Any circuit can be represented by the following equivalent circuit. The input of the circuit looks like a single resistor (the input impedance) and the output looks like the Thevinin-equivalent circuit as seen from the output.

The load is the electronic component that ultimately converts electrical energy into power. It is essentially the final input impedance of the system. Like input impedance, a circuit load is generically represented by a resistor.

The load of a public address system is the loudspeaker. The load of a radio transmitter is the antenna. The load of a computer power supply is the computer. The ultimate function of any circuit is to deliver power to the load. We will see this representation of a circuit load often in this series of books. As will be shown below, it is important to understand the interaction of the output impedance of a circuit and the circuit's load.

The term “load” is also used as a verb meaning to cause a significant current drain from the circuit, either by putting a useful load on the circuit or by unintentional means. The result of loading a circuit is always a drop in output voltage. How much drop and whether this is good, bad or indifferent depends on the interaction of the output impedance and the load.

Let's use a public address system as an example of a complex circuit. At each stage there is an output impedance coupled to an input impedance. For example, the output impedance of the microphone is coupled to the input impedance of the preamplifier. The output impedance of the preamplifier is coupled to the input impedance of the power amplifier.

Notice that the final input impedance in the public address system is the speaker (the speaker is the load of the circuit). The ultimate function of a public address system is to produce power in the speaker. The speaker moves air (mechanical motion), thus producing sound waves and therefore produces power (see Power above).

Output impedance determines the ability of a circuit to deliver current to the next circuit in a system. When current flows through the output impedance there is a drop in voltage across it. If a relatively large current is passed through a relatively high output impedance, the voltage drop could be enough that the next circuit in the system will not operate correctly. The basic rule is as follows:

A circuit with a high output impedance can only deliver a small amount of current. A circuit with a low output impedance can deliver a relatively high amount of current.

"Relatively high amount of current" does not necessarily mean several amps. Typically, the output of one circuit will deliver a few milliamps to the input of the next circuit. This is a relatively high amount of current compared to a circuit that can only deliver a few microamps to the next circuit.

Input impedance determines the relative amount of current required to drive a circuit. The basic rule is:

A circuit with a low input impedance requires a relatively high amount of current to operate. A circuit with a high input impedance requires a relatively low amount of current to operate.