BJT MCQs

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

Question 1

Electronic Devices & CircuitsBJT

For a BJT operating in the active region, the collector current is approximately related to base current by?

AIC = IB/beta
BIC = beta IB
CIC = IB + beta
DIC = beta/IB

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Correct Answer: B. IC = beta IB

Explanation: The common-emitter current gain beta is defined approximately as IC/IB in the active region, hence IC = beta IB.

Reference: Standard transistor electronics: BJT current gain

Question 2

Electronic Devices & CircuitsBJT

A transistor has beta = 100 and base current IB = 20 microampere. Neglecting leakage, its active-region collector current is approximately?

A0.2 mA
B2 mA
C20 mA
D200 mA

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Correct Answer: B. 2 mA

Explanation: IC = beta IB = 100 × 20 microampere = 2000 microampere = 2 mA.

Reference: Standard transistor electronics: BJT bias calculation

Question 3

Electronic Devices & CircuitsBJT

For a BJT, emitter current IE is related to collector and base currents by?

AIE = IC - IB
BIE = IC + IB
CIE = IC/IB
DIE = IB - IC

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Correct Answer: B. IE = IC + IB

Explanation: By current continuity in a BJT, the emitter current equals the sum of collector and base currents: IE = IC + IB.

Reference: Standard transistor electronics: BJT currents

Question 4

Electronic Devices & CircuitsBJT

The relation between common-base current gain alpha and common-emitter current gain beta is?

Abeta = 1 - alpha
Bbeta = alpha/(1-alpha)
Cbeta = (1-alpha)/alpha
Dbeta = 1/alpha

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Correct Answer: B. beta = alpha/(1-alpha)

Explanation: Using alpha = IC/IE and beta = IC/IB with IE = IC + IB gives beta = alpha/(1-alpha).

Reference: Standard transistor electronics: alpha-beta relation

Question 5

Electronic Devices & CircuitsBJT

If a transistor has alpha = 0.98, its beta is approximately?

A0.98
B49
C50
D98

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Correct Answer: B. 49

Explanation: beta = alpha/(1-alpha) = 0.98/0.02 = 49.

Reference: Standard transistor electronics: current-gain calculation

Question 6

Electronic Devices & CircuitsBJT

A BJT has beta = 99. Its common-base current gain alpha is approximately?

A0.50
B0.90
C0.99
D1.99

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Correct Answer: C. 0.99

Explanation: alpha = beta/(beta+1) = 99/100 = 0.99.

Reference: Standard transistor electronics: beta-alpha conversion

Question 7

Electronic Devices & CircuitsBJT

A transistor has IC = 4.95 mA and IB = 50 microampere. Its beta is approximately?

A9.9
B49.5
C99
D990

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Correct Answer: C. 99

Explanation: beta = IC/IB = 4.95 mA / 0.05 mA = 99.

Reference: Standard transistor electronics: BJT gain

Question 8

Electronic Devices & CircuitsBJT

In the BJT active region, the common-base current gain alpha is related to common-emitter beta by?

Aalpha = beta/(beta+1)
Balpha = beta+1
Calpha = 1/beta
Dalpha = beta-1

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Correct Answer: A. alpha = beta/(beta+1)

Explanation: Since IE=IC+IB and beta=IC/IB, alpha=IC/IE=beta/(beta+1).

Reference: Standard transistor electronics: alpha-beta relation

Question 9

Electronic Devices & CircuitsBJT

A BJT has beta=99. Its common-base current gain alpha is approximately?

A0.50
B0.90
C0.99
D1.99

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Correct Answer: C. 0.99

Explanation: alpha=beta/(beta+1)=99/100=0.99.

Reference: Standard transistor electronics: alpha calculation

Question 10

Electronic Devices & CircuitsBJT

The Early effect in a BJT refers primarily to variation of collector current with collector-emitter voltage due to?

ABase-width modulation
BGate-oxide tunneling
CZener regulation
DEmitter resistance only

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Correct Answer: A. Base-width modulation

Explanation: Increasing collector-base reverse bias widens the depletion region into the base, reducing effective base width and changing collector current; this is the Early effect.

Reference: Standard transistor electronics: Early effect