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How to measure the input capacitance and output capacitance of a MOSFET

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AMSA84

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Hi everyone,

Can someone explain to me what's the best way to simulate and get the input capacitances of aa mosfet? More, if I want to know the Cgs and Cgd how I can I do that?

All this using cadence.

Regards.
 

I see. But for example I could do an AC analysis for several operating point correct? But this way I would only know the Ciss = Cgs + Cgd.

Or I could plot the DCop but in this case the data would be only regarding the DC operating point without the frequency component.

I could do a transient analysis but in this case is not useful? Because I would only see the capacitance changing with time.

I can imagine that is possible again doing, for a certain operating point, a SP analysis and using the calculator and some manipulation extract each capacitances?
 

The capacitance values are defined by model parameters and can be calculated. Realistic MOSFET models are usually macro models with dedicated non-linear capacitors, it depends on the model complexity if the capacitance values can be easily extracted by hand calculation.

Measuring Cgs and Cds directly is simple with a respective test circuit.
 

Hi FvM. Thanks for your reply.

Can tou give me a tip on how to setup the test circuit then?
Regards.
 

Apply the same AC voltage to gate and drain, measure gate and drain current separately.
 

Yes, agree. But the thing is if I do that way, applying to the gate of the transistor I would get the Ciss = Cgs + Cgd. The same goes for the drain, I would get something like Coss = Cds + Cdg.

How can I get the respective capacitances separatly?

By the way, supposing that his is the correct of doing it, applying the AC to G and D respectively, can I do it for several bias points? And then if there is not a big discontinuity or a big difference in the pattern of the results can I do a C vs V curve and then interpolate?

Regards
 

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