Impedance is an important parameter used to characterize electronic circuits, components, and
the materials used to make components. Impedance (Z) is generally defined as the total opposition
a device or circuit offers to the flow of an alternating current (AC) at a given frequency, and
is represented as a complex quantity which is graphically shown on a vector plane. An impedance
vector consists of a real part (resistance, R) and an imaginary part (reactance, X) as shown in
Figure 1-1. Impedance can be expressed using the rectangular-coordinate form R+jX or in the
polar form as a magnitude and phase angle: |Z|∠θ. Figure 1 also shows the mathematical relationship
between R, X, |Z| and θ. In some cases, using the reciprocal of impedance is mathematically
expedient. In which case 1/Z= 1/(R+jX)= Y= G+jB, where Y represents admittance, G conductance,
and B susceptance. The unit of impedance is the ohm (Ω), and admittance is the
siemen (S). Impedance is a commonly used parameter and is especially useful for representing a
series connection of resistance and reactance, because it can be expressed simply as a sum, R and
X. For a parallel connection, it is better to use admittance (see Figure 1-2).