Coupling capacitance w.r.t voltage transition

[ksnmurthy]

Hi there,
Can someone pls explain the change in capacitance of a coupling capacitor (between two nets) with respect to changes in the voltage in both the nets?
Say, if voltages on both the nets have transition in i) same direction and ii) opposite direction, in both the cases, how does the capacitance changes in the coupling capacitor ?

Thanks
Murthy

 

[BradtheRad]

I guess you mean how does the capacitor charge change?

Whatever voltage differential is applied to the capacitor, that is really what matters.

Applying 10V at each end simultaneously has no effect on the charge.

Applying 10V at one end and -10V at the other, is the same as applying 20V at one end and 0 at the other.

The capacitor charge is added to whatever voltage differential is applied.
How fast the charge changes, depends on the net resistance seen by the capacitor. This is where the RC time constant comes in.

There is one behavior that may not be readily obvious... namely when an AC square wave is applied. Say it is +/- 10V.
If the frequency is not too high, the capacitor charges to +10V during one half of the cycle. Then there is a transition to -10V. The capacitor creates a 20V spike. It does this at each transition.

I have a Youtube video about capacitor behavior.

It is an animated simulation which shows the charge growing and shrinking in the capacitor, while current flows through it. It shows how it responds to various waveforms.

www.youtube.com/watch?v=eIWEU4pObJw