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[SOLVED] Finfet AC Analysis unexpected results

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Bakr.hesham

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Hi,
By making simulations on a BSIM-IGFET nfet model the DC analysis gives logical results but the AC analysis don't ,it gives the following results:
anyone can give reason.
thanks in advance.
CentOS-2017-06-12-15-09-03.png
 

I see nothing to say that the transistor is biased
at a point where there's any gain to be expected.
That all comes down to your choice of vin value.

Start with a DC sweep and then set vin to a value
near where maximum DC gain is found, for your AC
analysis.
 

Actually i did that , but what i couldn't understand is why the curve increase at high frequencies it should decays.CentOS-2017-06-13-11-00-48.png
 

Bakr.hesham said:
... what i couldn't understand is why the curve increase at high frequencies it should decay
Click to expand...

I think it's by the capacitive reach-through via the gate-drain capacitance, which constitutes a voltage divider together with the load resistance R0.
 

There is something wrong with the variables or the model.

Your DC sweep indicates that /out increases with vin.
above threshold (#5 attachment). It should be inverse.
You show /out greater than /vdd.

Your AC sweep indicates voltage gain of 8.4mV/100mV.
That's a lousy gain. But similar to the DC transfer gain
of ~35mV/500mV.

Your "nfet" model is not acting at all like a NMOSFET
should. Maybe try substituting any of a zillion known
good NMOSFET models for the "guts" and see whether
the circuit responds as it ought to.
 

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