Maximum Power Transfer
Real power sources include devices such as your audio amplifier. When a load is connected to any power source it is often desired to transfer the maximum power to the load (speakers for example). Because real sources have an internal source resistance, some power is ALWAYS lost within the source. In the example below we have attached a load RL to a voltage source VS with internal resistance RS.
What value
of RS is best? Reducing RL
increases the current in the circuit, but decreases the voltage across RL. Increasing RL increases the voltage, but decreases the current. Because the power delivered to the load PL is the product of the current and the voltage,
there must be a value of RL
that maximizes PL.
The maximum power transfer occurs when the load resistance equals the source resistance. Audio amplifiers specify the impedance of the speakers; this is precisely the same concept. For example, an amplifier designed to drive an 8 ohm speaker can also drive two 4 ohm speakers placed series or two 16 ohm speaks placed in parallel. Any combination that changes the equivalent load resistance will result in less volume.