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

Huy-Binh LE  Seung-Tak RYU  Sang-Gug LEE  

IEICE TRANSACTIONS on Electronics   Vol.E92-C   No.4   pp.587-588
Publication Date: 2009/04/01
Online ISSN: 1745-1353
DOI: 10.1587/transele.E92.C.587
Print ISSN: 0916-8516
Type of Manuscript: LETTER
Category: Electronic Circuits
electret microphone,  preamplifier,  switched-capacitor,  sigma-delta,  

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An on-chip CMOS preamplifier for direct signal readout from 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 µm CMOS technology, and the simulation results show 86 dB of dynamic range with 4.5 µVrms 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 µA and can operate over a wide supply voltage range of 1.6 V to 3.3 V.