On page 525, Eq. (18.3), you assume M1 is in saturation when the Schmitt trigger switches. However,

for M1 to be in saturation


Vx >= VinVTHN1 and thus Vx >= Vx + VTHN2VTHN1


Since VTHN2 increases due to the body effect we can write VTHN2 > VTHN1 so for M1 to be in saturation

0 >= to a positive number (so M1 is NOT operating in the saturation region).


The connection of M1 and M2 is a split-length device; see Problem 6.14 on page 160. So, M1 operates

in either the cutoff or triode regions. Your conclusion appears to be correct. However, as discussed

in section 6.3.2 starting on page 143, on the border between the saturation and triode regions (when VDS

= VDS,sat = VGSVTHN) the triode/saturation equations for the MOSFET’s drain current are equal. So,

what we need to show is that M1 is operating with Vx = VinVTHN1 (VDS,sat for M1) for it to be okay

to use the saturation equation as seen in Eq. (18.3). As pointed out above and in the book this is indeed

the case since VTHN1 = VTHN2 as M1/M2 turn on (M1/M2 behave as a single device, again, see problem