In a typical transmitter system, the basic carrier signal generated by the transmitter oscillator needs to be amplified several times before it is radiated out by the antenna.  This amplification process may require the passing of the signal from one stage to another.  The goal is to maximize the transfer of power from one amplification stage to the next.

A driving stage may be modeled as an RF generator with an internal impedance Zin, while the driven stage may be modeled as an impedance Zload, as shown in Figure 1. 

Figure 1.  Models for Driving and Driven Stages: Zin=Zload under ideal conditions

Recall that maximum transfer of power occurs when Zin = Zload. Thus, efficient transfer of power from one stage to the next can be achieved by using special circuits that match the impedance Zin of the driving stage to that of the driven stage, Zload.  These circuits are commonly referred to as impedance-matching circuits or impedance-matching networks, as shown in Figure 2.