Bipolar transistors

In a circuit using a transistor as a switch, what is the main purpose of a resistor in series with the base?

  1. ATo raise the current gain of the transistor
  2. BTo increase the collector current
  3. CTo limit the base current to a suitable value and protect the base-emitter junction and the driving circuit
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Answer: C. To limit the base current to a suitable value and protect the base-emitter junction and the driving circuit

The base-emitter junction is a forward-biased PN junction, so applying a voltage without a resistor would cause excessive current. The base resistor limits the current while being chosen to supply enough base current to drive the transistor firmly into saturation.

  • A. The current gain is a property of the transistor itself and is not raised by the base resistor.
  • B. The base resistor limits current; it is not meant to increase the collector current.
  • C. Correct. It limits base current and protects the junction and the driver.
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More Bipolar transistors questions

  1. What is the difference between the DC current gain hFE and the small-signal current gain hfe?
  2. Which model represents the large-signal behavior of a bipolar transistor with two diodes and current sources?
  3. How does the current gain hFE of a bipolar transistor depend on collector current?
  4. Which correctly relates BVCEO, the collector-emitter breakdown voltage with the base open, and BVCBO, the collector-base breakdown voltage with the emitter open?
  5. Which correctly describes the Miller effect that degrades the high-frequency performance of a common-emitter amplifier?