A 0.00055% THD + N Class-D Audio Amplifier With Capacitive Feedforward PWM and Wide-Band

Abstract

Linearity of Class-D audio amplifier based on conventional closed-loop pulsewidth modulat ion (PWM) is deteriorated by large swing of loop filter and input-dependent comparator delay. Furthermore, PWM-residual-aliasing has been revealed as a source of distortion, especially in low-power filter- less structure. This article presents a class-D amplifier (CDA) that utilizes an input-feedforward carrier to mitigate both loop filter swing and comparator delay coupling. As for the PWM- residual-aliasing, we propose a 2nd-order PWM-residual-aliasing reduction (PRAR) technique with loop compensation to achieve steeper filtering and wide-band linearity improvement. In addi- tion, a fully passive anti-aliasing filter is used to suppress the PWM residue leakage. This CDA was fabricated in a 0.18-µm CMOS 5 V process. It achieves a state-of-the-art total harmonic distortion plus noise (THD + N) of 0.00055% (−105.2 dB) and an FoM THD + N of 2119 while consuming a low quiescent current of 0.81 mA. Thanks to the wide-band-effective linearity enhancement, the THD + N across 20–20-kHz audio bandwidth is less than −100 dB.