20 Important BJT Previous Year Questions for Electronics JE Level
Topic: Bipolar Junction Transistor (BJT) – Theory, Numericals & Conceptual Questions
1. Relation among transistor currents
In a transistor, the relation among emitter, base, and collector currents is —
(A) \( I_E = I_C - I_B \)
(B) \( I_E = I_B + I_C \)
(C) \( I_C = I_E + I_B \)
(D) \( I_B = I_E + I_C \)
Answer: (B)
2. Input characteristic of CB configuration
The input characteristic of a CB configuration is similar to —
(A) PN junction forward bias
(B) PN junction reverse bias
(C) Zener diode
(D) Rectifier
Answer: (A)
3. Base current calculation
If a transistor has β = 100 and \( I_C = 5\,mA \), find \( I_B \).
\( I_B = \frac{I_C}{\beta} = \frac{5}{100} = 0.05\,mA = 50\,\mu A \)
Answer: 50 μA
4. Collector current variation
In CE configuration, if \( V_{CE} \) increases, the collector current —
(A) Increases slightly
(B) Decreases
(C) Remains constant
(D) Becomes zero
Answer: (A)
5. Relation between α and β
For a transistor, \( \alpha = 0.99 \). Find \( \beta \).
\( \beta = \frac{\alpha}{1 - \alpha} = \frac{0.99}{0.01} = 99 \)
Answer: 99
6. Configuration with maximum voltage gain
Which configuration provides maximum voltage gain?
(A) CE (B) CB (C) CC (D) Emitter follower
Answer: (A)
7. Base current from biasing network
Given \( V_{BE} = 0.7V, R_B = 200k\Omega, V_{BB} = 5V \).
\( I_B = \frac{V_{BB} - V_{BE}}{R_B} = \frac{5 - 0.7}{200k} = 21.5\,\mu A \)
Answer: 21.5 μA
8. Transistor as a switch
The transistor operates as a switch in —
(A) Active region (B) Cut-off (C) Both cut-off and saturation (D) Saturation only
Answer: (C)
9. Collector current in cut-off region
In cut-off region, collector current is —
(A) Maximum (B) Minimum (C) Zero (except leakage) (D) Equal to emitter current
Answer: (C)
10. Temperature dependence of leakage current
The leakage current \( I_{CBO} \) doubles for every —
(A) 1°C rise (B) 10°C rise (C) 20°C rise (D) 100°C rise
Answer: (B)
11. Active region condition
For silicon transistor in active region:
\( V_{BE} ≈ 0.7V \) and \( V_{CE} > 0.2V \)
Answer: True
12. Voltage gain of CC configuration
In a common collector configuration, voltage gain is —
(A) Very high (B) Unity (C) Less than 1 (D) Zero
Answer: (B)
13. Phase relationship in CE amplifier
In CE amplifier, the phase difference between input and output voltage is —
(A) 0° (B) 90° (C) 180° (D) 360°
Answer: (C)
14. Collector current calculation
If β = 200 and \( I_B = 40\,\mu A \), find \( I_C \).
\( I_C = \beta I_B = 200 \times 40\,\mu A = 8\,mA \)
Answer: 8 mA
15. Importance of biasing
In a transistor amplifier, biasing is necessary to —
(A) Stabilize operating point (B) Increase voltage gain (C) Avoid distortion (D) All of the above
Answer: (D)
16. Base current from emitter and collector currents
If \( I_E = 4.9\,mA \) and \( I_C = 4.8\,mA \), find \( I_B \).
\( I_B = I_E - I_C = 0.1\,mA \)
Answer: 0.1 mA
17. Junction biasing in saturation
In saturation, both emitter–base and collector–base junctions are —
(A) Reverse biased (B) Forward biased (C) One forward, one reverse (D) Open
Answer: (B)
18. Reason for thin base
In transistor, base is thin and lightly doped because —
(A) To reduce recombination (B) To increase α (C) To control \( I_C \) (D) Both A and B
Answer: (D)
19. Meaning of hfe
The transistor parameter \( h_{fe} \) represents —
(A) Voltage gain in CE (B) Current gain in CE (C) Power gain (D) Input resistance
Answer: (B)
20. Input resistance of CE amplifier
In CE amplifier, input resistance is —
(A) Low (B) High (C) Very high (D) Zero
Answer: (A)
Check also: MCQs on power ELECTRONICS
✅ Quick Formula Summary
- \( I_E = I_B + I_C \)
- \( \beta = \frac{I_C}{I_B} \)
- \( \alpha = \frac{I_C}{I_E} \)
- \( \beta = \frac{\alpha}{1 - \alpha} \)
- Active Region: Emitter–Base Forward, Collector–Base Reverse
- Saturation Region: Both Junctions Forward
- Cut-off Region: Both Junctions Reverse
These 20 BJT questions are ideal for JE-level exams like SSC JE, DRDO, IOCL, ISRO, ECIL, and BSNL JE
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