MEMS microphone fully-integrated CMOS cap-less preamplifiers

A pair of completely integrated capless preamplifiers (not requiring external capacitance) for MEMS microphones are presented. The devices exploit specific circuit solutions to implement a proper dc biasing of the pre-amplifier and a low frequency (< 1 Hz) high-pass pole. The two solutions adopted are a based on a transistor in the off state (OTP) and a switched-resistor (SRP), respectively. Since the devices are supposed to operate with different silicon microphones, the pre-amplifier gain has to be programmable. In spite of the single ended configuration and independently of the gain, both pre-amplifiers achieve a signal to noise ratio (SNR) lower than -100 dB. The total harmonic distortion (THD) is lower than -80 dB for the OTP and lower than -100 dB for the SRP. The devices are implemented in a 0.18-μm CMOS technology, with a supply voltage of 1.8-V. In both cases the power consumption is 230 μW. The bandwidth of interest ranges from 20 Hz to 20 kHz. Both solutions have variable gain configurations [-6, 0, 6, 12, 18] dB.