Hi,

   Nulling resistor is a common way to compensate the RHP zero in the two-stage miller compensation opamp. How to handel the PVT variation of the nulling resistor in a robust design? Except for trimming the nulling resistor, are there any advanced solution to do it? From my own opinion, I don't like to use the gm of a transistor as the nulling resistor (maybe I am wrong). Any comments? Thank you.

If we consider a source follower as the voltage buffer, I think it has the input voltage range, which may limit the output swing if the swing is too large.

Hi DanielLam,

   Thank you for your reply. I know some designers like to use the gds to generate the nulling resistor, but the transistor is normally biased at triode region. However the gm of the second stage input transistor is in saturation region. I would guess it is difficult to get a maching gm when two transistors are biased in different modes, especially with PVT. In theory I think it would work, but in practice it might not be easy. If just using a nulling resistor, I would prefer the poly resistor, just wonder if there is any advanced solution to tune the value of the resistor covering the PVT for both resistor and the gm of the input transistor of the second stage.
   Of course we can easily block the feed throught path to eliminate the zero by a CG stage. Just thinking how to compensate the PVT if a poly resistor is used as the nulling resistor.

First, I'll say I've monte-carlo'ed and sweep power + temperature on a 2 stage opamp with poly SI, and triode FETs as the nulling FET. If memory serves, the nulling FET was always better in schematic (you can mess up in layout). Matching a gm to a R should be worse than matching a gm to a gm.

Second, the remaining non-idealities are short channel effects.

Third, I think this statement is wrong

" But normally the Vgs of the input transistor in the second stage is not a copy voltage from the current mirror, and it is difficult to generate a same Vgs for the triode-region transistor,"

There are well-published ways to have those two transistors have the same Vgs. Razavi or Johns and Martin have a circuit you can directly use to do this.

I'm not replying anymore until you take a look at the textbook.