A systematic design procedure for log-domain filters based on nonlinear transconductance

Abstract

In this paper a new generalized method for synthesis of current-mode log-domain filters, using nonlinear transconductance is presented. The nonlinear transconductance is designed by employing floating gate MOS (FG-MOS) transistors that operate in weak-inversion. It allows deriving current-mode nonlinear transconductance-capacitor (G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> -C) structure from its corresponding conventional transconductance-capacitor (g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> -C) in a systematic way. In this method, synthesis complexity is independent of system size, and thus high-order systems can be readily realized. In addition, it keeps the transconductance-capacitor topology invariant, leading to a very regular structure. The filters are designed based on the FG-MOS transistors working in weak-inversion and feature, low supply voltage, low power consumption, immune from body effect and with low circuit complexity. For illustration of this new synthesis method, a biquad filter was designed and verified by simulation. The simulation results by Hspice show the validity of the proposed technique.