A linearly tunable low-voltage CMOS transconductor with improved common-mode stability and its application to gm-C filters

A linearly tunable low-voltage CMOS transconductor featuring a new adaptive-bias mechanism that considerably improves the stability of the processed-signal common-mode voltage over the tuning range, critical for very-low voltage applications, is introduced. It embeds a feedback loop that holds input devices on triode region while boosting the output resistance. Analysis of the integrator frequency response gives an insight into the location of secondary poles and zeros as function of design parameters. A third-order low-pass Cauer filter employing the proposed transconductor was designed and integrated on a 0.8-μm n-well CMOS standard process. For a 1.8-V supply, filter characterization revealed f_p=0.93 MHz, f_s=1.82 MHz, A_minn=44.08, dB, and A_max=0.64 dB at nominal tuning. Tuned by a dc voltage V_TUNE, the filter bandwidth was linearly adjusted at a rate of 11.48 kHz/mV over nearly one frequency decade. A maximum 13-mV deviation on the common-mode voltage at the filter output was measured over the interval 25 mV⩽V_TUNE⩽200 mV. For V_out=300 mV_PP and V_TUNE=100 mV, THD was -55.4 dB. Noise spectral density was 0.84 μV/Hz^1/2@ 1 kHz and S/N=41 dB@ V_out =300 mV_pp and 1-MHz bandwidth. Idle power consumption was 1.73 mW@ V_TUNE=100 mV. A tradeoff between dynamic range, bandwidth, power consumption, and chip area has then been achieved