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A High-Efficiency Self-Oscillating Class-D Amplifier for Piezoelectric Speakers

The design tradeoffs of the class-D amplifier (CDA) for driving piezoelectric (PZ) speakers are presented, including efficiency, linearity, and electromagnetic interference. An implementation is proposed to achieve high efficiency in the CDA architecture for PZ speakers to extend battery life in mobile devices. A self-oscillating closed-loop architecture is used to obviate the need for a carrier signal generator to achieve low power consumption.

The use of stacked-cascode CMOS transistors at the H-bridge output stage provides low-input capacitance to allow high-switching frequency to improve linearity with high efficiency. Moreover, the CDA monolithic implementation achieves 18 VPP output voltage swing in a low-voltage CMOS technology without requiring expensive high-voltage semiconductor devices. The prototype experimental results achieved a minimum THD + N of 0.025%, and a maximum efficiency of 96%. Compared to available CDA for PZ speakers, the proposed CDA achieved higher linearity, lower power consumption, and higher efficiency.