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Difficulty: MediumTransistors

A technician is configuring a PNP bipolar junction transistor to operate as a switch. To forward-bias the base-emitter junction and turn the transistor ON, what electrical condition must be established at the base terminal relative to the emitter?

  1. A
    The base voltage must be higher (more positive) than the emitter voltage.
  2. The base voltage must be lower (more negative) than the emitter voltage.Answer
  3. C
    The base must receive conventional current flowing into it from the main positive supply rail.
  4. D
    The base voltage must be held at the exact same potential as the collector terminal.

Answer

The base voltage must be lower (more negative) than the emitter voltage.
For a PNP bipolar junction transistor to turn ON, its base-emitter P-N junction must be forward-biased. Because the emitter is P-type semiconductor material and the base is N-type material, the base terminal must be held at a lower (more negative) electrical potential than the emitter terminal (typically by at least 0.7 V0.7\text{ V} in standard silicon transistors).

Step-by-Step Solution

1
Identify the transistor polarity and internal semiconductor structure.
A PNP transistor consists of an N-type base sandwiched between a P-type emitter and a P-type collector.
Understanding layer material types determines the required voltage polarities across the junctions.
2
Determine the biasing requirement for the base-emitter junction to allow conduction.
To forward-bias a P-N junction (P-type emitter to N-type base), the P-type region must be more positive than the N-type region.
Current can only flow across a forward-biased junction when the appropriate potential difference overcomes the barrier voltage (typically 0.7 V0.7\text{ V} for silicon).
3
Formulate the necessary base terminal condition relative to the emitter.
The base voltage must be pulled lower (more negative) relative to the emitter voltage, allowing conventional current to flow out of the base.
This establishes forward bias at the base-emitter junction, switching the PNP transistor ON.

Key Concept

PNP Transistor Biasing and Conduction Principles
Estimated Time:1m 0s
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Topics

Transistors

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