A MEMS-based precision operational amplifier

Abstract

Two main difficulties for amplifiers that attempt to make precision DC measurements are the inherent low-frequency noise of the amplifier and the leakage current of the amplifier input stage. This thesis presents a novel fully integrated operational amplifier design that addresses both measurement limitations by using a fully differential mechanical transductor input stage, fabricated using SOI-MEMS technology. The input stage of the amplifier is a MEMS structure that provides a variable capacitance to transduce a low-frequency input voltage into a high-frequency AC current. This up-modulation of the input signal is exploited to reduce offsets and low-frequency noise, and the dielectric isolation of the MEMS structure provides high input impedance and low leakage currents. To function, the MEMS-based amplifier includes two co-dependent feedback loops. The 'drive loop' utilizes closed-loop control to vibrate the MEMS structure at its mechanical resonant frequency to produce a modulating capacitance. The 'sense loop' senses the up-modulated signal from the MEMS structure, and provides gain and demodulation to this signal. Global feedback around the sense loop allows for accurate measurement of the input voltage.