The variation of bandgap voltage refernce circuit.

[leonken]

I design a conventional bandgap voltage reference. The output is around 1.2 V.
I already did corner analysis, such as TT, FF, SS, FS, SF.
I used mosfets operate in weak inversion to replace diodes.
The output reference is:
Vref=Vgs1 + nVtln(N)
when I do corner analysis, I found that the variation of Vth is 200 mv from F to S mode. However, the Vref vary only 0.1 mV.
Why the variation of Vref is so small compared with the Vth variation?
Thanks!!!

 

[paulux]

I guess, your circuit may counterbalance some variation. It would only be possible to analyze if you could provide the details of the circuit.