Transistor Configurations: circuit configurations

Transistor circuits use one of three transistor configurations: common base, common collector (emitter follower) and common emitter - each has different characteristics.


Transistor Circuit Design Tutorial Includes:
Transistor circuit design     Circuit configurations     Common emitter     Common emitter circuit design     Emitter follower     Common base    

See also: Transistor circuit types    


When designing or looking at a transistor circuit there are three different circuit configurations that can be used.

The three different transistor circuit configurations are: common emitter, common base and common collector (emitter follower), these three circuit configurations have different characteristics and one type will be chosen for a circuit dependent upon what is required.

2N3553 transistor in a TO39 metal can
2N3553 transistor in a TO39 metal can

Transistor circuit configurations

The naming of 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 showing the base connection common to both input and output circuits
    Transistor common base circuit configuration
      . . . . Read more about the Common base transistor amplifier
  • 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. In view of these characteristics, the emitter follower configuration is used as a buffer circuit providing a high input impedance to prevent loading of the previous stage, and a low output impedance to drive following stages.
    Common collector transistor configuration showing the base connection common to both input and output circuits
    Transistor common collector circuit configuration
    As can be seen from the diagram, in this transistor configuration, the collector electrode is common to both input and output circuits.   . . . . Read more about the Common collector transistor amplifier
  • 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 showing the base connection common to both input and output circuits
    Transistor common emitter circuit configuration
    As can be seen from the diagram, in this transistor configuration, the emitter electrode is common to both input and output circuits.   . . . . Read more about the Common emitter transistor amplifier

Transistor circuit configuration summary table

The table below gives a summary of the major properties of the different transistor configurations. Not only is gain a major aspect when designing a transistor circuit, but so too are parameters like input and output impedance.


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 180°
  Input resistance Low High Medium
  Output resistance High Low Medium

The most commonly used circuit configuration is the common emitter - this is used for many amplifier stages providing voltage gain. The emitter follower or common collector is also widely used. Providing a high input impedance and low output impedance it acts as a buffer and provides only current gain - its voltage gain is unity. The common base is used in more specialist applications and is seen considerably less.



More Circuits & Circuit Design:
Op Amp basics     Op Amp circuits     Power supply circuits     Transistor design     Transistor Darlington     Transistor circuits     FET circuits     Circuit symbols    
    Return to Circuit Design menu . . .