For normal transistor operation, how should the two p–n junctions of a BJT be biased to achieve the active region?
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Aboth junctions are forward biased
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Bboth junctions are reverse biased
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Cone junction is forward biased and the other is reverse biased
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Dnone of the above
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Eforward bias at collector–base and reverse bias at emitter–base
Answer
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