What is the cause of 1/gm smaller than rds?

[davison7]

hello everybody,

I read the analog integrated circuit text book, in that the author often assume the "1/gm is smaller than "rds" for the mos transistor. So I have a question what
is the cause of 1/gm smaller than rds?

 

[CrabMan]

This is because gm=change in current/change in Voltage

Gm is not inverse of on state resistance in case of transistors or switching devices.
It is the ratio of change in voltage which you apply at gate to the ratio of change in current that you see from drain to source which is very small as compared to Rds