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observe cgd, cgs etc as functions of time

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kparrish

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I'm designing a circuit that has a Vgs voltage swing that is large enough that i suspect cgd and cgs are changing with time. any suggestions on how I could observe what the values of these are? right now i can only look at the operating points which tells me the values at the bias but not while the voltage is swinging.
 

leo_o2

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Cgd, Cgs might change as the bias voltage.
 

kparrish

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Cgd, Cgs might change as the bias voltage.

I agree that the bias voltage would change Cgd and Cgs. my question is, with the DC bias voltage constant but the AC signal changing significantly (ie not really a 'small signal', therefore vgs is swinging), how can i observe the change in Cgd and Cgs when i'm doing a transient solution?
 

pancho_hideboo

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I'm using cadence 2009, spectre simulations, trying to perform a transient simulation that shows several cycles of the signal.
There is no tool whose name is cadence 2009.
Use correct terminologies.

If you use Cadence Spectre, there are two options available in transient analysis for your purpose.

infoname : Name of info analysis to be performed at each time point in the infotimes array.
infotimes=[...] : Times when the analysis specified by infoname is performed.
With these options, you can evaluate Transient Operating Points of devices by "ADE>Result>Print>Transient Operating Points".

If you use following options with tran analysis, you can do ac analyses at specific time points.
acnames=[...] : Names of ac, noise, sp, or xf analyses to be performed at each time point in the actimes array.
actimes=[...] : Times when analyses specified in acname array are performed.
So you can evaluate capacitance of devices by ac analysis based on specific time points.
For example, Cgs=imag(Ig_AC)/(2*pi*freq).
The Designer's Guide Community Forum - Cgs of MOS in Spectre
 
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kparrish

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Thanks, this answers my question.
 

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