There are three basic circuit configurations that can be used with transistors.
Known as common emitter, common base and common collector, these three circuit configurations have different attributes.
When designing a transistor circuit it is necessary to adopt the transistor circuit configuration that will provide the required attributes.
Transistor configuration basics
The terminology used for denoting the three basic transistor configurations indicates the transistor terminal that is common to both input and output circuits. This gives rise to the three terms: common base, common collector and common emitter.
The term grounded, i.e. grounded base, grounded collector and grounded emitter may also be used on occasions because the common element signal is normally grounded.
The three different transistor configurations are:
- Common base: This transistor configuration provides a low input impedance while offering 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. The other salient feature of this configuration is that the input and output are in phase.
Common base transistor configuration
As can be seen from the diagram, in this transistor configuration, the base electrode is common to both input and output circuits. - Common collector: This transistor configuration is also known as the emitter follower because the emitter voltage follows that of the base. Offering a high input impedance and a low output impedance it is widely used as a buffer. The voltage gain is unity, although current gain is high. The input and output signals are in phase.
Common collector transistor configuration
As can be seen from the diagram, in this transistor configuration, the collector electrode is common to both input and output circuits. - Common emitter: This transistor configuration is probably the most widely used. The circuit provides a medium input and output impedance levels. Both current and voltage gain can be described as medium, but the output is the inverse of the input, i.e. 180° phase change. This provides a good overall performance and as such it is often thought of as the most widely used configuration.
Common emitter transistor configuration
As can be seen from the diagram, in this transistor configuration, the emitter electrode is common to both input and output circuits.
Transistor circuit configuration summary table
The table below gives a summary of the major properties of the different transistor configurations.
| TRANSISTOR CONFIGURATION SUMMARY TABLE | |||
|---|---|---|---|
| TRANSISTOR CONFIGURATION | COMMON BASE | COMMON COLLECTOR (EMITTER FOLLOWER) | COMMON EMITTER |
| Voltage gain | High | Low | Medium |
| Current gain | Low | High | Medium |
| Power gain | Low | Medium | High |
| Input / output phase relationship | 0° | 0° | 180° |
| Input resistance | Low | High | Medium |
| Output resistance | High | Low | Medium |
The choice of the transistor configuration which is most applicable will depend upon many characteristics. Input impedance, output impedance, gain and also the phase relationships all have a bearing.
