A Class-D Amplifier With Pulse Code Modulated (PCM) Digital Input for Digital Hearing Aid

A class-D amplifier with pulse code modulated (PCM) digital input is developed for a low-power digital hearing aid. 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 (PDM) 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. The class-D amplifier with PCM digital input has been implemented in a standard 0.13-
$\mu{\rm m}$
CMOS process. With 160-
$\Omega$
load of speaker, the maximum output power delivered to the load is 1.14-mW while the efficiency of the power switch is 97-%. The
${\rm signal\mathchar"702D to\mathchar"702D noise}+{\rm distortion~ratio}$
(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.