The concept is that we have to replace the drain resistors by MOSFETs so that we can control the gain by varying the small signal trans-conductance gm since Av=gm*Rd. But my questions is how to vary the value of gm. I tried to give the inputs a differential mode sine source but the simulation showed an error. And when I also changed the input to a DC source, still the same error "unable to find nomd.lib". I have attached the schematic and simulation result as pdf.