Class D hearing aid amplifier
First Claim
1. In a hearing aid having a microphone, a power amplifier, and an output receiver transducer, said power amplifier being coupled to receive audio frequency input signals produced by said microphone and to drive said transducer in response to the input to said microphone, the improvement wherein:
- said power amplifier is of the pulse-width modulation type, and in response to the absence of an input signal from said microphone said power amplifier couples to said transducer 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 controlling the time duration of said positive and negative electrical pulses responsively to the amplitude of the audio frequency signal applied to the input of said power amplifier so as to provide to said transducer an audio frequency facsimile of said input signal.
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Abstract
A subminiature power amplifier for hearing aids configured to be completely self-contained and fit within the user'"'"'s ear features a Class D amplifier stage. By configuring all signal elements as digital units, circuit balance is readily maintained. A substantial economy of battery drain is achieved while preserving a low value of distortion at low signal levels. Oscillator frequency is governed by a single resistance element. The amplifier and an optional efficiency-improving inductor can be accommodated entirely within the residual volume of the motor chamber of a state-of-the-art earphone housing.
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Citations
17 Claims
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1. In a hearing aid having a microphone, a power amplifier, and an output receiver transducer, said power amplifier being coupled to receive audio frequency input signals produced by said microphone and to drive said transducer in response to the input to said microphone, the improvement wherein:
- said power amplifier is of the pulse-width modulation type, and in response to the absence of an input signal from said microphone said power amplifier couples to said transducer 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 controlling the time duration of said positive and negative electrical pulses responsively to the amplitude of the audio frequency signal applied to the input of said power amplifier so as to provide to said transducer an audio frequency facsimile of said input signal.
- View Dependent Claims (2)
- 3. An integrated circuit class D hearing aid power amplifier for amplifying audio frequency input signals and for driving a receiver transducer connected thereto, said amplifier having complementary metal oxide semiconductor (CMOS) output devices driven in switching mode, said amplifier including an oscillator for providing signals controlling the switching of said output devices, said switching oscillator comprising cascaded CMOS switching stages configured as a ring oscillator, each stage including first and second current control elements governing stage output rise and fall time respectively, a first and a second current mirror, said first and second control elements being controllably responsive to said first and second current mirrors respectively, said oscillator further comprising a resistance means connected to establish the current provided by said first current mirror, said second current mirror connected to said first current mirror to provide a replica of the current provided by said first current mirror, whereby said rise and fall times are substantially equal to provide symmetric operation at low current drain and so that the frequency of oscillation is controllably responsive to the value of said resistance means, said frequency of oscillation being chosen to be a multiple of the desired switching frequency, and means for dividing the oscillator output by said multiple to obtain symmetry of the oscillator output waveform.
- 12. A hearing aid including a microphone, a power amplifier, and a miniature receiver transducer, a housing for said receiver transducer, said housing disposed within said hearing aid, said power amplifier being coupled to receive audio frequency input signals produced by said microphone and to drive said transducer in response to the input to said microphone, said housing having a septum wall therein defining first and second chambers, a transducer motor and said power amplifier being disposed within said first chamber, said second chamber being acoustically excitable, an acoustical output port for said second chamber, said transducer motor coupled to drivingly acoustically excite said second chamber and provide acoustical output through said port.
Specification