the external load (210, 310).