A hearing aid is disclosed, which, in a test mode, can determine the impedance of the transducer that stimulates the anatomy of the patient. Impedance may be determined by simultaneous determination of the current flowing through the transducer and the voltage across the transducer. In some cases, the output amplifier of the hearing aid includes two outputs, with one being a scaled and/or summed replica of the other. The amplifier is driven with a periodic signal with a particular frequency and a known peak voltage. The periodic signal may be sinusoidal. The primary output of the amplifier is electrically connected to the transducer, with a known voltage given by the peak input voltage and a known gain of the amplifier. The current from the secondary output of the amplifier is measured. In an example measurement scheme, the secondary output is sent through a rectifier and then through a low-pass filter.