Bipolar transistors

Which bias condition is correct when an NPN transistor is used for amplification (operated in the active region)?

  1. ABase-emitter forward-biased, base-collector reverse-biased
  2. BBase-emitter reverse-biased, base-collector forward-biased
  3. CBoth base-emitter and base-collector forward-biased
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Answer: A. Base-emitter forward-biased, base-collector reverse-biased

In the active region, the base-emitter junction is forward-biased to inject carriers from the emitter into the base, and the base-collector junction is reverse-biased to sweep those carriers into the collector. When both junctions are forward-biased, the transistor is in the saturation region, which corresponds to a switch in the on state.

NPN transistor in the active region Emitter n⁺ Base p Collector n B-E junction: forward bias B-C junction: reverse bias Most electrons injected from the emitter cross the base to the collector A thinner base means less recombination and higher current gain
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More Bipolar transistors questions

  1. Which expression gives the DC current gain hFE of a bipolar transistor? (IC: collector current, IB: base current, IE: emitter current)
  2. What is the main reason for making the base of a bipolar transistor thin?
  3. When a bipolar transistor is used as a switch, which operating region corresponds to the on state?
  4. How does the phase of the output voltage of a common-emitter amplifier compare with the input voltage?
  5. What is the main reason for making the emitter of an NPN transistor more heavily doped than the base?