MOSFET hand analysis using BSIM

This article describes the set-up of the Liu small-signal circuit for hand analysis of circuits using MOSFETs with BSIM models, Liu´s circuit differs from standard textbook circuits in the use of a complex-valued transconductance and does not make the common, but inaccurate, simplification of neglecting the output drain-to-body capacitance. BSIM and Liu´s circuit agree completely for the frequency response of a common-source amplifier. This example shows that Liu´s complex transconductance is essential to find the correct corner frequency. The complex transconductance provides the same corner frequency regardless of the value selected for the BSIM parameter XPART, which is not true of a circuit with a real transconductance. The value selected for XPART affects somewhat the higher frequency pole of this amplifier, and affects radically the high frequency zero. As a result, uncertainty in the value of XPART translates directly into uncertainty in the high frequency response, and a corresponding uncertainty in design.