A Fully On-Chip Gm-Opamp-RC Based Preamplifier for Electret Condenser Microphones

Abstract

An on-chip CMOS preamplifier for direct signal readout front an electret capacitor microphone has been designed with high immunity to common-mode and supply noise. The Gm-Opamp-RC based high impedance preamplifier helps to remove all disadvantages of the conventional JFET based amplifier and can drive a following switched-capacitor sigma-delta modulator in order to realize a compact digital electret microphone. The proposed chip is designed based on 0.18 mu m CMOS technology, and the simulation results show 86 dB of dynamic range with 4.5 mu Vrms of input-referred noise for an audio bandwidth of 20 kHz and a total harmonic distortion (THD) of 1% at 90 mVrms input. Power supply rejection ratio (PSRR) and common-mode rejection ration (CMRR) are more than 95 dB at 1 kHz. The proposed design dissipates 125 mu A and can operate over a wide supply voltage range of 1.6 V to 3.3 V.