Is rds neglectiable?

[harry456]

How large is rds in the following circuit? How can I estimate the size od rds? The figure on the left side shows the small signal equivalent.

 

[Dominik Przyborowski]

Drain-source resistance value depends to many factors (channel length, technology, etc.). In first approximation You have to know the channel modulation factor and calculate rds as \[r_{ds}=( \lambda ~I_D ) ^{-1}\]

 

[LvW]

You only can think about neglecting the value of one specific resistance in comparison to another resistance which is in series or in parallel.

 

[dick_freebird]

Rds can be megohms to milliohms depending on the bias
conditions and device construction. Conduction losses on
the one end, leakage on the other, both matter to some.
gm*rout is gain and rout is the parallel value of internal
rDS and external load, and so on.

If you had a performance data table you could probably
find several attributes which directly depend on the
rDS under some test condition.

 

[harry456]

The NMOS in the figure is dioded-connected, isn't it? Doesn't this mean that its resistence is approx. rds=1/gm?