先看看Wikipedia上怎麼說 http://en.wikipedia.org/wiki/Input_impe ... io_systems
Generally in audio and hi-fi systems, amplifiers have an input impedance several orders of magnitude higher than the output impedance of the source device connected to that input. This concept is also called voltage bridging or impedance bridging. In this case,
Zload, or the input of the driven stage >> Zsource, or the output of the driving stage.
In general, this configuration will be more resistant to noise (particularly mains hum). Also the loading effects on the driving amplifier stage are reduced. In certain circuits a voltage follower stage is used to match the source and load impedance, which results in maximum power transfer.
