Hi Orient,
The equation gm = 2Ids/(Vgs-Vth) is derived from a first order model of MOSFET which operates in strong inversion region. The real life is not so simple. Most SPICE models adopted by Foundry or IDMs are BSIM3, BSIM4.5, MOS9, MOS11, and so on. Those models usually has around 100 parameters. The equations are very complicated.
I understand that you need bigger gm with specified Ids in a low power design. Please refer to the following paper.
Quote:F. Silveira, D. Flandre, and P. G. A. Jespers, “A gm/ID based methodology for the design of CMOS analog circuits and its application to the synthesis of a Silicon-on-Insulator micropower OTA,” IEEE J. Solid-State Circuits, vol. 31, no. 9, pp. 1314–1319, Sep. 1996.
You can google Gm over Id. There are some good literatures about the detailed simulation setup and so on. One of those papers is attached here.
Best Regards,
Yawei