Question: how to calculate vgs for mosfet?
Answer: In First Small Signal Equivalent Model of MOSFET, I have found out Vgs = Vg= (Vin R1//R2)/(Rg+R1//R2), because R1//R2 parallel with the Vin and Rg. Using Voltage divider, I found Vgs=Vg=(Vin R1//R2)/(Rg+R1//R2).