Introduction:
Correct DC biasing of a BJT establishes the operating (Q) point in the active region. This enables linear amplification. Knowing which junctions must be forward or reverse biased is a foundational concept in analog electronics.
Given Data / Assumptions:
- BJT operated in active region for amplification.
- Base–emitter (BE) junction forward biased.
- Second PN junction bias must be identified.
Concept / Approach:
In active mode: the BE junction is forward biased to inject carriers from emitter into base; the collector–base (CB) junction is reverse biased to sweep carriers from the base into the collector. This separation of biases provides transistor action with current gain.
Step-by-Step Solution:
Active-mode conditions: VBE > 0 (typically ~0.7 V for Si), VCB > 0 (reverse bias).Reverse-biased CB junction creates a strong electric field that collects carriers.Thus, proper operation requires CB reverse bias along with BE forward bias.Therefore the correct junction is the collector–base junction.
Verification / Alternative check:
Examining output characteristics (IC vs VCE) shows that with BE forward and CB reverse bias, the device exhibits near-constant IC for a given IB, the signature of active mode.
Why Other Options Are Wrong:
- collector–emitter junction: VCE is a terminal-to-terminal potential; active-mode condition is specifically CB reverse bias.
- base–emitter junction: This is already forward biased; reverse biasing it would move the device to cutoff or reverse-active region.
- emitter–substrate junction: Not a standard discrete BJT terminal pair in circuit bias discussions.
- base–collector junction (wording): Equivalent to collector–base; among duplicates, we choose the explicit correct phrasing.
Common Pitfalls:
- Confusing saturation (both junctions forward) with active region.
- Overlooking that reverse-active mode swaps roles and is rarely used in designs.
Final Answer:
collector–base junction
Discussion & Comments