New low-voltage electrically tunable triode-MOSFET transconductor and its application to low-frequency Gm-C filtering

Abstract

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 fT 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 (1.9mm 2 ) and power consumption (1µW), as desired for Low-Voltage Low-Power (LVLP) applications.