BJT Configuration

The base of transistor configurations indicates the transistor terminal that is common to both input and output circuits. This gives rise to the three configurations: common base, common collector and common emitter.

The three different transistor configurations are:

• Common base: when a base is common in input and output circuit it is term as common base configuration as shown in figure this transistor configuration provides a low input impedance and a high output impedance. Although the voltage is high, the current gain is low and the overall power gain is also low when compared to the other transistor configurations available.

• Common collector: when collector s common in input and output circuit it is term as common collector configuration as shown in the figure. This configuration is also known as the emitter follower because the emitter voltage follows that of the base. This configuration leads to high input impedance and a low output impedance it is commonly used as a buffer. The voltage gain is unity, although the current gain is high. The input and output signals are in phase.

• Common emitter: In This transistor configuration emitter is common in input and output circuit this configuration is the most widely used. The circuit provides a medium input and output impedance levels as compared to CB and CC configuration. Both current and voltage gain is also medium, but the output is the inverse of the input, i.e. 180° phase change. This provides a good overall performance and because of this CE configuration is the most commonly used configuration. 
  

Transistor circuit configuration summary table

The table is shown below summarize of the major properties of the various transistor configurations.