I am not sure I understood your question, but it seems you are asking how to generate differential inputs to simulate your amplifier. You can do that in Spectre/SPICE. Two sinusoidal power sources with a phase shift could generate Vin+ and Vin-. There are other ways too.

Like Samuel said, there are different way to make it. I don't have access to cadence now, so the schematic come from LTSpice.

I suggest you to use a Voltage generator for the common mode voltage, a voltage generator for a half of differential voltage and a voltage controlled voltage generator for the other half (be careful with the sign of the controlling voltage).

Remember also to put on V1 only an half of the desired amplitude of the differential voltage (for AC you can put 0.5 V in V1, so, as usual, from the output voltage you get directly the transfer function).

best

S.

As some other users said, it depends on your application but also km what you want to simulate. Should you need to asses the OTA‘s frequency response, you have to considere a ac voltage source. To do so add a voltage source from the analog library and assign it a ac magnitud aparte from a bias voltage. In the case you need to evaluate the time domain response, you have to add the same voltage source but define it as a time dependent voltage source (sine, pulse,etc) and also specify a dc bias for the same item. Hope this to be helpful