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BJT Transistors

A transistor is a device which acts like a controlled valve. The current flow permitted can be controlled.

   The bipolar junction transistor (BJT) is a three-terminal electronic valve - the output (collector) terminal current-voltage characteristics are controlled by the current injected into the input port (base). The BJT is a semiconductor device constructed from two pn junctions. There are two types of BJT: pnp and npn. Figures 91 and 92 show the circuit symbols and common current and voltage polarities during normal (active) operation.    

  

Figure 91: npn BJT circuit symbol.

  

Figure 92: pnp BJT circuit symbol.

The terminals are named: base (B), emitter (E), and collector (C), as shown.      

We will be looking mostly at npn transistors; pnp transistors operate similarly except for polarities.

Basic npn transistor relations are

$$\begin{array}{rl} i_E & = i_B + i_C \\ \\ v_{CE} & = v_{CB} + v_{BE} . \end{array}$$ (83)

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