Bipolar transistors

Which condition increases the small-signal input resistance (hie) of a common-emitter amplifier?

  1. AReducing the collector current, or using a transistor with a higher current gain
  2. BIncreasing the collector current
  3. CLowering the temperature
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Answer: A. Reducing the collector current, or using a transistor with a higher current gain

The common-emitter input resistance is roughly hie ≈ hFE × VT / IC. Reducing the collector current or using a transistor with higher hFE increases the input resistance. The transconductance decreases correspondingly, so the operating point is chosen to balance this against gain.

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More Bipolar transistors questions

  1. What is the advantage of an active load, which uses a transistor acting as a current source instead of a load resistor in an amplifier?
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