Impedance Matching Units

Maximum Power Transfer Theorem As we have seen previously the output of a power amplifier must transfer as much power as possible to the speaker. The Maximum Power Transfer Theorem requires the output impedance of the power amplifier to be equal to the impedance of the speaker.

Maximum Voltage Transfer Theorem For Maximum Voltage Transfer between stages the output impedance of the voltage source should be much lower than the input impedance of the stage it is driving. An ideal solution: The input impedance of a pre-amplifier should be at least ten times more than the output impedance of the microphone.

Conflicts We can see that these two theorems conflict with each other Voltage Source Driven Stage Impedance Maximum Voltage Transfer Maximum Power Transfer Input Output Voltage Source Driven Stage Input Output

Impedance Matching Units If the output impedance of a voltage source (e.g. crystal microphone) is too high for the following stage then a matching unit needs to be inserted. Example 1 As we know, the input impedance of an op amp is very high and will likely be higher than the output impedance of the voltage source. It is often the case that the voltage gain of the matching unit is unity i.e. V in = V out. This means that the majority of the signal is passed onto the next stage. They are called VOLTAGE FOLLOWERS because the voltage that comes out is the same as the input. Op Amp Voltage Follower