What is the gain of this n-MOS transistor?
For small-signal AC purposes R2 and R3 are in parallel to ground, so you have 50 ohms to ground in series with 100 ohms (R1) in series with V2.  So your input to your small signal gate model is your source in series with 150 ohms.  From there, you can figure out what Vgs is based on which small-signal model you are using.