Amplitude-modulated electro-mechanical feedback system for silicon condenser microphones

Abstract

Miniaturization of sensors and actuators has become a n important topic during the last decade. For silicon condenser microphones, narrowing the air gap as a part of the miniaturization process causes, however, a reduction of the bandwidth of the microphone. By introducing an actuator electrode on the diaphragm for electro-mechanical feedback, it is possible to increase the bandwidth of condenser microphones with a narrow air gap. In this paper a feedback system is presented, which uses an amplitudemodulated actuator signal with a carrier frequency far above the audio-frequency range. With feedback, the bandwidth of the microphone is increased by at least a factor IO.

Equivalent circuit representation of electromechanical transducers: I. Lumped-parameter systems

“…Instruments employing feedback are even often considered for use as secondary standards. Examples presented in the literature of micromechanical devices employing feedback techniques include capacitive accelerometers [16,17] and condenser microphones [18,19]. An arbitrary example of a system employing electrostatic feedback, that will be used to illustrate the principle of feedback and the use of equivalent circuits herein, is shown in figure 15(a).…”

“…The system can be used for measuring the force F m exerted directly on the mass (e.g. as in a microphone [18,19]) or it can be configured to measure the acceleration a in of the case which for this application is rigidly fixed to the vibrating body [16,17] (compare subsection 5.5). The upper capacitor senses the induced mass displacement resulting in a change of the plate charge, which can be detected by means of a charge amplifier.…”

Capacitive micromachined ultrasonic transducer arrays for minimally invasive medical ultrasound

“…Similar in structure to a MEMS microphone [22], CMUT devices use the deformation of a suspended membrane to transmit and receive ultrasounds.…”