A transistor has three terminals, which are called Emitter, Base, and Collector. By using these three terminals transistors can be connected in a circuit with one terminal for both input and output in three different possible configurations.
The three types of configurations are Common Emitter, Common Base, and Common Collector configurations. In each configuration, the emitter junction is forward biased and the collector junction is reverse biased.
Common Emitter CE Configuration
The emitter terminal is taken as the common terminal for both the input and output of the transistor.
This transistor configuration is the most widely used. The circuit provides medium input and output impedance levels. Both current and voltage gain can be described as medium, but the output is the opposite of the input, i.e. 180° phase shift. It offers good overall performance and as such, it is often the most widely used configuration.
In the CB configuration, the emitter junction is forward biased and the collector junction is reverse biased. The flow of electrons is controlled in the same way. Here the input current is the base current IB and the output current is the collector current IC
Base Current Amplification factor β
The ratio of the change in the base current $ΔIB$ to the change in the collector current $ΔIC$ is known as the base current amplification factor. It is denoted by β.
Relation between β and α
Derive the relation between the base current amplification factor and the emitter current amplification factor.
We can write,
Dividing by $$
We have
Therefore,
From the above equation, it is clear that, as α approaches 1, β approaches infinity.
Therefore, the current gain in the common emitter connection is very high. This is why this circuit connection is mostly used in transistor applications.
Expression for Collector Current
In common emitter configuration, IB is the input current and IC is the output current.
We know
And
If base circuit is open, i.e. if IB = 0,
The collector-emitter current with the base open is ICEO
Substituting its value in the previous equation, we get
Hence the equation of collector current is obtained.
Knee Voltage
In the CE configuration, keeping the base current IB constant, if the VCE is changed, the IC rises to about 1v of VCE and remains constant thereafter. This value of VCE up to which the collector current IC varies with VCE is called knee voltage. When transistors operate in the CE configuration, they operate above this knee voltage.
Characteristics of CE Configuration
- This configuration provides good current gain and voltage gain.
- Keeping VCE constant, the base current IB increases more rapidly than in the CB configuration, with a slight increase in VBE.
- For any value of VCE above the knee voltage, IC is approximately equal to βIB.
- The input resistance ri is the ratio of the change in base-emitter voltage $ΔIBE$ to the change in base current $ΔIB$ at the constant collector-emitter voltage VCE.
- Since the input resistance is of a very small value, a small value of VBE is sufficient to generate a large current of base current IB.
- The output resistance ro is the ratio of the change in collector-emitter voltage $ΔVCE$ to the change in collector current $ΔIC$ at constant IB.
- Since the output resistance of the CE circuit is less than that of the CB circuit.
- This configuration is commonly used for bias stabilization methods and for audio frequency applications.
Common Base CB Configuration
The base terminal is the common terminal for both the input and output of the transistor.
This transistor configuration is the least used but provides the advantage that the base that is common to the input and output is grounded and it helps reduce unwanted feedback between output and input for various RF circuit design applications. There are advantages. This happens because the base, which is the electrode physically between the emitter and collector, is grounded, creating a barrier between the two.
This transistor configuration provides low input impedance while offering high output impedance. Although the voltage is high, the current gain is low and the overall power gain is also low compared to other transistor configurations available. The other main feature of this configuration is that the input and output are in phase.
In an NPN transistor in CB configuration, when voltage is applied to the emitter, it is forward biased, so the electrons from the negative terminal repel the emitter electrons and travel through the emitter and base to contribute to the collector current. Current flows. It happens. During this, the collector voltage VCB is kept constant.
In CB configuration, the input current is the emitter current IE and the output current is the collector current IC.
Current Amplification Factor α
When the collector voltage VCB is kept constant, the ratio of the change in collector current $ΔIC$ and emitter current $ΔIE$ is called the current amplification factor. It is denoted by α.
Expression for Collector current
Some expression for collector current, emitter current flowing, and some amount of base current Ib that flows through the base terminal due to electron-hole recombination. Since the collector-base junction is reverse biased, there remains another current that flows due to the minority charge carriers. It is the leakage current that is indicated as Ileakage. This is due to minority charge carriers and is therefore very small.
The emitter current reaching the collector terminal is
Total collector current
If the emitter-base voltage VEB = 0, however, a small leakage current flows, which can be called an ICBO collector-base current, with the output open.
Therefore the collector current can be expressed as
Hence the above derivative is the expression for collector current. The value of the collector current depends on the current amplification factor of the transistor in use as well as the base current and leakage current.
Characteristics of CB configuration
- This configuration provides voltage gain but no current gain.
- With VCB being constant, the emitter current IE increases, with a slight increase in the emitter-base voltage VEB.
- Emitter current IE collector voltage is independent of VCB.
- Collector voltage VCB can only affect collector current IC at low voltages when VEB is kept constant.
- The input resistance ri is the ratio of the change in emitter-base voltage $ΔVEB$ to the change in emitter current $ΔIE$ at the constant collector base voltage VCB.
- Since the input resistance is of a very small value, a small value of VEB is sufficient to generate a large current of emitter current IE.
- The output resistance ro is the ratio of the change in collector base voltage $ΔVCB$ to the change in collector current $ΔIC$ at constant emitter current IE.
- Since the output resistance is of very high value, a large change in VCB produces very little change in the collector current IC.
- This configuration provides good stability against rising temperatures.
- The CB configuration is used for high-frequency applications.
Common Collector CC Configuration
The collector terminal is taken as the common terminal for both the input and output of the transistor.
The collector circuit configuration is known as an emitter follower because it follows the emitter voltage base, although the voltage is less than the equivalent of the base-emitter junction’s turn-on voltage.
Collector and emitter followers provide high input impedance and low output impedance. The voltage gain is unity, although the current gain is high. Input and output signals are in phase.
In CB and CE configurations, the emitter junction is forward-biased and the collector junction is reverse-biased. The flow of electrons is controlled in the same way. Input current is base current IB and outputs current is emitter current IE.
Current Amplification Factor γ
The ratio of the change in base current $ΔIB$ to the change in emitter current $ΔIE$ is known as the current amplification factor in the common collector CC configuration. It is denoted by γ.
- The current gain in the CC configuration is the same as in the CE configuration.
- In CC configuration the voltage gain is always less than 1.
Relation between γ and α
γ and α some relation between:
Taking the value of IB, we get
Dividing by ΔIE
Expression for collector current
We know
The above is the expression for the collector current.
Characteristics of CC Configuration
- This configuration provides current gain but no voltage gain.
- In the CC configuration, the input resistance is high and the output resistance is low.
- The voltage gain provided by this circuit is less than 1.
- The sum of collector current and base current is equal to the emitter current.
- Input and output signals are in phase.
- This configuration works as a non-inverting amplifier output.
- This circuit is mostly used for impedance matching. This means, running a low impedance load from a high impedance source.
Transistor circuit configuration summary table
The table below summarizes the key properties of different transistor configurations. Not only is it a major aspect when designing a transistor circuit, but so are parameters such as input and output impedance.
Related Tutorial: Transistors and their Operations, and Factors.