Equivalent resistance from source to drain in such configuration

[simbaliya]

The gate resistor value is R, can somebody help me understand how to calculate the equivalent Rds?

 

[Kuldeepluvani]

Rds=Vds/Id.

This is the formula we can calculate Rds.

 

[simbaliya]

Rds=Vds/Id.

This is the formula we can calculate Rds.

I tried to use small signal model and introduce a measuring voltage between source and drain, but I can not get a solvable equation of Vds/Id or V_measure/I_measure.

 

[FvM]

You get a non-linear relation Vds=f(Ids), you need to specify either Vds or Ids to calculate a small signal rds = dVds/dIds or large signal Rds = Vds/Ids. In any case, they don't depend on gate resistor R.

 

[Kuldeepluvani]

Rds=Vgs(off)/[2Idss(Vgs-Vgs(off))]

Think this could be helpful to you.

 

[simbaliya]

You get a non-linear relation Vds=f(Ids), you need to specify either Vds or Ids to calculate a small signal rds = dVds/dIds or large signal Rds = Vds/Ids. In any case, they don't depend on gate resistor R.

Thank you for the reply, but could you please explain more on why gate resistor will not affect Rds?

 

[FvM]

No current through gate = no voltage drop at R. The circuit simply makes Vgs = Vds.

 

[simbaliya]

No current through gate = no voltage drop at R. The circuit simply makes Vgs = Vds.

Thanks a million! Now I am clear about this question.