skippy
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If you don't know the capacitances, how did you figure out Cdb? Cgd and Cds would have a significant effect as well. Don't forget the Miller effect as well.
By having the small-signal equations you can then find the model in which to fit your data. Using some form of optimization, you can then get the capacitance values. The conductances in your model should come right out from the DC i-v characteristic or a low frequency AC analysis. The remaining unknowns should be your capacitances. Don't forget to include your bias circuit in the calculations. By changing these bias resistors, you can even choose which capacitance would be dominant in your measurement.
This is much like what your trying to do with your v=i/(sC) equation, but you would be accounting for more small-signal elements.
If you use a digital spectrum analyzer with a random noise source, you would see the frequency response all at once.
You could also switch the device on and off and look at the resultant output waveform in the time domain. Looking at the time delay, you may then figure out the switching Resistance and the output capacitance.
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