A 12.5 MHz CMOS continuous time bandpass filter

The authors describe a continuous-time bandpass filter integrated in a 3- mu m CMOS technology, with an f/sub 0/ of 12.5 MHz and a 2% fractional bandwidth. The four-pole filter is modeled on two L-C resonators coupled by mutual inductance, resistively terminated at the input and output for maximum power transfer. Each resonator is simulated on the IC by two tunable integrators in negative feedback and is terminated by MOS resistors to define the filter bandwidth. Regulation of f/sub 0/ is obtained by locking an oscillating, consisting of a third identical integrator pair without termination resistors, to an externally supplied reference frequency. The circuits were fabricated on a double-metal, single-polysilicon, 3- mu m CMOS IC. Operation of the chip at a 12.5-MHz center frequency was verified, showing the desired 2% fractional bandwidth.<>