In
Fig. 26.41 it says that this is NOT
the way to implement gain enhancement (GE)
in an
op-amp, however, I've seen papers
that use this topology (are they wrong?).
Would
you comment?
As
indicated in the text (page 889)
describing Fig. 26.41 this is not the way to
implement
GE unless the added GE
amplifiers, with differential inputs/outputs,
employ
common-mode feedback (CMFB) to
balance their outputs.
Note
that no deep thought is needed...when
you have fully-differential outputs you
must
employ a CMFB circuit (don't let
anyone tell you otherwise...yes in SPICE
where
the devices are perfectly matched
things may work fine without CMFB, see
discussion
in the beginning of Ch. 26).
If
the papers you've seen employ CMFB
for both the added GE amplifiers and the main
amplifier
then I don't see a problem.
I
prefer to avoid the use of fully
differential GE amplifiers because they add two
more
CMFB paths (and the associated
problems with stability and settling) and don't
provide
any additional benefits over
the topology using single-ended output GE
amplifiers
seen in Fig. 26.42.
Note
that power isn't an issue since
the added GE amplifiers can be slow and thus
low
power amplifier topologies (see
Sec. 24.4 starting on page 808 for a discussion
of
the requirements of the GE
amplifiers).