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BJT Amplifiers MCQs

Practice BJT Amplifiers MCQs with four options, answers and explanations for focused Electronics & Telecommunication Engineering revision.

Question 1
Electronic Devices & CircuitsBJT Amplifiers

A common-emitter amplifier normally produces an output voltage that is approximately how far out of phase with the input voltage?

  • A0 degrees
  • B90 degrees
  • C180 degrees
  • D360 degrees only
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Correct Answer: C. 180 degrees
Explanation: A common-emitter voltage amplifier is inverting, so its output voltage is approximately 180 degrees out of phase with its input.
Reference: Standard analog electronics: common-emitter amplifier
Question 2
Electronic Devices & CircuitsBJT Amplifiers

Which BJT configuration typically has high voltage gain, high current gain, and a 180-degree phase reversal?

  • ACommon base
  • BCommon emitter
  • CCommon collector
  • DEmitter follower with unity current gain
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Correct Answer: B. Common emitter
Explanation: The common-emitter configuration provides both voltage and current gain and introduces phase inversion, making it widely used for amplification.
Reference: Standard analog electronics: transistor configurations
Question 3
Electronic Devices & CircuitsBJT Amplifiers

The common-collector BJT configuration is also called an?

  • AEmitter follower
  • BCollector follower
  • CBase follower
  • DDifferential pair
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Correct Answer: A. Emitter follower
Explanation: In a common-collector stage, the output is taken from the emitter, which approximately follows the input voltage; hence the name emitter follower.
Reference: Standard analog electronics: emitter follower
Question 4
Electronic Devices & CircuitsBJT Amplifiers

Emitter degeneration using an unbypassed emitter resistor in a common-emitter amplifier generally provides?

  • APositive feedback and unstable bias
  • BNegative feedback and improved bias stability
  • CInfinite voltage gain
  • DZero input resistance
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Correct Answer: B. Negative feedback and improved bias stability
Explanation: An emitter resistor develops degenerative negative feedback, helping stabilize the operating point and improving linearity, though it reduces gain.
Reference: Standard analog electronics: emitter degeneration
Question 5
Electronic Devices & CircuitsBJT Amplifiers

A common-base BJT amplifier typically has a current gain that is?

  • AMuch greater than beta
  • BApproximately alpha and slightly less than unity
  • CExactly zero
  • DAlways negative
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Correct Answer: B. Approximately alpha and slightly less than unity
Explanation: The common-base current gain is alpha = IC/IE, normally close to but less than 1.
Reference: Standard analog electronics: common-base configuration
Question 6
Electronic Devices & CircuitsBJT Amplifiers

The voltage gain of an ideal emitter follower is approximately?

  • A-100
  • B-1
  • C0
  • D+1
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Correct Answer: D. +1
Explanation: A common-collector or emitter-follower stage has a non-inverting voltage gain slightly less than unity in practice and approximately +1 ideally.
Reference: Standard analog electronics: emitter follower
Question 9
Electronic Devices & CircuitsBJT Amplifiers

In a common-emitter amplifier, bypassing the emitter resistor with a sufficiently large capacitor generally causes the midband voltage gain magnitude to?

  • ADecrease substantially
  • BIncrease
  • CBecome exactly zero
  • DBecome independent of transistor parameters in all cases
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Correct Answer: B. Increase
Explanation: At signal frequencies where the capacitor effectively bypasses the emitter resistor, emitter degeneration is reduced, so the voltage-gain magnitude generally increases.
Reference: Standard analog electronics: emitter bypass capacitor
Question 10
Electronic Devices & CircuitsBJT Amplifiers

A transistor amplifier has voltage gain Av = -50. The negative sign indicates?

  • APower loss only
  • BA 180-degree phase inversion between input and output voltage
  • CThe transistor is necessarily in cutoff
  • DThe output frequency is negative
Report an error
Correct Answer: B. A 180-degree phase inversion between input and output voltage
Explanation: A negative voltage gain indicates that the output voltage is inverted relative to the input, corresponding to approximately 180 degrees phase difference.
Reference: Standard analog electronics: amplifier phase inversion