GATE Electrical – Analog Electronics
Page 8: BJT Operating Regions
A Bipolar Junction Transistor operates in different regions depending on the biasing conditions of its two PN junctions.
The two junctions in a transistor are:
- Emitter–Base Junction
- Collector–Base Junction
Depending on how these junctions are biased, the transistor operates in different regions.
1. Cutoff Region
In the cutoff region both PN junctions are reverse biased.
- Emitter–Base Junction → Reverse biased
- Collector–Base Junction → Reverse biased
In this condition:
- No base current
- No collector current
- Transistor behaves like an open switch
IC ≈ 0
2. Active Region
In this region the transistor operates as an amplifier.
- Emitter–Base Junction → Forward biased
- Collector–Base Junction → Reverse biased
Small changes in base current produce large changes in collector current.
IC = β IB
This region is used for:
- Amplifier circuits
- Signal processing
- Analog electronics applications
3. Saturation Region
In saturation region both PN junctions are forward biased.
- Emitter–Base Junction → Forward biased
- Collector–Base Junction → Forward biased
In this region the transistor behaves like a closed switch.
VCE ≈ 0.2 V
This region is used in:
- Digital switching circuits
- Logic circuits
- Power switching applications
4. Summary Table
| Region | Emitter-Base Junction | Collector-Base Junction | Application |
|---|---|---|---|
| Cutoff | Reverse | Reverse | OFF State |
| Active | Forward | Reverse | Amplifier |
| Saturation | Forward | Forward | Switch ON |
Important GATE Points
- Active region is used for amplification.
- Saturation and cutoff regions are used for switching.
- Collector current depends on base current.
- Small IB controls large IC.
Next Page → BJT Configurations (CB, CE, CC)
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