When a transistor is active, the current gain β is large, in the range of 100 or more. This means that we can use the transistor as an amplifier to increase the power of a signal. The amplifier may be considered the single most important device in communications electronics, and it is key to both receivers and transmitters. Developing amplifiers has been a central focus of electrical engineering from the days of the first vacuum tubes, and it is just as important today. There are many issues to consider in designing an amplifier. In transmitters, we are very interested in efficiency. High efficiency makes it easier to dissipate the heat and allows long battery life in portable transmitters. In receivers, it is important to add as little noise as possible to the signal. In this chapter, we study linear amplifiers, where the amplitude of the output tracks the amplitude of the input. In the next chapter, we consider saturating amplifiers, where only the frequency of the output follows the input.

Common-Emitter Amplifier

The basic transistor amplifier is shown in Figure 9. 1a. It uses an npn transistor with a load resistor R at the collector. The supply voltage is written as Vcc. It is traditional to double the subscript of a supply voltage to distinguish it from an AC voltage. This circuit is called a common-emitter amplifier. You do have to be on your guard with amplifier names.