Transistor Configurations

A transistor is a two-port device. It can take an input and deliver an output. For both input and output, two terminals are needed. This can be done in a transistor by making one of the three terminals common.

  • When emitter is common to both input and output circuits - common emitter (CE) configuration
  • When base is common to both input and output circuits - common base (CB) configuration
  • When collector is common to both input and output circuits - common collector (CC) configuration

In each of these configurations, the transistor characteristics are unique. The CE configuration is used most widely because it provides voltage, current and power gains. In the CB configuration, the transistor can be used as a constant current source while the CC configuration is usually used for impedance matching.

For each configuration, you can plot three different characteristics:

  1. Input characteristics
  2. Output characteristics
  3. Transfer characteristics

Like a p-n junction diode, transistors are designated with two letters followed by a serial number. The first letter gives an indication of the material. A is for germanium and B is for silicon. The second letter indicates the main application: C is used for audio frequency transistors, D for power transistors and F for radio-frequency transistors. The serial number consists of digits assigned by the manufacturer for identification. For example, AC125 represents germanium transistor for AF applications.