Input resistance into a MOSFET amplifier

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 Rsi 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
 

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

CWatters

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

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