Impedance matching consists of transforming a load impedance, Z _L, in the optimal working impedance of the signal source Z (Figure 2-47). Depending on the specific purpose of the circuit, the optimal working impedance ( Z) may assure maximum power delivered to the load, maximum efficiency or power gain, minimum distortion of the signal across the load and more. In a specific case, this optimal impedance may be the complex conjugate of the source impedance, ^[8] assuring a maximum power transfer, as is usual in small-signal amplifiers. In RF power amplifiers, the optimal working impedance of the signal source is different in most cases from the complex conjugate of the source impedance. However, as an almost general rule, the reactive component of the source impedance must be compensated by a convenient reactance seen at the input of the matching network, so the signal source operates into a purely resistive load. As was discussed, mismatching in RF power amplifiers may cause reduced efficiency and/or output power, increased stresses of the active devices, distortion of the output signal and so on.

Figure 2-47: The general problem of impedance matching.

Considering the specific problems of RF PAs, the matching circuits must meet, as much as possible, the following requirements.

1. Transform the load impedance in the optimal working impedance of the signal source.

2. Maintain the specified amplitude- and phase-frequency response over a certain frequency range in accordance with what is being transmitted, and in compliance...