When you couple some amplifier stages or connect electronic appliances, the input/output impedance of every stage will effect the efficiency.
The internal resistance/impedance of the signal source (R1) and the loudspeaker (R4) is fixed. Only R2 and R3 can be designed to match the impedance.
Between signal source and amplifier it is important to transfer the highest possible voltage.
In order to get the highest, possible voltage (V2) let's try several values for R2.
Ex: V = 10V, R1 = 100 Ω
R2 = 0.1 * R1 --- >
R2 = R1 --- >
R2 = 10 * R1 --- >
R2 = 100 * R1 --- >
The input impedance of an amplifier should be much bigger than the internal resistance of the signal source.
Between amplifier and loudspeaker it is most important to transfer the highest possible power.
Ex: Power in the load under different values of R4.
V = 10V, R3 = 8Ω
PL = R4 * I2
Input impedance of the load and the output impedance of the amplifier should have the same value.
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