In This Chapter

In this chapter, we cover some basic transistor topologies, including the common-emitter amplifier, emitter-follower, common-base and differential amplifiers. The important issue of biasing connection of the transistor so that it operates in the forward-active region is also discussed in detail.

The Issue of Transistor Biasing

In order to achieve useful amplification from a transistor, the transistor is generally biased in the forward-active region (FAR). The FAR is the region of operation of the transistor where amplification can occur because the transistor provides current gain. Biasing is the process by which one sets the DC operating point of a transistor amplifier to a known and repeatable point in this forward-active region. If you do your biasing correctly, your amplifier output bias level will not drift significantly with time, temperature or component variations.

Let s consider the simple bias circuit shown in Figure 5-1a. The bias voltage V _BB = 5V sets the base current as follows:

This initial calculation assumes that the base-emitter voltage V _BE = 0.7V, which is approximately correct if the transistor is biased in the forward-active region and the base-emitter junction is forward biased. Let s assume that this transistor has a DC current gain ? _F = 175. This means that the collector current is:

With a load resistor R _L = 240 ?, there is a 6.3V drop across this load resistor and hence V _CE = 5.7V. The operating point, labeled Q for quiescent operating point, is shown...