Curioustab Logo Curioustab
Home » Electronics and Communication Engineering » Electronic Devices and Circuits

For normal transistor operation, how should the two p–n junctions of a BJT be biased to achieve the active region?

Difficulty: Easy

Correct Answer: one junction is forward biased and the other is reverse biased

Explanation:


Introduction / Context:
The BJT has two junctions: emitter–base (E–B) and collector–base (C–B). The mode of operation (cutoff, active, saturation, reverse-active) is determined by the biasing of these junctions. Correct biasing is essential for amplification in the active region.


Given Data / Assumptions:

  • Active region: transistor used as a linear amplifier.
  • E–B junction: forward biased to inject carriers from emitter into base.
  • C–B junction: reverse biased to collect injected carriers into the collector.


Concept / Approach:
In active operation, the emitter injects majority carriers into the base (forward bias). The collector–base junction reverse bias creates a strong field that sweeps the minority carriers across the depletion region into the collector, enabling controlled, nearly linear amplification governed by the base current (or base–emitter voltage in transconductance terms).


Step-by-Step Solution:

Set VEB forward (≈ 0.7 V for Si, ≈ 0.3 V for Ge).Set VCB reverse (positive for n–p–n, negative for p–n–p collector with respect to base in conventional bias).This biasing yields the active region necessary for amplification.


Verification / Alternative check:

Compare other modes: both forward → saturation; both reverse → cutoff; swapped (reverse-active) rarely used for amplification.


Why Other Options Are Wrong:

Both forward/reverse bias correspond to saturation/cutoff respectively, not linear amplification.Specified “forward at C–B and reverse at E–B” describes reverse-active mode.


Common Pitfalls:

Confusing collector polarity with bias type; always think in terms of E–B and C–B junctions.


Final Answer:

one junction is forward biased and the other is reverse biased
← Previous Question Next Question→

More Questions from Electronic Devices and Circuits

Discussion & Comments

No comments yet. Be the first to comment!
Join Discussion