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. To do this we need to employ Power Amplifiers which can output relatively large amounts of current & high voltages. 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. (Except, of course, for the room & your ears!) We can use this as a typical example of the use of a power amplifier.

As with all complicated electronic circuits we can start to understand this system by considering it in chunks:- the loudspeaker ‘load’, the current/power section (the transistors, Q1 & Q2), and the voltage amplifier or control section (the op-amp & the resistors, R1 & R2). Lets start with the loudspeaker. This will enable us to discover what the rest of the circuit has to do.

If you look through a Hi-Fi magazine or an audio manufacturer's catalogue you'll see loudspeakers referred to using terms like, “8 Ohms nominal impedance”, “25 Watt power handling”, and “Sensitivity of 85 dBa/W at 1 metre”. 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.

Looking at this we can see that the speaker is far from being an 8 Ohm resistor! Its impedance (both magnitude & phase) varies in a complex way with the signal frequency. The nominal impedance value is only meant as a general guide. It represents a sort of typical value averaged over the audible frequency range (from about 20 Hz to 20 kHz.) It tells us that when we play music the ‘typical’ current/voltage ratio will be similar to that we'd see if we replaced the loudspeaker with an 8 W resistor. 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.

The power handling (or power rating) value indicates the largest steady power level the speaker can cope with for a reasonable period of time without bursting into flames or flying apart! In practice, this value is like the nominal impedance & should be taken with a pinch of salt. Most speakers can take more power than this for short musical ‘peaks’ — although they might sound awful when asked to ‘shout’ like this!

Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
using HTMLEdit2 on a StrongARM powered RISCOS machine.
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.