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Negative Capacitances by Cadence

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aryajur

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When I see the value of Cgs from the Result Browser in Cadence why do I see a negative value??
 

Caps can be negative in spice matrix solving, just like any other spice circuit matrix data, that is voltages, currents, resistances, inductances, etc.
Just as a simplified illustration, voltages in Kirchoff equations can be negative or positive, right? Their derivatives can be positive or negative too, right? Mathematically speaking, caps are just coefficients, connecting currents with voltage derivatives, right? Now you see that caps can be positive and negative, at least in spice circuit matrix. Nothing more than that.
 
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aryajur said:
So that means the value is the absolute value...?
Click to expand...

Well, treat them as the circuit matrix components which incidenatally have some resemblance to capacitors. Note that sometimes they are not symmetrical. For example, Cdg is not necessarily equal to Cgd, even neglecting the minus sign.
 
I think that is because of the transcapacitance modeling in SPICE. The definition for Cgs is
Cgs=-dQg/dVs. The "-" here is just the definition of the direction of charge flow.
 

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