the nature of capacitance

fez

Joined Dec 6, 2009
47
I am having a lot of trouble understanding the idea, usage, and implications of capacitance. I know what a capacitor is and what it does, but the whole realm of transistor/parasitic capacitances, and interconnect capacitances is so mysterious to me. I don't really know those capacitances are there (and why they aren't somewhere else). I don't know which capacitance exists but has been ignored (deemed insignificant), and I don't know whether a capacitance I expect to exist actually exists at all.

A very rudimentary approach I take is to use the idea of opposite charges attracting each other. So whereever you have current, there is charge, and that charge is attracting other charge (be it on a wire, gnd, or semiconductor device).. so basically you have a capacitance to each and every other electrical object.

Could somebody please point me to a book or resource where the whole nature of capacitance is dealt in detail?

steveb

Joined Jul 3, 2008
2,431
Could somebody please point me to a book or resource where the whole nature of capacitance is dealt in detail?
To truly understand capacitance, you need to go beyond circuits and consider electromagnetic field theory. In particular, electrostatics is the subtopic. Any good book on field theory will deal with basic capacitance, but the following book is one of the few that deals with the general theory of capacitance. The general theory of capacitance includes mutual capacitance and more general electrostatic effects. You need a strong math background to understand the full description.

Principles of Electromagnetics, by Schwartz, section 2.8

http://www.amazon.com/Principles-Electrodynamics-Dover-Books-Physics/dp/0486654931/ref=sr_1_1?ie=UTF8&qid=1331475241&sr=8-1

If you are weak in math, then the above may not be appropriate for you. It is possible to understand the principles of general capacitance without all the complicated math, but it probably requires one-on-one tutoring from someone who understands section 2.8 of this book.

crutschow

Joined Mar 14, 2008
23,157
Capacitance occurs between any two conductors that are insulated from each. Thus any two wires on a circuit that carry different signals or between any wire and ground (common) have capacitance. The amount of this capacitance is generally small and determined by how close the conductors are together and the surface area they have exposed to each other. Thus two conductors parallel to each other have more capacitance than two conductors at right angles.

Wiring capacitances are small but whether they are important depends upon the frequency of the signals. Since capacitance reactance is inversely proportional to frequency, the higher the frequency, the lower the reactance, and the more effect the capacitance can have on the signal. It all depends upon the particular circuit.

All wires also have inductance. This is small for short pieces of wire, but since inductive reactance is directly proportional to frequency, its effect increases as the signal frequency gets higher. The stray capacitance and capacitance in a circuit form a resonant circuit which can cause damped oscillation "ringing" on the fast edges of digital signals or alter the frequency response of high frequency AC signals.

Here's an article that may help you further.