New Low-Voltage Electrically Tunable Triode-MOSFET Transconductor and its Application to Low-Frequency Gm-C Filtering

A new low-voltage electrically tunable transconductor is presented. Its transconductance can be settled by means of a ratio between a reference current and a reference voltage rendering the circuit independent of technological parameters, to a first order approach. This property allows, to some extent, reusing the transconductor in several CMOS processes. A kind of linear current division strategy turns also the transconductance inversely proportional to a product of two ratios between transistors' sizes that can be chosen so as to meet the desired Gm order of magnitude. This feature, together with a low-current biasing policy, is exploited in order to get a transconductance in the range of 1nS, as needed for very low-frequency filters. For a 2V supply and a 20pF load capacitor, an integrator characterization in a 1.6μm CMOS technology revealed a unity-gain frequency f_T of 10Hz, a current consumption of 220nA and an input-referred noise of 2.2μVrms. Using this transconductor, a 10Hz third-order low-pass transconductance-capacitor (Gm-C) Butterworth filter was designed. It is intended for smoothing the output of a chopper amplifier. The filter shows acceptable performance in terms of die area (1.9mm²) and power consumption (1μAV), as desired for Low-Voltage Low-Power (LVLP) Applications.