Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
haoyun said:Hi yschuang,
I think , gm/Id=2/(Vgs-Vt) is valid for all inversion.
But by simulation I don't understand why it's no valid.
Thanks your reply.
deeply understanding. Thanks.sutapanaki said:It would have been really nice if gm/Id was equal to 2/(Vgs-Vt) everywhere. This way you make very small overdrive voltage and you get huge efficiency of the transistor - analog designer's dream. Seriously, however, you should probably know that when transistor is in saturation it can be either in strong inversion or weak inversion or inbetween those two. That formula is based on the square law of operation of the MOS transistor and valid in strong inversion only. For modern technologies even in strong inversion and long channel devices there is probably 15-30% discrepancy too, because of velocity saturation. It is definitely not true in moderate inversion and weak inversion. In weak inversion MOS transistor works like a bad bipolar transistor and hence current is exponentially changing with Vgs. For bipolar transistors gm/Id=q/kT and for MOS in weak inversion it is q/nkT where n=1 to 2.
And in moderate inversion it should be somewhere inbetween those asymptotes.
Hope this helps.
DenisMark said:All guys are right exept you, This formula is only for strong inversion.
If you'd like be more advanced in analog design take a look for Gm/Id methodology. There're many guys on forum who are interested in.