In analog lib ,you will see "Vsin" ,put it in series with the input voltage source V3 at vmais;
Vmenos=1.2V DC only,remove the feedback loop.
For Sinusoidal source ,take Amplitude=Magnitude=10mV,Freq=according to your application(e.g. 1M).
Now do AC analysis,sweep frequency from 1 to 1GHz in log scale with 100 points per decade for good resolution of graph.Go to Output--> to be plotted: select on schematic vmais ,vmenos and vout on the schematic window.(Select the nets not the nods).Netlist and run.
Now you have AC analysis window.Go to calculator,Select wave,First select vmais,then select vmanos,then press minus(-) sign,press eval.
Now select vout and Vmais-vmanos on the graph,and press divide(/) sign on calculator,Click eval.see the gain at freq=1Hz.this is DC gain.
Now select the vout/vmais-vmanos and press db20 and eval.you will get the db curve.
Now select the vout/vmais-vmanos again and press phase and eval.
simiarly u can find phase margin,gain margin,gain bandwidth product,3 db freq.........and so on
Keep it in mind that Calculator in cadence takes the expression like this:
Operand1,operand2,operator