CMOS R-MOSFET-C fourth-order Bessel filter with accurate group delay

A fully differential R-MOSFET-C fourth-order Bessel active lowpass filter employing passive resistors and current-steering MOS transistors as a variable resistor is proposed. The implementation relies on the tunability of the current-steering MOS transistors operating in triode region which counteract the concert deviation of resistance in integrated circuit manufacturing technology in order that the group delay of the Bessel active filter can be realised accurately. A 0.75 μs group delay 520 kHz fourth-order Bessel lowpass filter based on a passive doubly terminated RLC prototype was designed and fabricated using 3.3 V power supply and 0.5 μm CMOS technology. Chip test results demonstrate better than 65 dB THD with a 100-kHz, 1.65-Vpp input signal, frequency tuning range of more than ten decades from 0.6 kHz to 550 kHz, chip area of 0.32 mm² and power consumption of 13.3 mW.