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.