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.
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.
. . . . 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.
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.
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
|Input / output phase relationship||0°||0°||180°|
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.
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