How to get the Ro of nmos in spectre or in Hspice

[wangkes9]

I want to get the Ro of nmos when it is in saturation in a common source circuit to calculate the gm*Ro.

In cadence spectre DC sim ,I open the calculator -> hit the 'op' option-> select the nmos -> and select the Ron in the list. But the result seems not good. The value of Ron will increase with Vds. But I think in high Vds voltage, the Ron will decrease.

the Ron in spectre is really the output resistance of mos in saturation , or just only in linear region. If not ,how to sim it.

Thanks.

 

[Fom]

How to get the Ro of nmos in spectre sim

I am not sure but I think Ron is for linear region.
If you want get Ro use calculator "deriv" function.
Select IS of NMOS drain terminal. Select VS of NMOS drain node and click on "deriv" function and "plot" or "erplot".

 

[SergeyBondarev]

Re: How to get the Ro of nmos in spectre sim

Connect the voltage source Vds to the drain of the transistor and change it in DC or TRAN. Gate voltage is fixed. Calculate the current Ids through the voltage source. The output resistance will be equal to Vds/Ids.

 

[solidstate]

How to get the Ro of nmos in spectre sim

Instead of taking the parameter 'ron' from the model, try 'gds', the drain-source conductance.

 

[rania_hassan]

Re: How to get the Ro of nmos in spectre sim

you can take gds from the dc operating point

 

[drabos]

How to get the Ro of nmos in spectre sim

usually you can find a value for gm/gds also in the DC operating value table, but the names vary a little bit with transistor models, that is why I do not say an exact name

 

[wangkes9]

Re: How to get the Ro of nmos in spectre sim

Thanks a lot.

Then anyone know if it is easy to get it in Hspice ?