Help comparing two fully differential circuits with CMFB

[PEEMONGER]

I am trying to figure out problem 26.4 from Baker's book, CMOS: Circuit Design, Layout, and Simulation. I am taking an introductory CMOS design course and I could use some help with this question. I am trying to compare these two circuits:

My initial guesses had to do with the difference in influence of CMFB between the two circuits and how that impacts stability. That or the difference in the relationship between Vcmfb and the commond-mode output voltage.

In transient simulation, both circuits function similarly and when I put in a very small step response, they both seem to stop functioning in the same way (the output common mode voltage increases drastically). The only confident distinction I can make is that increasing Vcmfb causes output common-mode to decrease in top circuit and increase in bottom circuit.

Link to chapter:

https://bjpcjp.github.io/pdfs/cmos_layout_sim/ch26-op-amps-p2.pdf

If you have any insight into what I should expect, I would appreciate it. I do not know if my designs are at fault or if the topologies are flawed, or both. Thank you!

 

[dick_freebird]

The one with CMFB sinks and diff tails all slabbed together
means CMFB messes with everything. Maybe some things do
not need to be messed with and are better off controlled for
other care-abouts (or don't-want-to-care-abouts).

 

[D.A.(Tony)Stewart]

The lower example has independent output Nch bias voltage control with the mirrored currents on the differential inputs while Vcmfb allows independent current mirror control on the high side Pch for feedback error correction would appear to be more stable with greater tolerance on Vt and Beta differences.