Class D hearing aid amplifier

In a hearing aid having a microphone, a power amplifier, and an output receiver transducer, said power amplifier having positive and negative supply terminals adapted to be connected to a source of direct voltage, said amplifier being coupled to receive audio frequency input signals produced by said microphone for driving said transducer from amplifier output terminals in response to the input to said microphone, the improvement comprising:

• a power amplifier of the pulse-width modulation type, so that in response to the absence of an input signal from said microphone said power amplifier provides to said output terminals an alternating electrical signal pulse train of positive and negative electrical pulses of substantially zero average value at an ultrasonic frequency, said amplifier including means for providing at said output terminals an audio frequency facsimile of said input signal by controlling the time duration of said positive and negative electrical pulses responsively to the received audio frequency input signal, said power amplifier including output polarity switching means for producing a peak-to-peak voltage provided at said output terminals which is greater than the magnitude of said direct voltage by connecting said supply terminals to said output terminals at a given polarity to produce said positive pulses and at an opposite polarity to produce said negative pulses.