Design of a low-distortion 22-kHz fifth-order Bessel filter

A linearity improvement technique employing passive resistors and current-steering MOS transistors as a variable resistance element is proposed to implement a low-distortion filter in CMOS technology. This proposed implementation relies on the linearity of the passive resistors and the tunability of the current-steering MOS transistors operating in the triode region to overcome the limited linearity performance in continuous-time electronically tunable filters. By using the existing systematic feedback loops in the active filters and placing the nonlinear elements inside the feedback, the distortion resulting from the nonlinear devices is greatly reduced by the filter loop gain. A 22-kHz fifth-order Bessel filter, its dynamic range optimized by applying Karmarkar´s rescaling algorithm and self-tuned with a switched-capacitor reference resistor, demonstrates better than -90-dB THD with a 2-kHz, 4-V_pp signal in 5-V 2-μm CMOS