Hi,
are you sure that it is an op-amp? I'm not an expert on op-amps, but it appears to be some kind of fixed gain cell.
The inner core of the amplifier seems to be a super source follower if you take the transfer function from the inputs to the voltage across M13. i.e, the voltage across M13 follows the input very closely. If M13 is considered a linear resistor, then the Gm of the inner core is quite linear and depends on M13.
The outer part of the amplifier seems to simply be copying the action of M9, M10 and M13. So the gain of the cell appears to be W7/W9.
I'm not an expert on op-amps so I might be way off
![Cheesy Cheesy](https://designers-guide.org/forum/Templates/Forum/default/cheesy.gif)
. But I suggest you try simulating the circuit to see. I'm not sure, but it may need some form of CMFB.
cheers,
Aaron