Common Emitter Configuration

In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-junction-transistor (BJT) amplifier topologies, typically used as the voltage amplifier.

In this circuit the base terminal of the transistor serves as the input, the collector is the output, and the emitter is common to both (for example, it may be tied to ground reference or a power supply rail), hence its name.

A change in base current $\Delta I_B$ produces a corresponding change in collector current, $\Delta I_C$, and the common emitter gain $\beta$ is then defined by

$$\beta = \frac{\text{Change in collector current}} {\text{Change in base current}} = \frac{\Delta I_C}{\Delta I_B} |_{V_{CE}\text{ = CONSTANT}}$$

之所以讲共射电路，是因为后面大多数时候我们能遇到的电路都是这个，而且后面知识的大体框架是一样的，只是细节不一样