Design considerations for large dynamic range MOSFET-C filters for direct conversion receivers

The tradeoffs involved in the design of MOSFET-C filters for direct conversion are discussed, with emphasis on linearity and tunability vs. temperature. A high IIP3 MOSFET-C lowpass filter with an automatic cutoff frequency tuner is implemented in a 0.35 µm Bi-CMOS process. To improve IIP3 performance, the gate control voltage of the MOSFET-C filter is set around 3 V using a charge-pump circuit. The 5th-order elliptic filter achieves +8 dBV in-band IIP3, +33 dBV out-of-band IIP3, +80 dBV out-of-band IIP2 and -79 dBV integrated input-referred noise, and dissipates 2.9 mW from a 2.7 V supply.