Amplifier designs at RF differ significantly from the conventional low-frequency circuit approaches and thus require special considerations. In particular, the fact that voltage and current waves impinge upon the active device necessitates appropriate matching in order to reduce the VSWR and avoid undesirable oscillations. For this reason a stability analysis is usually the first step in the design process and, in conjunction with gain circles and noise figures, form the basic ingredients needed to develop amplifier circuits that meet the often competing requirements of gain, gain flatness, output power, bandwidth, and bias conditions.

This chapter builds upon the material covered in Chapters 2 and 3 where power relations of terminated transmission lines are investigated. However, unlike the passive circuits presentation in these two chapters, Chapter 9 deals with active devices where gain and feedback considerations assume central importance. Issues such as transducer gain, uni-lateral and bi-lateral circuit designs and their graphical display in the Smith Chart constitute the starting point of an extensive analysis into quantifying the high-frequency transistor amplifier performance. The reader will note the flexibility of the Smith Chart which allows constant gain, VSWR, and stability circle displays to be superimposed over the reflection coefficient and impedance representation discussed in Chapter 3. Moreover, even a noise analysis can be conducted by converting the noise figure of an amplifier into circle equations that are displayed in the Smith Chart.

After covering the basic design tools, Chapter 9 also investigates various classes of power amplifiers and their intermodular distortion as well as the differences between single- and multi-stage amplifiers.