Power Amplifier Principles and Modern Design Techniques

Enabled by Lee de Forest's invention of the vacuum tube triode in 1906, power amplification of electrical signals has played a key function in electronic systems ever since. Mundane devices we take for granted such as the telephone, the radio, or the television would not exist without this capability. Given such a wide application space, it is not surprising that early on electrical engineers have worked out the details of designing good power amplifiers (PAs), first with vacuum tubes, and then with discrete transistors [1]. They did such a fine job that by the second part of the twentieth century, the art of designing PAs became a mature electrical engineering (EE) specialty, which seemed to have little room left for breakthroughs or major innovations. However, the late-century market explosion of mobile digital communication systems and devices, such as cellular phones and wireless local area networks (LANs), and the massive introduction of integrated circuit (Ie) technology in everyday life have changed the electronic landscape dramatically, opening new challenges and opportunities for PAs. In this chapter, the issues and appropriate techniques for modern PAs are discussed, focusing on Ie implementations for wireless communication systems. To familiarize the reader with the general PA design approach, which is rather different from the regular analog circuit approach, a few important points are clarified, as a prerequisite for the following material. Then, the classical theory of PA design in the case of constant magnitude signals is reviewed and the trade-offs for different classes of transistor operation are pointed out. The important class AB case is discussed in more details. Next, the PA design problem from a unified, general point of view based on the internal PA signal harmonic content is revisited. This will give the reader a further insight into the PA design problem and high-level solution possibilities. The following section concerns the important topic of efficiency in the presence of back-off and briefly mentions other important design considerations. Finally, recent PA results are reviewed and conclusions drawn.