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

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 addition, a fully passive anti-aliasing filter is used to suppress the PWM residue leakage. This CDA was fabricated in a 0.18-mu m 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.