And this is my testbench.

Vence,

 Attached is an example of how to setup your testbench. This
testbench replicates how the circuit will be used, i.e., it includes
feedback, etc. The tricky point is that you need perform the
dc operating first with the circuit setup for "sampling": sample
the short input to reference level and short the common mode
input to the common mode reference level, and then run the
stability analysis in "gain": apply an input signal and connect
the common mode level to the feedback. This is pretty straight
forward using ADE-XL [IC61].

                                                           Best Regards,

                                                              Sheldon