Operation of BJT in Active, Saturation And Cutoff Region

Understanding the active region, saturation region, and cutoff region of a Bipolar Junction Transistor (BJT) is fundamental for both analog and digital circuit design. These regions define how the transistor behaves—whether it acts as an amplifier, a closed switch (ON), or an open switch (OFF).

The transistor can be used as a switch or as an amplifier by forward or reverse biasing the emitter-to-base and base-to-collector junctions. Based on biasing conditions, the transistor operates in one of the three regions:

• Cutoff Region
• Active Region
• Saturation Region

These regions define the transfer characteristics and determine whether the transistor is ON, OFF, or amplifying signals.

Transistor Working Regions Overview

A Bipolar Junction Transistor (BJT) operates in three major regions depending on how the junctions are biased:

• Cutoff Region – Both junctions reverse biased
• Saturation Region – Both junctions forward biased
• Active Region – Emitter-Base forward biased, Base-Collector reverse biased

These regions are crucial to understand transistor operation in switching and amplification applications.

Transistor as a Switch

When used as a switch, the transistor operates in either cutoff or saturation mode.

Cutoff Region of BJT

In the cut-off region, both the emitter-to-base (EB) and base-to-collector (BC) junctions are reverse-biased. This prevents current flow through the transistor.

• Base current (IB) = 0
• Collector current (IC) ≈ 0
• Emitter current (IE) ≈ 0
• Collector-emitter voltage (VCE) = High

The transistor acts like an open switch (OFF state).

In the off region, the output of the transistor VCE, IC, IB, and IE=0

Saturation Region of BJT

In the saturation region, both the EB and BC junctions are forward-biased.

• The base current is sufficient to allow maximum collector current.
• VCE is very low, typically around 0.2V.
• The transistor behaves like a closed switch.

Transistor as an Amplifier

When the transistor is used as an amplifier, it operates in the active region.

Active Region of BJT

In the active region:

• The emitter-to-base junction is forward-biased
• The base-to-collector junction is reverse-biased

This biasing allows the transistor to act as a linear amplifier, where the collector current is controlled by the base current.

The relationship:
IC = β × IB

Where:

• IC = Collector current
• IB = Base current
• β = Current gain (hFE)

Read detailed article on: Bipolar Junction Transistor (BJT) as an Amplifier

Comparison Table: Active, Saturation, and Cutoff Regions

The table below summarizes the transistor’s behavior in the three main working regions:

Related Articles:

• Bipolar Junction Transistor – Definition, Construction, Types & Applications
• Bipolar Junction Transistor (BJT) as an Amplifier
• Why Core Saturation happens in Transformers and What are its Effects
• Difference between Active and Passive Components
• Magnetic Saturation: What is it?