Transistor criteria to heck of in saturation and subthreshold

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Hi all. I would like to know if someone could enlight me how to check or how to assess/what criteria do you use and what to target for when sizing the MOSFET, via the op point information (vgs, vds, vdsat, vth, etc)?

For example:
Vgs>Vth
Vds>Vdsat (but for example by how much?

Anything else? There is Bob but not sure what to compare it against with and which value should we target.
Thanks in advance.
 

Well, in general, it is a simple two stage amplifier pmos input.
 

If you're messing with device size with an eye to
"being in saturation", that's all about Rout as an
element of stage DC gain. You'd see this in the OP
as gds (1/Rout) I expect.

You're not targeting a specific value of overdrive,
that's all "rule of thumb" type stuff which you will
need to go beyond in optimizing (there are likely
other care-abouts like bandwidth, stability, power,
area, matching, noise which must have their say).
 

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