This paper presents a fully differential R-MOSFET-C 4th-order Bessel active low-pass filter with group delay adjusted by tuning the channel transconductance of CMOS transistors worked in weak inversion region.The tuning can counteract the deviation of resistor in the manufacture of integrated circuit

therefore the group delay of Bessel active filter can be designed and realized accurately .The active filter can be synthesized according to the state equation of passive filter.Computer simulation of 4th-order Bessel low-pass filter with 0.75μs group delay indicate that the proposed circuits are both effective using 3.3V、0.5μm CMOS technology

and suitable for integration.