1) In
Ch. 20, on page 629 in Fig.
20.22, I am unclear of the reason Vreg
is
connected
to the gate of M2. Without
that connection it would still work.
2)
There is also a second feedback due
to the cross-coupled MOSFET. What
is
the function? How would that affect
the DC operating point and stability?
Vreg
is connected to the gate of M2 to make Vreg
a low impedance node. In Fig.
20.19
Vreg is a
high impedance node (meaning only drain
connections). By diode
connecting
M2 (connecting the gate and
drain of M2 together so that its small
signal
resistance is 1/gm)
we only have one
high-impedance node (in Fig. 20.22),
that
is, Vbiasp.
The reason for
doing this is to make the feedback in the circuit
easier
to make stable. The circuit in
Fig. 20.19 is a two stage op-amp, a diff-amp
and a
common-source amplifier made with
M4. These topics are discussed in detail
in
Ch. 24.
The
reason I put that circuit in Ch.
20 instead of waiting until after we discuss
those
topics in Ch. 24 is so that we
have a practical biasing circuits for
designing
amplifiers and current
mirrors in Chs. 21-23.
Practical biasing allows
us to
use real circuits for analysis
and design.
The
second cross-coupled MOSFET is used
so that we have something to compare to
Vreg.
We want the drains of M1 and M2 to be the same potential to minimize
the
effects
of VDD variations.