Design of a tunable frequency CMOS fully differential fourth-order Chebyshev filter

A new fully differential amplifier and a fully differential R-MOSFET-C fourth-order Chebyshev active lowpass filter employing passive resistors and current-steering MOS transistors as variable resistors are proposed. The implementation relies on the tunability of current-steering MOS transistors operating in the triode region which counteract the deviation of resistors in integrated circuit manufacturing technology in order that the cutoff frequency of Chebyshev active filter can be realized accurately tunable. The amplifier is not only with voltage common-mode negative feedback (VCMFB), but also with current common-mode negative feedback (CCMFB), which will benefit for the stability of its DC operating point. A cutoff frequency of 138 kHz fourth-order Chebyshev lowpass filter was designed and fabricated using 3.3 V power supply and 0.35 μm CMOS technology. Chip test results demonstrate better than −68 dB THD with 70 kHz, 2.0Vpp signal, frequency turning range of more than 14,000 from 3 Hz to 420 kHz, chip area of 0.36 mm2 and power consumption of 16 mW.