Learning Outcomes:

● Become familiar with the r e , hybrid, and hybrid p models for the BJT transistor.
● Learn to use the equivalent model to find the important ac parameters for an amplifier.
● Understand the effects of a source resistance and load resistor on the overall gain and characteristics of an amplifier.
● Become aware of the general ac characteristics of a variety of important BJT configurations.
● Begin to understand the advantages associated with the two-port systems approach to single- and multistage amplifiers.
● Develop some skill in troubleshooting ac amplifier networks.

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

Lesson Plan:

Lecture 01: Common-Emitter Fixed-Bias Configuration and Voltage-Divider Bias

Lecture 02: CE Emitter-Bias Configuration and Emitter-Follower Configuration

Lecture 03: Common-Base Configuration and Collector Feedback Configuration

Lecture 04: Collector DC Feedback Configuration and Effect of R_L and R_s

Lecture 05: Determining the Current Gain

Lecture 06: The Hybrid Equivalent Models