GATE Electrical – Analog Electronics
Page 9: BJT Configurations
A Bipolar Junction Transistor (BJT) can be connected in three different ways depending on which terminal is common between input and output circuits.
The three configurations are:
- Common Base (CB)
- Common Emitter (CE)
- Common Collector (CC)
1. Common Base (CB) Configuration
In this configuration the base terminal is common between input and output.
- Input is applied between Emitter – Base
- Output is taken between Collector – Base
Important Characteristics
- Current Gain (α) is less than 1
- Very high voltage gain
- Low input impedance
- High output impedance
α = IC / IE
Typical value of α ≈ 0.95 – 0.99
2. Common Emitter (CE) Configuration
In this configuration the emitter terminal is common between input and output.
- Input is applied between Base – Emitter
- Output is taken between Collector – Emitter
Important Characteristics
- High current gain
- High voltage gain
- Moderate input impedance
- Moderate output impedance
β = IC / IB
Typical value of β = 50 to 300
This is the most widely used configuration for amplifiers.
3. Common Collector (CC) Configuration
In this configuration the collector terminal is common.
- Input between Base – Collector
- Output between Emitter – Collector
Important Characteristics
- Current gain is very high
- Voltage gain approximately 1
- Very high input impedance
- Very low output impedance
This configuration is also called:
Emitter Follower
Comparison of BJT Configurations
| Configuration | Current Gain | Voltage Gain | Input Impedance | Output Impedance |
|---|---|---|---|---|
| CB | Low | High | Low | High |
| CE | High | High | Medium | Medium |
| CC | Very High | ≈ 1 | Very High | Very Low |
Important GATE Points
- CE configuration is most widely used.
- CC configuration is used for impedance matching.
- CB configuration is used in high-frequency circuits.
- Relationship between gains:
β = α / (1 − α)
Next Page → BJT Characteristics (Input & Output Curves)
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