Input resistance into a MOSFET amplifier

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Discussion Overview

The discussion revolves around understanding the input resistance of a MOSFET amplifier circuit, specifically why the input resistance is considered to be the parallel combination of bias resistors R1 and R2, and the role of the source resistance Rsi.

Discussion Character

  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions why Rsi does not contribute to the input resistance, referencing a text that states the input resistance is primarily determined by the bias resistors due to the high input resistance of the MOSFET gate.
  • Another participant suggests that Rsi is associated with the signal source.
  • A further contribution reiterates the modeling of the circuit, indicating that the input resistance Ri is defined at a specific point in the circuit, with Rs representing the source resistance to the left of that point.
  • A later reply acknowledges the clarification provided by the previous posts.

Areas of Agreement / Disagreement

Participants appear to agree on the interpretation of the circuit diagram and the role of Rsi as related to the signal source, but the initial question regarding the input resistance remains unresolved.

Contextual Notes

The discussion does not clarify the assumptions regarding the definitions of input resistance or the conditions under which the MOSFET's input resistance is considered infinite.

Who May Find This Useful

Students and practitioners in analog electronics, particularly those studying MOSFET amplifiers and input resistance concepts.

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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Likes   Reactions: 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.
 
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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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