Digital Circuit Boards: Mach 1 GHz

1.7 Capacitance

When a charge is moved to a new position in an electric field, the work that is done is stored in the electric field. If the electric field is confined to a region between two conductors, the conductor geometry is called a capacitor. The capacitance of this geometry is the ratio of stored charge to potential difference.

N.B.

Capacitance is the ability to store electric field energy. A capacitor is a conductor geometry that is designed to store electric field energy. The energy is stored in space and not in the conductors.

A volume in space with an electric field stores field energy. This volume has capacitance, although it may not be in a capacitor. It is important to appreciate that nearly all field energy is stored in space. (Remember that in most practical circuits it takes very little E field in the conductors to cause current to flow. This means that there is very little field energy stored in the conductors.) Our hope is that we can confine this energy to very small regions under and between traces and in capacitors. This limits the amount of “loose” energy involved. This is how we control radiation. Conductor geometry defines the shape and extent of the field. If it is confined by conductors, it cannot radiate.

N.B.

At low frequencies (below 100 kHz) most of the field energy generated by a circuit returns to that circuit.

The intensity of the electric field is reduced by the presence of a dielectric. If the charge in a capacitor is held constant ...