a problem about the temperature dependence of the voltage

[huashizng]

I use a buffer opamp to increase the 1.2v bandgap voltage to 4.1v. in the divided resistors, i add a trimming resistors network. when the switch NMOS transistors's bulk are connected to the ground, the temperature dependence of the 4.1v voltage is worse. when these switch NMOS transistors' bulk are connected to their source respectively, the temperature dependence of the 4.1v voltage becomes well. how to explain this phenomenon?

 

[sunking]

does all the switch nmos Ron is small enough?

 

[huashizng]

Re: a problem about the temperature dependence of the voltag

sorry, you can go to another same topic,,, the circuit schematic is attached. this topic is a copy by a mistake. the Ron of switch is about 60ohm