Design of fully differential fourth-order Bessel filter with accurate group delay

A fully differential R-MOSFET-C fourth-order Bessel active lowpass filter employing passive resistances and current-steering MOS transistors as a variable resistance is proposed. This proposed implementation relies on the tunability of the current-steering MOS transistors operating in the triode region counteracting the concert deviation of resistance in integrated circuit manufacturing technology in order that the group delay of Bessel active filter could be designed accurately. 0.75 us group delay fourth-order Bessel lowpass filter, which is synthesized according to passive doubly terminated RLC prototype lowpass filter, demonstrates better than -65 dB THD with 100-kHz, 1.65-Vpp signal in 3.3 V 0.5 um CMOS technology.