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We propose an adaptive microphone preamplifier that adjusts its power consumption according to the input signal's bandwidth. A chip prototype of the adaptive preamplifier for speech processing was fabricated in a 0.5-μm CMOS process. The adaptive preamplifier's measured input referred noise is 3 μV rms and its total harmonic distortion is -40 dBc for an 80-mV rms input amplitude. These noise and distortion specifications remain virtually constant over the preamplifier's range of bandwidth adaptation. The adaptive preamplifier consumes 65 μW of power. This is achieved with no degradation in speech quality, when compared to a conventional preamplifier of fixed bandwidth, and as measured by the perceptual evaluation of speech quality (PESQ) score.