A Biquadratic Cell Based on the Flipped-Source-Follower Circuit

This brief presents a novel biquadratic cell (biquad) based on the flipped-source-follower (FSF) circuit. The main idea is to exploit the FSF circuit as a basic building block for a low-pass second-order filter, taking advantage of its well-known strengths, like low-output impedance, low-noise, large in-band linearity, and low power. Thanks to the very essential FSF circuit, the resulting biquad is a power-efficient broad bandwidth stage, with very low-noise performance. In order to validate the biquad design idea, extensive circuital simulation results will be presented. The filter design example synthesizes a third-order low-pass transfer function and consumes 2.3 mW from a single 1.8 V@V-DD. Input noise spectral density is 7 nV/vHz. Linearity has been evaluated in terms of IIP3 (25 dBm) and THD (-40 dBc at 380 mV(0-PEAK) output voltage swing, resulting in 62-dB SNR). Hence, a very promising figure-of-merit (i.e., 165 J(-1)) has been achieved.