First I did this circuit and the response of Vref(°C) was close to the one in the paper :
Response with first circuit :
But, when I am trying to do the next step (ie : having an Opamp replacing the 2 NMOS at the center), I have a very bad response while it should normally be better than the precedent one.
The response :
For the Opamp, I am using a 2 stages OTA with open-loop gain of 85 dB. Do you have any clue of what could be the problem please ?