Bipolar transistors

How does the current gain hFE of a bipolar transistor depend on collector current?

  1. AIt peaks at moderate currents and falls at both low and high currents
  2. BIt is constant regardless of collector current
  3. CIt increases monotonically with collector current
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Answer: A. It peaks at moderate currents and falls at both low and high currents

At low currents, recombination current in the base-emitter depletion layer makes up a large part of the base current, lowering hFE. At high currents, high-injection effects and the Kirk effect (effective widening of the base) lower hFE. hFE is therefore highest at intermediate currents.

  • A. Correct. It falls on both the low-current and high-current sides.
  • B. hFE clearly changes with current.
  • C. hFE drops at high currents.
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More Bipolar transistors questions

  1. Which correctly relates BVCEO, the collector-emitter breakdown voltage with the base open, and BVCBO, the collector-base breakdown voltage with the emitter open?
  2. Which correctly describes the Miller effect that degrades the high-frequency performance of a common-emitter amplifier?
  3. What is the aim of a transistor with a Schottky barrier diode connected across its base-collector junction (a Schottky clamp)?
  4. Which bias condition is correct when an NPN transistor is used for amplification (operated in the active region)?
  5. Which expression gives the DC current gain hFE of a bipolar transistor? (IC: collector current, IB: base current, IE: emitter current)