I had
a quick question on deriving
voltage gain of a common-source amp with source degeneration.

Please
see attached PDF
file. I
derived the voltage gain using the regular method (method 1) and by

inspection
(method2). I get different
answers from the 2 methods, although upon plugging in the

numbers
the answers are numerically
about the same (same up to the second decimal). I am unable

to
algebraically reduce the symbolic
results to show they are equal.

Method
1 in your PDF is exact
but it doesn't lend itself to intuitive understanding. Method
2 is easy

to
understand intuitively but it isn't
exact. Yes, as you noted the results are about the same.

Why
the difference? When you look at
the resistance looking into the source of the MOSFET it isn't

exactly
1/*g _{m}* as
specified in Method 2 but rather

(1 + *R _{o}*/

If *r _{o}*
>>

In
later printings of the book I
changed the more advanced discussion of the resistance looking into the

source
with a very large load *R _{o}*,
see pages 688-690 here,
to keep things more
general. Specifically see Eq.

(21.81)
and Fig. 21.37.

For
the question at the bottom of the PDF,
superposition allows you to look at sources separately so it is

fine
to short-out the input source when
determining the resistance looking into the drain. For exact

analysis
make sure you use the same
circuit for all analyses. Don't lump the resistance looking into the

drain
into a single resistor connected
to the output, like we do for the fast/intuitive analysis, and then try

to
calculate the exact gain.