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Difficulty: Very hardTransistors

A technician is troubleshooting a solid-state switching circuit that utilizes a silicon PNP bipolar junction transistor. The emitter terminal is connected directly to a +12V DC power source, and the collector is wired in series with a ground-referenced load. In order to drive this transistor into saturation and fully power the load, what specific voltage condition must exist at the base terminal, and what is the direction of conventional current flow through the device?

  1. A
    The base must be at a higher voltage potential than the emitter; conventional current flows from the collector to the emitter.
  2. B
    The base must be at a higher voltage potential than the emitter; conventional current flows from the emitter to the collector.
  3. The base must be at a lower voltage potential than the emitter; conventional current flows from the emitter to the collector.Answer
  4. D
    The base must be at a lower voltage potential than the emitter; conventional current flows from the collector to the emitter.

Answer

The base must be at a lower voltage potential than the emitter; conventional current flows from the emitter to the collector.
The correct response accurately synthesizes both fundamental rules for a PNP transistor. First, to achieve saturation (full conduction), the base must be at a lower potential than the emitter to forward-bias the emitter-base junction. Second, unlike NPN devices, conventional current in a PNP transistor enters through the emitter and exits through the collector, following the direction of the emitter arrow on its schematic symbol.

Step-by-Step Solution

1
Identify the type of transistor and its fundamental biasing requirements.
The device is a PNP transistor. To turn it on (saturation), the base must be negative relative to the emitter.
PNP transistors operate with opposite polarities compared to NPN transistors. Forward-biasing the emitter-base junction requires the P-type emitter to be more positive than the N-type base.
2
Determine the direction of conventional current flow through the component.
Conventional current flows from the emitter to the collector.
The schematic symbol for a PNP transistor has an arrow on the emitter pointing inward (towards the base), which indicates the direction of conventional current flow from emitter to collector.
3
Synthesize the findings to select the correct operational state.
Base voltage must be lower than emitter voltage, with current flowing emitter-to-collector.
Combining the biasing rule (V_base < V_emitter) and the current path establishes the correct parameters for full conduction.

Key Concept

PNP Transistor Biasing and Current Flow
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Topics

Transistors

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