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The Multilevel Technique for Improving Filterless class-d Audio amplifiers

Class-D amplifier features very high efficiency on power delivery because its switching operation consumes tiny static power on very low on-resistance. In this work, a multilevel technique is presented to improve total-harmonic-distortion (THD) and signal-to-noise-ratio (SNR) of pulse-width-modulation (PWM) filterless class-D amplifiers. The proposed method consists of a multilevel converter and a time division adder (TDA) followed by PWM modulator. The PWM-modulated signal is arranged into several time divisions and then integrated and encoded to a set of parallel control signals for multilevel converter. Instead of the two-level PWM signal, the output signal of a multilevel converter is as stairway with less transient variation. The performance of THD and SNR are therefore improved because the instantaneous variation of signal is greatly reduced. To demonstrate the proposed method, a filterless audio amplifier was implemented by TSMC 5 V–0.35 μm CMOS technology. With 8 Ω speaker and 550 mW maximum power, experiment results show that the THD, SNR and power efficiency can be achieved over 0.02%, 85 dB and 85%, respectively.