Input resistance into a MOSFET amplifier

AI Thread Summary
The discussion centers on understanding the input resistance of a MOSFET amplifier, specifically why it is determined solely by the parallel combination of bias resistors R1 and R2. It is clarified that the input resistance looking into the gate of the MOSFET is considered infinite at low frequencies, meaning Rsi does not affect the input resistance. The confusion arises from the role of Rsi, which is associated with the signal source rather than the amplifier's input. The explanation emphasizes that the input resistance is defined at the gate, independent of the source resistance Rs. Overall, the key takeaway is that in this configuration, only the bias resistors influence the input resistance of the MOSFET amplifier.
Abdulwahab Hajar
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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.
 
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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 modeled a real source using an ideal voltage Vi and resistance Rs.
 
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CWatters said:
+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
 

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