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spectre captab: 1) negative capacitance 2) not reciprocal values

ZoltanT

HI everyone,

I wanted to get some node-to-node capacitance values of a transistor with the help of spectre's captab option. Unfortunately I faced two strange issues:

1) In some cases I got negative capacitance values for the drain-source capacitance of a transistor.

2) C_AB differ from C_BA where B and A are two arbitrary nodes. In some cases the error is more than 10x.

The results are unsettling to me. Either I set up something wrong, or the transistor models behave very strange.The formula for calculating the capacitances is written in the Spectre's User Guide. What I do not know, how is it determined? Do Spectre run AC analysis on the circuit to determine the capacitances? How error-proof the process is? Is it easy to miss something?

Thanks in advance!
Zoltan

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  • ZoltanT

    I really try to understand this. One of my problems with it is that how can we speak about charge change if we regard only the capacitance (one side has to be driven with a voltage source and the other grounded at least), or if we take the whole transistor into account, than we see the transistors effect in the charge change, e.g. if the gate voltage is increased, than charges will be drawn from the drain and its capacitances, which is much more than the cap itself. 

    What I am looking for, and I believe, but can not be sure, that other designers are also curious about the capacitances what one would add to the DC model to get the AC behaviour. I often derive an analytical model of the circuit I am designing for optimization, and for the verification of such models I need the caps from circuit design textbooks. Do you have any workaround how to get reliable capacitance values for that?

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  • ZoltanT

    I really try to understand this. One of my problems with it is that how can we speak about charge change if we regard only the capacitance (one side has to be driven with a voltage source and the other grounded at least), or if we take the whole transistor into account, than we see the transistors effect in the charge change, e.g. if the gate voltage is increased, than charges will be drawn from the drain and its capacitances, which is much more than the cap itself. 

    What I am looking for, and I believe, but can not be sure, that other designers are also curious about the capacitances what one would add to the DC model to get the AC behaviour. I often derive an analytical model of the circuit I am designing for optimization, and for the verification of such models I need the caps from circuit design textbooks. Do you have any workaround how to get reliable capacitance values for that?

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