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Simulators >> Circuit Simulators >> Question about the plot for gm/Id design methodology
https://designers-guide.org/forum/YaBB.pl?num=1240299092

Message started by Yutao Liu on Apr 21^st, 2009, 12:31am

Title: Question about the plot for gm/Id design methodology
Post by Yutao Liu on Apr 21^st, 2009, 12:31am

Hi everyone,
I am confused about why the Vgs is set to be Vth+200mV when plotting gm*ro versus Vds.

first, why the Vgs should be Vth+200mV while ploting gm*ro versus Vds?

second, while sweeping Vds, the actual threshold voltage would change as well, how should I define the value of "Vth" in the "Vth+200mV"? And how should I implement this simulation in Spectre?

Title: Re: Question about the plot for gm/Id design methodology
Post by analogue_guy on Apr 22^nd, 2009, 3:10pm

Hi,

I guess you want to bias your transistor in strong inversion, that is why you use Vth+200 mV. The standard textbook definition of strong inversion: VGS> Vthreshol + 6* Thermal voltage (kT/q).
If you want some safety margin, you will end up at Vthreshold+200 mV.

regards