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Difficulty: HardTransistors

An electronics technician is analyzing an NPN bipolar junction transistor (BJT) configured as an amplifier in a common-emitter circuit. For the transistor to operate properly in its linear active region, which combination of internal junction biasing conditions must be established?

  1. A
    Forward-biasing both the base-emitter junction and the collector-base junction
  2. Forward-biasing the base-emitter junction and reverse-biasing the collector-base junctionAnswer
  3. C
    Reverse-biasing the base-emitter junction and forward-biasing the collector-base junction
  4. D
    Reverse-biasing both the base-emitter junction and the collector-base junction

Answer

Forward-biasing the base-emitter junction and reverse-biasing the collector-base junction
For any bipolar junction transistor (BJT) to function as a linear amplifier in the active region, the input base-emitter PN junction must be forward-biased to permit carrier injection, and the output collector-base PN junction must be reverse-biased to attract and collect these carriers across the junction.

Step-by-Step Solution

1
Identify the operational requirement for linear amplification in a BJT
Linear amplification requires the transistor to operate strictly in the active mode.
Operating outside the active region causes either signal clipping in saturation or complete signal cutoff.
2
Determine the required bias across the base-emitter (input) junction
The base-emitter junction must be forward-biased (positive base relative to emitter in an NPN transistor).
Forward bias reduces the potential barrier across the base-emitter junction, allowing majority carriers (electrons) to be injected from the emitter into the thin base layer.
3
Determine the required bias across the collector-base (output) junction
The collector-base junction must be reverse-biased (positive collector relative to base in an NPN transistor).
Reverse bias widens the collector-base depletion region and sets up a strong electric field that collects the injected electrons passing through the base into the collector.

Key Concept

BJT Active Region Biasing Conditions
Estimated Time:1m 15s
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Topics

Transistors

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