CE Transistor Regions of Operation: In which region of a common-emitter (CE) bipolar junction transistor is the collector current almost constant?

Introduction / Context:
Bipolar junction transistors (BJTs) operate in distinct regions: cutoff, active, saturation, and breakdown. Recognizing which region provides linear amplification with nearly constant collector current is essential in amplifier circuit design.

Given Data / Assumptions:

• Configuration: Common-emitter (CE).
• Focus is on collector current (IC) behavior with varying collector–emitter voltage (VCE).

Concept / Approach:

In the active region, IC is controlled primarily by the base current (IB) or base–emitter voltage (VBE), and remains nearly constant over a wide range of VCE. This property makes the active region suitable for amplification. In contrast, in saturation IC depends strongly on VCE, and in breakdown IC rises uncontrollably.

Step-by-Step Solution:

Verification / Alternative check:

Characteristic IC–VCE curves for BJTs show flat regions (active) where IC is almost constant, validating this answer.

Why Other Options Are Wrong:

• Saturation: IC depends on VCE strongly.
• Breakdown: Destructive runaway.
• Both saturation and active: Misleading, only active region shows flat IC vs VCE behavior.
• Cutoff: No conduction at all.

Common Pitfalls:

• Confusing 'digital saturation' (switch closed) with 'linear constant current' behavior.
• Assuming collector current is constant in both saturation and active regions; in reality only active qualifies.

Final Answer:

Active region