Abstract
A low-distortion three-stage class-AB audio amplifier is designed to drive a 16-Ω headphone speaker load. High power efficiency is achieved using fully differential internal stages with local common-mode feedback and replica biasing of the output stage. The threshold voltage of nMOS transistors was made comparable to that of pMOS transistors by negatively biasing the p-substrate in order to achieve high linearity. Multiple compensation networks guarantee the stability of the audio amplifier when driving a wide range of capacitive loads from 10 pF to 5 nF. Peak power delivered to the load is measured as 93.8 mW (corresponding to 46.9 mW RMS) with -77.9-dB total harmonic distortion; quiescent power is only 1.43 mW. The power-supply rejection ratio from both ±1.5-V supplies exceeds 63 dB over the entire audio frequency range. The design is implemented in a 0.5-μm CMOS process and occupies 0.34 mm2 of area.
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