Current mode MOSFET-only third order Butterworth low pass filter with DTMOS tuning technique

Abstract

A new low-voltage, MOSFET-only, third order low-pass filter is proposed. The circuit employs only MOSFETs operating in saturation region. The transconductance gains and the parasitic gate to source capacitances of the MOSFETs represent resistive and capacitive elements of the filter. Using TSMC 0.18 µm technology parameters the circuit is simulated, non-ideal effects have been investigated and dynamic threshold voltage MOS tuning technique has been developed for the filter circuit to suppress the non-idealities. In this tuning technique, bulk terminals of MOS transistors are used to adjust the biasing point of the circuit by changing the threshold voltages of the MOS transistors. This gives the designers more flexibility than conventional tuning methods and allows low voltage operation when several transistors are stacked over each other. The resulting circuit is capable of operation at high frequencies with low power consumption due to the usage of significantly less number of transistors than conventional active block-based filtering circuits.