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).