Chapter 5: BJT AC Analysis

The basic construction, appearance, and characteristics of the transistor were introduced in Chapter 3. The dc biasing of the device was then examined in detail in Chapter 4. We now begin to examine the ac response of the BJT amplifier by reviewing the models most frequently used to represent the transistor in the sinusoidal ac domain.



One of our first concerns in the sinusoidal ac analysis of transistor networks is the magnitude of the input signal. It will determine whether small-signal or large-signal techniques should be applied. There is no set dividing line between the two, but the application—and the magnitude of the variables of interest relative to the scales of the device characteristics—will usually make it quite clear which method is appropriate. The small-signal technique is introduced in this chapter, and large-signal applications are examined in Chapter 12.



There are three models commonly used in the small-signal ac analysis of transistor networks: the re model, the hybrid model, and the hybrid equivalent model. This chapter introduces all three but emphasizes the re model.

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