about generation current from bandgap circuit

[kavitha_rapolu]

Hi ,

I would like to learn and use the circuit in the attached fig for generating ix =200uA from vx=1.25 ( ie bandgap voltage) . Are there any material/books/paper you can refer to about this circuit.

What is the purpose of NMOS ? and in which region should it be?
what happens if it is in saturation?
What happens if it in subthreshold region?
Why is its size usually large? (if i am correct)

 

[dick_freebird]

NMOS is the current-error-free cascode. Closing the loop
on Rx voltage enforces a current that has nowhere to go
but out the NMOS drain (diode leakages aside, which on
a crazy big FET at high temp is not to be neglected).

Operating saturated. your op amp has a lot less work to
do in rejecting power supply noise (a better source
follower). If you have the headroom then subthreshold
probably doesn't buy you anything (and to operate in
subthreshold at such a current means an even bigger,
leakier-at-high-temp FET).