ABSOLUTE-VALUE IMPEDANCE

Sometimes you’ll read or hear that the “impedance” of some device or component is a certain number of ohms. For example, in audio electronics, there are “8-ohm” speakers and “600-ohm” amplifier inputs. How can manufacturers quote a single number for a quantity that is two-dimensional and needs two numbers to be completely expressed? There are two answers to this.
First, figures like this generally refer to devices that have purely resistive impedances. Thus the “8-ohm” speaker really has a complex-number impedance of 8+j0, and the “600-ohm” input circuit is designed to operate with a complex-number impedance at or near 600+j0. Second, engineers sometimes talk about the length of the impedance vector, calling this a certain number of “ohms.” If you talk about “impedance” in this way, then theoretically you are being ambiguous because you can have an infinite number of different vectors of a given length in the RX half-plane. The expression “Z = 8 ohms,” if no specific complex impedance is given, can refer to the complex vectors 8+j0, 0 + j8, 0 - j8, or any vector in the RX half-plane whose length is 8 units. This is shown in Fig. 1. There can exist an infinite number of different complex impedances with Z = 8 ohms in a purely technical sense.

If you’re not specifically told which particular complex-number impedance is meant when a single-number ohmic figure is quoted, it’s best to assume that the engineers are talking about nonreactive impedances. This means that they are pure resistances and that the imaginary, or reactive, factors are zero. Engineers often will speak of nonreactive impedances as “low-Z” or “high-Z.” There is no formal dividing line between the realms of low and high impedance; it depends to some extent on the application. Sometimes a reactance-free impedance is called a pure resistance or purely resistive impedance.

Fig. 1. Vectors representing an absolute-value impedance of 8 ohms.

Purely resistive impedances are desirable in various electrical and electronic circuits. Entire volumes have been devoted to the subject of impedance in engineering applications. Insofar as basic physics is concerned, we have gone far enough here. A somewhat more detailed yet still introductory treatment of this subject can be found in Teach Yourself Electricity and Electronics, published by McGraw-Hill. Beyond that, college textbooks in electrical, electronics, and telecommunications engineering are recommended.