INTRODUCTION

The primary purpose of most amplifiers used in radio frequency systems is to increase signal magnitude. This is necessary in receivers because propagation path loss results in a signal arriving with voltage or current which is far too small to be digitized or directly demodulated. Amplification appears in transmitters when it is convenient to synthesize a signal at low power and amplify it, as opposed to synthesizing signals directly at the intended transmit power; the latter is difficult for all but very simple analog modulations.

This chapter is organized as follows. Section 10.2 introduces the transistor, its use as a gain device, and provides a brief summary of transistor types and device technologies. The design of narrowband single-transistor amplifiers is introduced in four parts: biasing (Section 10.3), small-signal design to meet gain requirements (Section 10.4), designing for noise figure (Section 10.5), and designing for voltage standing wave ratio (Section 10.6). Section 10.7 presents a design example intended to demonstrate the principles of amplifier design as laid out in the previous sections. Section 10.8 introduces the concepts of multistage, differential, and broadband transistor amplifiers. Section 10.9 describes the implementation of transistor amplifiers as integrated circuits.

TRANSISTORS AS AMPLIFIERS

The fundamental building block of modern radio frequency amplifiers is the transistor. A transistor is a three-terminal semiconductor device in which the current between two terminals can be manipulated using the voltage or current at a third terminal. The application to RF design is most easily explained in the context of a particular device – for our purposes, the silicon (Si) bipolar junction transistor (BJT) is a good place to start.

Bipolar Transistors

A BJT is depicted in Figure 10.1. The three terminals of a BJT are known as the base, the collector, and the emitter. The associated terminal currents and voltages are as defined in Figure 10.1. The Si BJT comes in two varieties, known as “NPN” and “PNP,” referring to the geometry of semiconductor materials within the device. The NPN variant is somewhat more common in RF applications and is specifically considered here (and shown in Figure 10.1); the PNP variant is relatively easily understood once the NPN variant is mastered.