Bipolar transistors

What is the difference between the DC current gain hFE and the small-signal current gain hfe?

  1. AhFE is the DC ratio IC/IB, while hfe is the ratio of small changes around the operating point, ΔIC/ΔIB
  2. BhFE is the ratio of small-signal changes, while hfe is the DC ratio
  3. CThey are just different names for the same quantity
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Answer: A. hFE is the DC ratio IC/IB, while hfe is the ratio of small changes around the operating point, ΔIC/ΔIB

hFE is the ratio of DC collector current to base current, IC/IB. hfe is the ratio ΔIC/ΔIB when the signal changes slightly around a given operating point. The two are often similar but generally not equal, because hFE varies with current.

  • A. Correct. It is the difference between the DC ratio and the small-signal ratio.
  • B. Uppercase F denotes DC and lowercase f denotes small-signal, so this is reversed.
  • C. They are defined differently and are generally not equal.
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More Bipolar transistors questions

  1. Which model represents the large-signal behavior of a bipolar transistor with two diodes and current sources?
  2. How does the current gain hFE of a bipolar transistor depend on collector current?
  3. Which correctly relates BVCEO, the collector-emitter breakdown voltage with the base open, and BVCBO, the collector-base breakdown voltage with the emitter open?
  4. Which correctly describes the Miller effect that degrades the high-frequency performance of a common-emitter amplifier?
  5. What is the aim of a transistor with a Schottky barrier diode connected across its base-collector junction (a Schottky clamp)?