A low-power self-biased rail-to-rail preamplifier as a readout circuit for a capacitor-type microphone
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TECHNICAL PAPER
A low-power self-biased rail-to-rail preamplifier as a readout circuit for a capacitor-type microphone Tse-Yi Tu • Paul C. -P. Chao • Yueh-Teng Mai
Received: 31 October 2012 / Accepted: 21 April 2013 / Published online: 5 May 2013 Ó Springer-Verlag Berlin Heidelberg 2013
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 twostage or rail-to-rail MOS operational amplifier at the second stage. The proposed circuit is implemented by using the technology of TSMC 0.35 lm 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 lW. Experiments are finally conducted to validate the performance of the designed readout circuit from 20 to 20k Hz with appropriate add-on high- and lowpass filters. T.-Y. Tu P. C.-P. Chao (&) Y.-T. Mai Department of Electrical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, ROC e-mail: [email protected] P. C.-P. Chao Institute of Imaging and Biophotonics, National Chiao Tung University, Tainan, Taiwan
1 Introduction Recent advances in the technology of micro-electromechanical systems (MEMS) (ITRC 2005) have accelerated design and application a micro-sensors and actuators based on electrostatic actuation. The MEMS microphone becomes popular electronic device to be implemented in cell phones. As the traditional microphone, the MEMS microphone (Bergqvist and Rudolf 1994; Bernstein and Borenstein 1996; Scheeper et al. 1994; Kuhnel and Hess 1992) could be capacitive-type, piezo-resistive, piezo-electronic or optical (Bergqvist 1993; ITRC 2005; Voorthuyzen et al. 1989; Hsu et al. 1998; Hsieh et al. 1997). For the MEMS microphone, it is generally the capacitivetype microphone (Bergqvist and Rudolf 1994; Bernstein and Borenstein 1996; Scheeper et al. 1994; Kuhnel and Hess 1992). A capacitive-type microphone has a dielectric material that has been charged or discharged. Special material and structure are involved in the device, making the electronic characteristics complex beyond linearity. This work is focused on designing
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