Input resistance into a MOSFET amplifier

[Abdulwahab Hajar]

Hello everyone
I was hoping you could help me with something, in my analog electronics course..

In the diagram below, is displayed a circuit and its equivalent...
I don't understand why the input resistance is only the parallel combination of R₁ and R₂??
Why doesn't R_si contribute to the input resistance?

The text explains it as follows "Since the low-frequency input resistance looking into the gate of the MOSFET is essentially infinite, the input resistance is only a function of the bias resistors." but I don't get it. Maybe I wrongly understood what the input resistance is?

any explanation would be greatly appreciated

 

[willem2]

I think R_si is meant to belong to the signal source.

 

[CWatters]

+1 On the lower diagram they show the point at which Ri is defined and Rs is to the left. They have modeled a real source using an ideal voltage Vi and resistance Rs.

 

[Abdulwahab Hajar]

+1 On the lower diagram they show the point at which Ri is defined and Rs is to the left. They have modeled a real source using an ideal voltage Vi and resistance Rs.

I see, thank you