Transistors: Characteristics of a transistor in CB, CE and CC mode, α and β of BJT, common emitter amplifier
10 questions on each with explained answers
## Transistor Characteristics in CB, CE, CC Modes
**1. Which configuration has the lowest input resistance?**
a) Common Emitter
b) Common Collector
c) Common Base
d) All equal
**Answer: c) Common Base** [1]
**Explanation:** CB mode uses forward-biased emitter-base junction with low resistance, typically 20-50 ohms. [1]
**2. In CE configuration, output is 180° phase shifted from input.**
a) True
b) False
c) Only in saturation
d) Only in cutoff
**Answer: a) True** [2]
**Explanation:** CE provides phase inversion due to collector current variation opposing input signal. [2]
**3. Common Collector is also known as?**
a) Emitter follower
b) Base follower
c) Collector grounded
d) Voltage inverter
**Answer: a) Emitter follower** [1]
**Explanation:** Output taken from emitter follows base input voltage with unity gain. [1]
**4. CB output characteristics plot Ic vs?**
a) VCE
b) VCB at constant IE
c) VBE
d) VCC
**Answer: b) VCB at constant IE** [1]
**Explanation:** Collector current vs collector-base voltage with fixed emitter current shows early effect. [1]
**5. CE configuration provides medium input/output impedance.**
a) Low input, high output
b) High input, low output
c) Medium both
d) Very low both
**Answer: c) Medium both** [2]
**Explanation:** Balances properties for general amplification applications. [2]
**6. In CC mode, current gain is?**
a) Less than 1
b) Equal to β + 1
c) Equal to α
d) Infinite
**Answer: b) Equal to β + 1** [1]
**Explanation:** High current gain due to emitter current nearly equaling base plus collector. [1]
**7. Voltage gain in CB configuration is?**
a) Less than 1
b) High, nearly equal to CE
c) Unity
d) Zero
**Answer: b) High, nearly equal to CE** [1]
**Explanation:** Provides good voltage amplification with low input impedance. [1]
**8. Power gain highest in which configuration?**
a) CB
b) CE
c) CC
d) All same
**Answer: b) CE** [3]
**Explanation:** Combines high voltage and current gains for maximum power transfer. [3]
**9. CB used for high frequency due to?**
a) High input capacitance
b) Low Miller effect
c) High power
d) Unity gain
**Answer: b) Low Miller effect** [1]
**Explanation:** Common base minimizes feedback capacitance multiplication. [1]
**10. Input resistance order from highest to lowest?**
a) CC > CE > CB
b) CB > CE > CC
c) CE > CB > CC
d) CC > CB > CE
**Answer: a) CC > CE > CB** [3]
**Explanation:** CC very high, CE low-medium, CB very low. [3]
## α and β of BJT
**1. α is defined as?**
a) IC / IB
b) IC / IE
c) IE / IC
d) IB / IE
**Answer: b) IC / IE** [1]
**Explanation:** Common base current gain, typically 0.95-0.99. [1]
**2. β equals?**
a) α / (1 - α)
b) α / (1 + α)
c) 1 / α
d) α - 1
**Answer: a) α / (1 - α)** [4]
**Explanation:** Derives from IB = IE - IC relation. [4]
**3. If α = 0.9, then β = ?**
a) 9
b) 10
c) 0.9
d) 0.1
**Answer: a) 9** [4]
**Explanation:** β = 0.9 / (1 - 0.9) = 9. [4]
**4. Typical β value range?**
a) 0-1
b) 20-1000
c) 1-10
d) Infinite
**Answer: b) 20-1000** [5]
**Explanation:** Common emitter current gain varies by transistor type. [5]
**5. If α changes 0.5% from 0.9, % change in β?**
a) 0%
b) 2.5%
c) 5%
d) 7.5%
**Answer: c) 5%** [6]
**Explanation:** Small α change causes larger relative β shift due to inverse relation. [6]
**6. α is always?**
a) Greater than 1
b) Less than 1
c) Equal to 1
d) Negative
**Answer: b) Less than 1** [4]
**Explanation:** Some base recombination prevents perfect transmission. [4]
**7. β = 20, then α = ?**
a) 0.952
b) 0.5
c) 20
d) 21
**Answer: a) 0.952** [4]
**Explanation:** α = β / (1 + β) = 20/21 ≈ 0.952. [4]
**8. In CE amplifier, current gain is?**
a) α
b) β
c) 1/α
d) γ
**Answer: b) β** [5]
**Explanation:** IC = β IB defines amplification. [5]
**9. Relation IE = ?**
a) IB + IC
b) IC - IB
c) β IC
d) α IB
**Answer: a) IB + IC** [1]
**Explanation:** Emitter current sums base and collector components. [1]
**10. γ (common collector gain) equals?**
a) α
b) β + 1
c) 1/β
d) 1 - α
**Answer: b) β + 1** [1]
**Explanation:** IE / IB = 1 + IC/IB. [1]
## Common Emitter Amplifier
**1. Input terminals of CE amplifier?**
a) Base-emitter
b) Collector-base
c) Emitter-collector
d) Base-collector
**Answer: a) Base-emitter** [7]
**Explanation:** Signal applied across forward-biased base-emitter junction. [7]
**2. CE provides large?**
a) Current gain
b) Input impedance
c) Output impedance only
d) Voltage gain only
**Answer: a) Current gain** [7]
**Explanation:** High β provides significant current amplification. [7]
**3. Phase shift in CE output?**
a) 0°
b) 180°
c) 90°
d) 360°
**Answer: b) 180°** [2]
**Explanation:** Collector voltage falls as base current rises. [2]
**4. Output resistance in CE?**
a) Low
b) High
c) Medium
d) Zero
**Answer: b) High** [7]
**Explanation:** Due to collector circuit configuration. [7]
**5. CE used for?**
a) Impedance matching only
b) Voltage amplification
c) Current buffering
d) High frequency only
**Answer: b) Voltage amplification** [7]
**Explanation:** High voltage gain makes it ideal. [7]
**6. Grounded emitter refers to?**
a) CB
b) CE
c) CC
d) All
**Answer: b) CE** [7]
**Explanation:** Emitter common to input/output. [7]
**7. Voltage gain Av in CE depends on?**
a) RC / r_e
b) RE only
c) β only
d) VCC
**Answer: a) RC / r_e** [2]
**Explanation:** Load resistance over emitter dynamic resistance. [2]
**8. Input impedance of CE?**
a) Very high
b) Low
c) Infinite
d) Zero
**Answer: b) Low** [3]
**Explanation:** Base-emitter junction forward biased. [3]
**9. Power gain in CE compared to others?**
a) Lowest
b) Highest
c) Medium
d) Unity
**Answer: b) Highest** [3]
**Explanation:** Product of high Av and β. [3]
**10. CE amplifier output taken from?**
a) Emitter
b) Base
c) Collector
d) Junction
**Answer: c) Collector** [2]
**Explanation:** Across collector-emitter with load. [2]
