Junction Transistors (BJT) Questions and Answers

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Q11
Which configuration provides maximum input impedance for a BJT amplifier?
  • A Common-collector (emitter follower)
  • B Common-base
  • C Common-emitter
  • D Differential pair
Answer: Option A
Explanation: The common-collector (emitter follower) configuration yields high input impedance and low output impedance.
Q12
In a common-emitter amplifier, a 180° phase shift appears between input and output because?
  • A Because collector and base currents are equal
  • B Because collector voltage is inverted relative to base input
  • C Because emitter is floating
  • D Because base-emitter junction conducts in reverse
Answer: Option B
Explanation: Common-emitter inverts signal; collector voltage swings opposite to base/emitter input, causing 180° phase shift.
Q13
What happens to collector current if base-emitter forward bias increases slightly (in active region)?
  • A Collector current decreases with VBE
  • B Collector current increases approximately exponentially with VBE
  • C Collector current is linear with VBE
  • D Collector current independent of VBE
Answer: Option B
Explanation: Collector current increases approximately exponentially with base-emitter voltage (IC ≈ Is * e^(VBE/VT)).
Q14
Which parameter is reduced by adding emitter degeneration resistor in series with emitter?
  • A Voltage gain increases
  • B Gain is reduced and linearity improved
  • C Transistor becomes unstable
  • D Input impedance drastically decreases
Answer: Option B
Explanation: Emitter degeneration reduces gain but improves linearity and stability by negative feedback.
Q15
Which junctions are present in a PNP transistor?
  • A p-type emitter, n-type base, p-type collector
  • B n-type emitter, p-type base, n-type collector
  • C All n-type regions
  • D Metal-semiconductor junction only
Answer: Option A
Explanation: PNP transistor has p-type emitter, n-type base, p-type collector — junctions are p-n and n-p accordingly.
Q16
What is the approximate VBE (base-emitter voltage) for silicon BJTs in forward active region?
  • A Approximately 0.7 V
  • B Approximately 5 V
  • C Approximately 0.01 V
  • D Approximately 2 V
Answer: Option A
Explanation: For silicon BJTs VBE is typically around 0.6–0.8 V at normal bias and currents (≈0.7 V commonly used).
Q17
Which effect causes collector current to slightly depend on collector-base voltage even in active mode?
  • A Early effect (base-width modulation)
  • B Zener effect
  • C Hall effect
  • D Photoelectric effect
Answer: Option A
Explanation: Base-width modulation (Early effect) makes IC dependent on VCB; represented by Early voltage.
Q18
What is the usual role of the base region in a BJT?
  • A To be thin and lightly doped so carriers pass with minimal recombination
  • B To be heavily doped so it supplies carriers
  • C To act as a perfect insulator
  • D To supply majority holes only
Answer: Option A
Explanation: Base is thin and lightly doped to allow majority carriers from emitter to diffuse through to collector with minimal recombination.
Q19
Why is the emitter usually heavily doped compared to base?
  • A To increase injection efficiency and gain
  • B To reduce collector-emitter voltage
  • C To make base thicker
  • D To convert transistor to FET
Answer: Option A
Explanation: Heavy emitter doping increases injection efficiency so more carriers are injected into base, increasing transistor gain.
Q20
Which BJT configuration has unity voltage gain approximately but high current gain?
  • A Common-collector has unity voltage gain and high current gain
  • B Common-base has unity current gain
  • C Common-emitter has unity voltage gain
  • D None of the above
Answer: Option A
Explanation: Common-collector (emitter follower) has ~unity voltage gain and high current gain, used as a buffer.

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