Hello,

Given a MOS device at saturation, how to estimate the small signal output resistance rout?  For example, if Id doubles, rout will decrease anti-proportionally (almost) according to approximation rout=Va/Id, with Va is related to geometry parameters and Vds. However, if taking a MOS device model for a  simulation, the Gds hardly changes if Id changes. So it seems rout is not simply the relation was just outlined. Note that the simulation ensures the device is under saturation.

Thanks.

Hi Geoffrey,

For circuit design purpose, I guess Rout is then assumed to be constant with respect to Id. In achieving enough voltage gain, we sometimes need Rout large. But it seems Rout is not well approximated by the pointed relation. Well, longer device can increase Rout.

Thanks for the information,
Steven