can someone please tell me how did they derive with the above equation? my point of view: Vgs = Vin Vds = Vout so how did they even get Vgs = Vds? below shows the Small signal circuit: might be a stupid question to ask. but i am really confuse on when to look at the original circuit, when to look at the AC circuit, when do i remove the load, etc. Thanks for helping and sorry for asking such a simple question here.