디지털 보청기를 위한 저전력 D-클래스 음성 증폭회로

Abstract

This dissertation presents a low-power class-D amplifier for a digital hearing aid which operates with a pulse code modulated (PCM) digital input. A 16-bit 40-kbps PCM digital input is noise-shaped by a third-order digital sigma-delta modulator (SDM) which provides 1.5-bit digital output. The 1.5-bit digital output of the digital SDM is converted to a three-level analog signal by a simple digital-to-analog converter (DAC) and then applied to an analog SDM. The analog SDM provides pulse density modulated (PCM) signal to drive a power switch. The PDM output is fed back to the input of the analog SDM in order to suppress the noise of the power switch. While the integrators of the analog SDM are implemented with switched-capacitor (SC) circuits for a well-defined frequency response of the modulator loop filter, the feedback path from the power switch output is realized with a continuous-time (CT) integrator for effective noise suppression. Unlike the conventional three-level switching scheme, the “0” state is realized by shorting-circuiting the differential outputs and charge sharing, which improves the power efficiency and reduces electro-magnetic interference (EMI) emission. The class-D amplifier with PCM digital input has been implemented in a standard 0.13-μm CMOS process. With 160-Ω load speaker, the maximum output power delivered to the load is 1.14-mW. The signal-to-noise+distortion ratio (SNDR) and dynamic range (DR) of the class-D amplifier are measured to be 80.6-dB and 87-dB, respectively. The class-D amplifier consumes 0.38-mW from a 1.2-V power supply including the driving power of the power switch. The analog SDM with shared op-amps to save power for a digital hearing aid has also been developed. Applied to a class-D amplifier for a digital hearing aid, the fifth-order analog SDM with shared op-amp consumes only 0.2-mW from a 1.2-V power supply. The total harmonic distortion+noise (THD+N) and the DR are achieved to be 0.0038-% and 96-dB, respectively.