OPAMP common mode half circuit with its gain
This is a degenerated amplifier, where the transistors M14 and M15 act as the degeneration resistors.

Are you sure that M14 and M15 are supposed to be PMOS?  The gate of M14, which is connected to V1 (or the gate of M4), is higher than its source/drain --> it is off.  This configuration usually has M14 and M15 in its triode region (using NMOS).