A 1 V 1.8 MHz CMOS switched-opamp SC filter with rail-to-rail output swing

The switched-capacitor technique which implements analog functions in CMOS technology requires capacitors, switches, and opamps. Supply voltage reduction does not affect capacitors. On the other hand, turning MOS switches on and off and maintaining proper opamp operation are difficult with reduced supply voltage. Reducing the supply voltage reduces MOS switch overdrive so complementary switches are not effective. Three solutions have been proposed: low-threshold devices, on-chip voltage multipliers, and switched-opamps. Technologies with low-threshold devices are costly. On-chip voltage multiplication is not possible in scaled-down technologies where devices cannot sustain the multiplied voltage. Switched opamps have the potential to solve the problem by eliminating critical switches in series with opamp output nodes and realizing their function by turning the opamp on and off. The authors present the structure of a prototype biquad band-pass filter (with fo/fs=0.25 and Q=7) implemented in 0.5 /spl mu/m CMOS technology. The area is about 0.15 mm/sup 2/ and the unit capacitance is 0.25 pF. The filter consumes about 160 /spl mu/W.