Feasibility study of a low-cost feedback damping scheme for a micromachined capacitive microphone

This paper reviews the need for feedback to damp the resonance of a low-noise capacitive microphone, evaluates alternative feedback structures with different measurement mechanisms for their control effectiveness, and finally investigates the feasibility of a low-cost mechanism that uses the voltage across the capacitive sensor as the feedback variable. This capacitive sensing mechanism leads to a stiff system with weak controllability, indicated by the Hankel singular values of the small-signal microphone model. The outcome of an initial attempt in linear controller design for small signal operation is presented. The linear controller is verified through simulations in closed-loop with a nonlinear microphone model. The design is shown to have eliminated the resonant peak and extended the microphone´s bandwidth. A second degree of freedom is introduced to robustify the achieved nominal performance level in the presence of uncertainties attributed to the micro-fabrication variability of the microphones.