A low-power self-biased rail-to-rail preamplifier as a readout circuit for a capacitor-type microphone

Abstract

This study is dedicated to design a readout circuit to extract the output signal of a capacitive-type MEMS microphone. A low-power preamplifier is forged for a capacitive-type microphone to meet the demands on cell phone applications. The design starts with modeling the electro-mechanical behavior of a biased capacitive-type microphone using combination of AV and DC voltages sources. This aims to enable co-simulation of the microphone and the readout circuit design. The output signal of a capacitance-to-voltage converter is normally small and may cause substantial noise in the output signals. Therefore, a preamplifier is designed and applied to amplify the signal to an acceptable level for the convenience of ensuing signal processing. The designed readout circuit consists of two main sub-circuits, responsible for functions in two different stages. One is a newly-designed self-bias circuit in the structure of a capacitance-to-voltage converter at the first stage, while another is a low-voltage low-power two-stage or rail-to-rail MOS operational amplifier at the second stage. The proposed circuit is implemented by using the technology of TSMC 0.35 A mu m Mixed-Signal MODE (2P4M, 3.3 V/5 V) POLYCIDE. The supply voltage is fixed at low voltage (3 V), the total power dissipation is merely 200 mu W. Experiments are finally conducted to validate the performance of the designed readout circuit from 20 to 20k Hz with appropriate add-on high- and low-pass filters.