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This question is related to MOSFET.

NMOSFET's resistance was till now defined in many different ways, for example as: enter image description here

or enter image description here which value varies from 1-50k Ohm.

And there is also drain-source on-state resistance enter image description here which is usually lesser than 1 Ohm.

Which one can I actually use in an equation like this one which applies for common source configuration?

enter image description here

Keno
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    **You're mixing up two different things.** r0 is the **small signal** output impedance. It applies to the NMOS being biased at a certain Id (current) and a certain Vds across it. It is the **derivative** of the Vds/Ids curve, you cannot measure r0 with a multimeter. Rds(on) is a **large signal** parameter, it is the resistance **you can measure with a multimeter** between drain and source when Vgs has a certain (large) value. – Bimpelrekkie Mar 25 '17 at 22:14
  • @FakeMoustache: When I'm dealing with small signal transistor circuit, I apply (for common source) r0 value to upper equation. But when I'm dealing with large signal transistor circuit, I can apply Rds(on) value to upper equation? – Keno Mar 26 '17 at 09:58

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@Keno Let's put it this way. When you operate your MOSFET in the saturation regime, as an amplifier, you use r0 in the small-signal analysis of the circuit. If you operate MOSFET as a switch (as in digital circuitry), and the switch is turned ON, you can use Ron, as long as the transistor is in the triode region. For detailes on MOS regimes of operation, i suggest looking up one of great books from Baker, Sedra & Smith, Gray & Meyer etc., because you can't really cut corners with these stuff if you want to build proper circuits.

Mladen
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