Up until now we've considered signals which require just enough power to carry information from one piece of equipment to another. However, there are many situations when we use electronics to drive or control some useful physical process. Its also often useful to build systems which compare the effect they're producing with what's actually being asked for and adjust their output to achieve the desired result. The arrangement illustrated in figure 9.1 represents the 'final link in the chain' of a Hi-Fi system. We can use this as a typical example of the use of a power amplifier.
Lets start with the loudspeaker.
This will enable us to discover what the rest of the circuit has to do. These values determine what the power amplifier is going to have to do for us to hear music through the speaker. A more detailed look at the impedance behaviour of a typical speaker will produce something like the graphs shown in figure 9.2. The nominal impedance value is only meant as a general guide. We can therefore use the nominal impedance as an indication of the load the power amplifier will have to drive, although the real behaviour of the speaker is more complicated.