Mixed-integrator biquad for continuous-time filters

A mixed-integrator biquad (MIB) combining the advantageous features of a gm-C integrator and a source-follower-based (SFB) integrator is proposed. Unlike the conventional pure SFB biquad suffering from significant passband gain loss, the MIB employs a linearised gm-C integrator at the front, overcoming the gain loss, and an SFB integrator at the back, guaranteeing good linearity and low-output impedance. The complex poles are synthesised through a negative feedback technique avoiding the need of a common-mode feedback circuit. An experimental fourth-order MIB-based Butterworth lowpass filter fabricated in 0.18 mu m CMOS shows improved figure-of-merit with respect to the state-of-the-art, while occupying a small die area.