A low-distortion 22 kHz 5th-order Bessel filter

A low-distortion MOSFET-C filter design is proposed. The proposed technique relies on the linearity of the passive resistors and the tunability of current-steering MOS transistors operating in the triode region. Placing the nonlinear elements inside the feedback loops and degenerating the MOSFETs by passive resistors, greatly reduces the distortion resulting from the nonlinear devices. A prototype 22-kHz Bessel filter automatically tuned to a switched-capacitor reference resistor has control voltage feedthrough below -100 dB, and measured linearity better than -90-dB THD (total harmonic distortion), with a 2-kHz, 4-V/sub pp/ voltage swing. Measured THD demonstrates an improvement of at least 30 dB over a conventional MOSFETC filter. The chip consumes 40 mW with a 5-V supply, and the active die area is 7 mm/sup 2/ in 2- mu m double-poly CMOS.<
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