Chapter 9: RF Transistor Amplifier Design
Amplifier designs at RF differ significantly from conventional low-frequency circuit approaches, and consequently require special considerations. In particular, the fact that voltage and current waves impinge on the active device necessitates appropriate matching to reduce the VSWR. In addition, we will find that most amplifiers can oscillate when terminated with certain source and load impedances. Matching networks can help stabilize the amplifier by keeping the source and load impedances in the appropriate range. For this reason, a stability analysis is usually the first step in the design process and, in conjunction with gain and noise figure circles, is a basic ingredient needed to develop amplifier circuits that meet the often competing requirements of gain, gain flatness, output power, bandwidth, and bias conditions.
This chapter expands upon the material covered in Chapters 2 and 3, where power relations of terminated transmission lines are investigated. However, unlike the passive circuit presentations, Chapter 9 deals with active devices, where gain and feedback considerations assume central importance. Issues such as power gain, unilateral and bilateral circuit designs and their graphical display in the Smith Chart constitute the starting point of an extensive analysis into quantifying 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 noise analysis can be conducted by converting the noise figure of an amplifier into circles that are displayed in the Smith Chart.
After covering the basic design tools, Chapter 9 also investigates various types of power amplifiers and their characteristics such as gain flatness, bandwidth, and intermodulation distortion. We will also examine the differences between single- and multistage amplifiers.