Are the PMOS loads in Fig. 24.21 called "composite cascodes?"


As mentioned on page 636 a cascode structure is formed by CASCading a common-source

amplifier (common-cathODE from tube days) with a common-gate amplifier (common-grid

amplifier). Hence the name CASCODE.


Since the devices M3T, M4T, and M7T in Fig. 24.21 are operating in the triode region (not

common-source amplifiers) it would be incorrect to call the PMOS devices seen in that figure

cascode structures. A correct name is "split-length" devices since the resulting devices are

really just one MOSFET with an effective channel length equal to the sum of the two

MOSFETs (e.g., M3 has a W/L of 100/2, see problem 6.14 on page 160).


Further, the devices M3B, M4B, and M7B don't have their gates connected to AC ground

so they are not common-gate amplifiers. Again, a split-length device should not be called a

"composite cascode" since it's not a cascade structure.


Note that the structure seen in Fig. 20.35 is also a split length device. It's not a "composite

cascode" device with 4 levels of cascoding!