I need more information.
Part numbers.
What is Vbg? How is this voltage made?
Power supply pins on op-amp?
Resistor values?
Where are the capacitors?
Load current?
So the op-amp is powered from Vdd, the same as on the source of the MOSFET.
What op-amp are you using? I need to know input range. If you give a part number I can find the numbers I am looking for.
What MOSFET are you using? I need to know gate turn on voltage. N or P?
Please plot Vdd and Vout, Gate of MOSFET. I want to see the output of the opamp.