I
have some questions about the SF gain
calculations on page 670 and 691

As I
am going through Ch. 21, I have a
question on the *A _{v}*
calculation for the

SF
Amp on page 691. If I use the
earlier approach in calculating the gain,

shouldn't
it be something like

*v _{out}*
= (1/

*v _{in}*
= (1/

Therefore,
*v _{out}*/

close
to zero). What am I doing wrong?

On
page 670 the load is a gate-drain
connected device with a small-signal

resistance
of approximately 1/*g _{mn}*
so
what you've written for

is
correct for Fig. 21.15. On page 691
the load is infinite (that is why

we've
included the body effect on pages
691-692). So for *v _{in}*
in the NMOS

SF on
page 670, and assuming 1/*g _{mn}*
<<

*v _{in}*
=

knowing
*v _{out}*
=

this
so let me try and answer your
question a different way.

In
Fig. 21.40 note that the MOSFET’s
output resistance isn't included in the

calculation.
The *r _{on}*
ends up having a
small effect compared the body effect. If

you
think of the MOSFET’s output resistance
going from drain to source of the

MOSFET
(drain is at AC ground) then you
shunt the ideal DC current source

with *r _{on}*.

Now,
you can write, neglecting body
effect,

*v _{out}*
=

*v _{in}*
=

*v _{out}*/

which
is approximately 1 and what we
derived for the SF with current source

load,
Fig. (21.39) and Eq. (21.89).

Why
wasn't *r _{on}*
included in the
calculation on page 691? Because with body

effect
we get a gain of, for example,
0.7 while with the MOSFET’s output

resistance
included in the gain
calculation, with no body effect, we get to 0.98.