Bipolar transistors

Check the basics of bipolar transistors — NPN/PNP structure, bias conditions and current gain — with multiple-choice questions.

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Bipolar transistors: key points

A bipolar transistor has a three-layer structure of emitter, base and collector, and both electrons and holes take part in its operation. The base current controls the collector current, amplifying it.

For amplification (the active region), the base-emitter junction is forward-biased and the base-collector junction is reverse-biased. Understand the current gain hFE, the relationship between base width and recombination, and the meaning of the saturation and cutoff regions.

All Bipolar transistors questions (50)

  1. Which bias condition is correct when an NPN transistor is used for amplification (operated in the active region)?
  2. Which expression gives the DC current gain hFE of a bipolar transistor? (IC: collector current, IB: base current, IE: emitter current)
  3. What is the main reason for making the base of a bipolar transistor thin?
  4. When a bipolar transistor is used as a switch, which operating region corresponds to the on state?
  5. How does the phase of the output voltage of a common-emitter amplifier compare with the input voltage?
  6. What is the main reason for making the emitter of an NPN transistor more heavily doped than the base?
  7. What is the approximate common-emitter current gain β of a bipolar transistor whose common-base current gain α is 0.99?
  8. Which correctly describes the Early effect in a bipolar transistor?
  9. Which is a characteristic of a Darlington connection of two transistors?
  10. In a PNP transistor, which carriers are injected from the emitter into the base and mainly carry the current?
  11. Which is a characteristic of a common-collector amplifier (emitter follower)?
  12. Which is a characteristic of a common-base amplifier?
  13. What does the transition frequency fT of a bipolar transistor mean?
  14. Which correctly describes thermal runaway in a bipolar transistor?
  15. Which is a characteristic of a SiGe HBT (heterojunction bipolar transistor)?
  16. What is the typical collector-emitter voltage of a silicon bipolar transistor in the saturation region?
  17. Which circuit uses two matched transistors to make the current in one transistor equal to the current in the other?
  18. What does a differential pair, with the emitters of two transistors connected to a common current source, do?
  19. What is the approximate transconductance gm of a bipolar transistor operating at a collector current of 1 mA? (Room temperature, thermal voltage VT ≈ 26 mV)
  20. What is the main use of a photocoupler, which combines an LED and a phototransistor in one package?
  21. What is the main reason for using a current-feedback bias circuit with an emitter resistor in a transistor amplifier?
  22. In an amplifier, what is the DC voltage and current state of the transistor with no input signal called?
  23. Which distortion tends to occur in a class B push-pull amplifier in which NPN and PNP transistors handle the positive and negative half-cycles?
  24. What is the role of a coupling capacitor placed between amplifier stages?
  25. How does the current gain hFE of a silicon bipolar transistor generally change as temperature rises?
  26. Which condition increases the small-signal input resistance (hie) of a common-emitter amplifier?
  27. 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?
  28. When a switching transistor is turned off from deep saturation, what is the main reason it takes time to turn off?
  29. Which is a typical family of logic ICs built with bipolar transistors?
  30. Which pairing of a technology that integrates bipolar transistors and CMOS on the same chip with its purpose is correct?
  31. Which set correctly names the three terminals of a bipolar transistor?
  32. In the circuit symbol of an NPN transistor, which way does the arrow on the emitter point?
  33. Which correctly relates the emitter current IE, base current IB and collector current IC of a bipolar transistor?
  34. A transistor with DC current gain hFE = 100 has a base current of 10 μA. Roughly what is the collector current in the active region?
  35. What is the state of the junctions when a bipolar transistor is in the cutoff region (off)?
  36. What is the typical base-emitter voltage VBE of a silicon bipolar transistor operating in the active region?
  37. What is the collector dissipation of a transistor operating at a collector-emitter voltage of 5 V and a collector current of 100 mA?
  38. Which correctly describes the difference in how bipolar transistors and MOSFETs are controlled?
  39. In a common-emitter amplifier with transconductance gm = 40 mS and collector resistor RC = 5 kΩ, roughly what is the magnitude of the voltage gain? (Ignore the load.)
  40. What is the state of the junctions when a bipolar transistor is in the saturation region?
  41. When the output characteristics (IC-VCE) of a common-emitter transistor are plotted with base current IB as a parameter, what is characteristic of the active region?
  42. What does the load line drawn on the output characteristics of a transistor amplifier represent?
  43. Which value is closest to the maximum power efficiency of an ideal class B push-pull amplifier?
  44. In a circuit using a transistor as a switch, what is the main purpose of a resistor in series with the base?
  45. What is the difference between the DC current gain hFE and the small-signal current gain hfe?
  46. Which model represents the large-signal behavior of a bipolar transistor with two diodes and current sources?
  47. How does the current gain hFE of a bipolar transistor depend on collector current?
  48. Which correctly relates BVCEO, the collector-emitter breakdown voltage with the base open, and BVCBO, the collector-base breakdown voltage with the emitter open?
  49. Which correctly describes the Miller effect that degrades the high-frequency performance of a common-emitter amplifier?
  50. What is the aim of a transistor with a Schottky barrier diode connected across its base-collector junction (a Schottky clamp)?

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