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RON vs Vds

 

If I want to plot RON vs Vds is nmos in ltspice... Should I do d(Vds)/d(Id) or VDS/ID?... Please help as whether RON is a large signal parameter or small signal? 
 

See, for instance, 

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Dave (Nick) Daniel
KC0WJN
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On Thu, Dec 19, 2024 at 14:44 abanindra.kumarmandal.ece24 via groups.io <abanindra.kumarmandal.ece24=heritageit.edu.in@groups.io> wrote:

If I want to plot RON vs Vds is nmos in ltspice... Should I do d(Vds)/d(Id) or VDS/ID?... Please help as whether RON is a large signal parameter or small signal? 
 

The equations are opposite right? 
 

On 19/12/2024 20:43, abanindra.kumarmandal.ece24@... wrote:
If I want to plot RON vs Vds is nmos in ltspice... Should I do d(Vds)/d(Id) or VDS/ID?... Please help as whether RON is a large signal parameter or small signal? 
You should use d(Vds)/d(Id) because it is a non-linear function.

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Tony
 

But RON is defined for linear region right? 
 

On Thu, Dec 19, 2024 at 09:41 PM, <abanindra.kumarmandal.ece24@...> wrote:
But RON is defined for linear region right? 
Not necessarily.
 
If it is linear, then you know that the small-signal and large-signal definitions would be the same, so there would be no question.
 
Andy
 
 

What will be the expression for RON then? 
 

On Thu, Dec 19, 2024 at 10:03 PM, <abanindra.kumarmandal.ece24@...> wrote:
What will be the expression for RON then? 
Either one.  They would be the same.  (But unlikely.)
 
Andy
 
 
 
 

On Thu, Dec 19, 2024 at 09:41 PM, <abanindra.kumarmandal.ece24@...> wrote:
But RON is defined for linear region right? 
Even in the FET's linear region, it is nonlinear.  It is unlikely that a JFET or MOSFET has an Rds that is actually linear, in the so-called linear region.  It "bends", even there.
 
But if you found a FET whose Rds was strictly linear, then there would be no difference between small-signal and large-signal Rds.
 
Andy
 
 

I suppose that Rds depends on die temperature, too, so large signal Rds is a thicket, not a value.

On 2024-12-20 12:25, Andy I via groups.io wrote:
On Thu, Dec 19, 2024 at 09:41 PM, <abanindra.kumarmandal.ece24@...> wrote:
But RON is defined for linear region right? 
Even in the FET's linear region, it is nonlinear.  It is unlikely that a JFET or MOSFET has an Rds that is actually linear, in the so-called linear region.  It "bends", even there.
 
But if you found a FET whose Rds was strictly linear, then there would be no difference between small-signal and large-signal Rds.
 
Andy
 
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