2.1: Introduction

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Amplifiers increase the power of an RF signal by converting DC power to AC power. Amplifiers can be optimized for low noise, moderate to high gain, high efficiency, low distortion, or specific output power. At the same time stability must be assured, which is a problem with feedback due to parasitics and the internal feedback of transistors. However, it is not possible to optimize all of the parameters simultaneously. This has led to several amplifier design strategies and amplifier topologies trading off design complexity and performance. In this chapter the major active devices and linear amplifiers based on them are examined. A critical common aspect is minimizing noise, maximizing the efficiency of power conversion to RF, and ensuring stability.

A common characteristic of linear amplifier design, the subject of this chapter, is that the operation of the amplifier at RF is similar to its operation at low frequencies. The design strategy is based on small signal design and it is sufficient to use the small-signal \(S\) parameters of a device at the chosen operating point. In contrast, the design of power amplifiers considered in a future chapter is more complicated, as the amplifier operation is dependent on the signal level and design must use the large signal model of transistors and use nonlinear simulators.