Is rds neglectiable?

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harry456

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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.

 

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 \[r_{ds}=( \lambda ~I_D ) ^{-1}\]
 

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

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.
 

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

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