Saturation vs Cutoff vs Active Region in BJTs Explained with Examples

The Bipolar Junction Transistor (BJT) is a three-terminal device widely used in amplification and switching. A BJT operates in different regions depending on the applied voltages. Understanding cutoff, active, and saturation regions is essential for both design and interview preparation.

1. BJT Basics

A BJT has three terminals: Emitter (E), Base (B), and Collector (C). Current relationships:

I_C = β I_B (in active region)

2. Regions of Operation

a) Cutoff Region

Condition: V_BE < 0.7 V (for silicon) and V_BC < 0.7 V
Transistor is OFF, only leakage currents flow. Equivalent to an open switch.

b) Active Region

Condition: V_BE > 0.7 V (forward-biased) and V_BC < 0.7 V (reverse-biased)
Transistor acts as an amplifier. Collector current is controlled by base current.

Equation: I_C ≈ β I_B

c) Saturation Region

Condition: Both base-emitter and base-collector junctions forward-biased.
Transistor is fully ON, behaves like a closed switch.

I_C is no longer β I_B but limited by external circuit resistances.

3. Example

A BJT has β = 100. Base current = 20 µA. Expected I_C = β × I_B = 2 mA.

• If supply allows only 1 mA → transistor enters saturation.
• If base current is 0 → transistor is in cutoff.
• If conditions are met → transistor is in active region.

4. Applications

• Cutoff: Transistor OFF in switching circuits.
• Active: Amplification in analog circuits.
• Saturation: Transistor fully ON in digital logic or switches.

5. Interview Questions

• What are the three regions of operation of a BJT?
• How do you bias a BJT to operate in active region?
• What happens to β in saturation?
• Explain the use of cutoff and saturation in switching circuits.

Conclusion

Understanding cutoff, active, and saturation regions is essential for applying BJTs in amplifiers and switches. These concepts are heavily tested in analog design interviews and real-world circuit design.