gm/Id method

The traditional circuit bias and sizing method using square law, as described in textbooks, is cumbersome and not very accurate in today’s sub-micron technologies. Unfortunately, the prevalent textbooks being rather old, still teach the square law method.

A more modern method using “gm/Id” of the transistor has been known since 1996 (Silviera, et. al. 1996), but hasn’t made its way into many designers’ toolkits. Instead of trying to explain it here, I suggest reading Bernhard E Boser’s (2011) lecture notes.

If you’d like to try this yourself, I have created an example testbench for ngspice using 45nm predictive technology models from ASU and a python script to post-process it.

If you find an error, have an idea for improving it, or just want to discuss this generally, please leave a reply or contact me.

Silveira, F., 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 Journal of Solid-State Circuits 31, no. 9 (September 1996): 1314–19. doi:10.1109/4.535416.
Boser, Bernhard E. “Analog Design Using gm/Id and Ft Metrics,” 2011.

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