A high dynamic range CMOS variable gain filter for ADSL

A 3/sup rd/ order, 1.1 MHz, lowpass Butterworth Gm-c filter has been combined with a low noise variable gain amplifier to form the variable gain filter (VGF). In this VGF an improved transconductor is used and a new method is used to adjust the transconductance gain for tuning application. A continuous variable gain current-to-current converter is used to tune the transconductor value. The THD of the VGF is -85 dB for 1 V/sub ppd/ input signal and -50 dB for 3 V/sub ppd/ input signal. VGF gain can be varied from 0 to 31 dB with 1 dB variation steps. Input referred noise is 20 nV//spl radic/(Hz) for maximum gain. All the circuits are designed based on a 0.25 /spl mu/m CMOS process technology.