Bipolar transistors

When a bipolar transistor is used as a switch, which operating region corresponds to the on state?

  1. ACutoff region
  2. BActive region
  3. CSaturation region
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Answer: C. Saturation region

As a switch, the off state corresponds to the cutoff region, where both junctions are reverse-biased, and the on state to the saturation region, where both are forward-biased. In saturation, the collector-emitter voltage is small, close to a closed switch. The active region is used for amplification.

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  1. How does the phase of the output voltage of a common-emitter amplifier compare with the input voltage?
  2. What is the main reason for making the emitter of an NPN transistor more heavily doped than the base?
  3. What is the approximate common-emitter current gain β of a bipolar transistor whose common-base current gain α is 0.99?
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