Low-Gm CMOS Transconductors with Wide Tuning Range for Bioimpedance Spectroscopy

Two techniques to design a low-transconductance (G
m
) widely-tunable transconductor are described. The operation principle consists in both cases in the subtraction of two nominally equal transconductances, one of which can be electronically tuned. The circuit implementation differs, being one solution based on the use of source followers (SF), a linearization resistor, and a tunable current mirror, and consisting the other approach of differential pairs (DP) with resistive source degeneration, and an electronically programmable current biasing. Both transconductors have been designed in 180 nm CMOS technology to operate with 1.8 V. The SF and DP solutions achieve a minimum G
m
nominal value of 1.38 nA/V and 6.03 nA/V, respectively, with a simulated transconductance tuning range of 729.6× and 7129.4×, also respectively, obtained from a Montecarlo analysis. The application of the transconductors to the design of a second-order G
m
− C bandpass filter for signals separation in multi-sine bioimpedance spectroscopy is also addressed.