Impedance Matching

Figure below demonstrates the significance of correctly matching input and output impedances between stages of the circuit. The AC signal source demonstrated in the left half of our fig below could be the output of a common-emitter amplifier, a laboratory signal generator output, the “line”

output of a FM radio receiver, or numerous of other possible sources.

Here the important point is that all signal sources will possess an internal impedance; shown by Z_s in the example. Internal source impedances can range from less than of an ohm to many thousands or even in millions of ohms in value. In this case, assume that the internal impedance is 1000 ohms. The signal level we want to supply to R_L is 10 volts peak to peak. Notice that Z_s is in series with R_L. We know that, in a series circuit, the drop in voltage will be proportional to the values of resistance.

Therefore, in this circuit, about 9.1 volts P-P will be dropped across the internal source impedance and just about 0.9 volt P-P will actually be applied to RL. This is unwanted because over 90% of the signals has been lost.

The circuit illustrated in the right half of Figure below demonstrates that same signal source joined to the input of a common-collector amplifier stage.

(note that there is a triangular symbol for the common-collector amplifier. This is the symbol mostly used for amplifiers in the block diagrams.) Because of the amplifier`s input impedance of 10,000 ohms, only 0.9 volt P-P will be dropped across the internal source impedance, and 9.1 volts P-P will be applied to the amplifier. This is much reasonable because the 9% signals are lost.

Here are a  small number of general rules to remember concerning impedance matching. For the purpose of maximum transfer of voltage the output impedance should be as low as possible and the input impedance should be as high as it can be possible possible. For the purpose of maximum transfer of power, the output impedance should have tha same value like the input impedance. In case for maximum transfer of current, the output impedance and input impedance should be kept as low as possible.