A 0.4-mA-Quiescent-Current, 0.00091%-THD+N Class-D Audio Amplifier With Low-Complexity Frequency Equalization for PWM-Residual- Aliasing Reduction

• Published in: IEEE journal of solid-state circuits 2022-02, Vol.57 (2), p.423-433
• Main Authors: Qiu, Yi-Zhi, Chien, Shih-Hsiung, Kuo, Tai-Haur
• Format: Article
• Language: English
• Subjects: Aliasing; Audio amplifier; Class-D amplifier; Complexity; Delays; Distortion; Equalization; Feedforward systems; Figure of merit; frequency equalization; Harmonic distortion; Intermodulation distortion; Operational amplifiers; Phase shift; Pulse duration modulation; Pulse width modulation; pulsewidth modulation (PWM)-residual-aliasing distortion; quiescent current; Receivers & amplifiers; Switches; Switching; Total harmonic distortion; total harmonic distortion plus noise (THD+N)
• ISSN: 0018-9200; 1558-173X
• DOI: 10.1109/JSSC.2021.3093309

This article proposes a low-complexity frequency equalization technique for pulsewidth modulation (PWM)-residual-aliasing reduction in closed-loop Class-D audio amplifiers to achieve both low total harmonic distortion plus noise (THD+N) and low quiescent current. Based on the comprehensive analysis on the phase shift delay between the audio input and the loop filter's output for the prior PWM-residual-aliasing reduction technique, the proposed technique minimizes the non-idealities via a frequency-equalization path to enhance the cancellation ability of high-frequency PWM switching components. In this way, the PWM-residual-aliasing distortion is greatly suppressed, thereby permitting Class-D audio amplifiers to achieve a further-reduced THD+N for the entire audio band while operating at a low switching frequency ( f_{\mathrm {SW}} ), leading to less quiescent current consumption. Moreover, due to the minimized phase shift delay, the required bias current of the loop filter's first operational amplifier for the target THD+N can be reduced, resulting in even lower quiescent current. Compared with other state of the arts, this work's high-fidelity second-order Class-D audio amplifier, fabricated with cost-efficient 0.5- \mu \text{m} CMOS technology, not only achieves a competitive minimal THD+N of 0.00091% for a 1-kHz input as well as the lowest maximal THD+N of 0.0027% over the entire audio band while operating at a low f_{\mathrm {SW}} of 168 kHz but also features both the highest figure of merit (FOM) of 2536 and the lowest quiescent current of 0.4 mA, surpassing the prior arts by a factor of 2.3.