Previously, the micromachined electrothermal structures were used for mass flow sensors, rms-dc converters, infrared sensors, etc. These applications take advantage of the structures such as low thermal conductance for effective temperature sensing and large thermal capacitance for low frequency filtering. Generally, micromachined structures can give these desirable characteristics implemented by IC compatible processes in a small size. We have demonstrated the filter applications of the micromachined electrothermal structure in the audio frequency range with driving circuitry. In general, audio-band electrical filters have been designed using Gm-C or switched capacitor techniques. However, they have some drawbacks. Gm-C filters require large capacitors and low-gain transconductors while switched capacitor filters typically have high switching noise and aliasing effects. By using the large effective time constant that can be obtained in temperature domain, the realization of audio frequency range filters be comes much easy and simple. Test results of the fabricated electrothermal structures and filters demonstrated followed by the discussion of the audio frequency filter design using the micromachined electrothermal structures.

Fabricated Electrothermal Structure

Principles of Electrothermal Filter Operation and Typical Frequency Responce of The Electrothermal Structure

Low-pass and band-pass electrothermal filters have been fabricated, tested and tuned from 30Hz to 300Hz. It has been showed that electrothermal filters can be realized in the audio frequency range (from 100Hz to 10kHz) when the materials with a TCR higher than 2%K-1 are used for the resistors in the thermal structure. These electrothermal filters can be applied to audio band filter applications such as implementation of basilar membrane models and loop filters in the automatic adaptation technique.

Frequency Response of 2nd-order Electrothermal Bandpass Filter