Maximizing Power Transfer Given A Fixed Source Impedance
A related case, but one which does not occur often in audio, is to optimize the power delivered to the load when the source impedance is unchangeable. In such cases, power delivery is maximized when the load impedance is matched to the source. The only typical audio application where power delivery (as opposed to voltage delivery) is important is the situation mentioned above of an amplifier driving a loudspeaker. A loudspeaker's impedance is a function of its various electrical and mechanical characteristics, and this impedance typically takes on a value between 2 to 16 ohms without much room for variation. It is however, relatively easy these days to design an audio amplifer with any of a range of output impedances, even down to nearly zero. In a hypothetical situation with an amplifer having an output impedance much higher than zero, say 8Ω, then it is true that maximum power will be delivered if the loudspeaker also has an 8Ω impedance. However, given the same 8Ω loudspeaker, even more power is delivered (and in fact, the maximum possible amount) when the amplifer has an output impedance of zero. This is assuming that the amplifier is modeled by VS and ZS as shown above, and that the VS between the two example amplifiers have the same value.
The matched impedance situation is encountered much more in non-audio related situations, for example in antenna design where the impedance at the antenna terminals takes on a set value due to requirements in its geometry. In such cases, the impedance of the circuit stage connected to the antenna should be matched to the antenna terminal impedance in order to maximize power transfer. Such cases frequently arise in RF circuits, where also transmission line effects dictate impedance matching.
Read more about this topic: Impedance Bridging
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