BJT & Biasing
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๐ Comprehensive Syllabus & Examination Guide
BJT & Biasing
Official curriculum roadmap, subject/topic distribution, negative marking rules, pacing guidelines, and solved sample questions.
๐ฏ Mapped Subjects & Topic Question Distribution
Total Question Pool
100%
35 MCQs
Combined Active Syllabus
BJT & Biasing
35 MCQs
Topic Pool
๐ Question Pool Structure
35 MCQs across fundamental, intermediate, and advanced concept tiers.
โก Recommended Pacing
45 to 60 seconds per MCQ. Flag complex problems and preserve 10 minutes for final revision.
โ๏ธ Scoring & Negative Marking
+1 mark per correct answer. In competitive tests with negative marking, -0.25 applies for incorrect guesses.
๐ก Strategic Preparation & Exam Hall Guidelines
To maximize your score on BJT & Biasing, candidates are advised to follow a structured three-pass approach. In the First Pass, solve all direct recall and formula-based questions within 30 seconds each to secure foundational marks. In the Second Pass, tackle multi-step analytical and quantitative reasoning problems. In the Third Pass, review marked questions and verify calculations.
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๐ Pre-Rendered Solved Sample Questions & Detailed Solutions
Review the solved problems below to understand question phrasing, answer choices, and step-by-step solution logic prior to starting the full interactive practice drill:
Sample Question 1
BJT & Biasing
Easy • Electronics Engineering
In a Bipolar Junction Transistor (BJT), the three terminals and their relative doping concentrations are:
A
Emitter (heavily doped), Base (lightly doped and very thin), Collector (moderately doped and physically largest)
B
Base (heavily doped), Emitter (lightly doped), Collector (thin)
C
Collector (heavily doped), Emitter (moderately doped), Base (wide)
D
All three regions have identical doping concentrations
โ Correct Answer: A - Emitter (heavily doped), Base (lightly doped and very thin), Collector (moderately doped and physically largest)
๐ Step-by-Step Solution & Conceptual Rationale:
The Emitter is heavily doped to inject large numbers of majority carriers. The Base is lightly doped and extremely thin to minimize carrier recombination (< 1-2%). The Collector is moderately doped and physically largest to dissipate heat generated by carrier collection.
Sample Question 2
BJT & Biasing
Easy • Electronics Engineering
For a BJT to operate in the Active (Linear Amplification) region, the junction biasing conditions must be:
A
Emitter-Base junction forward-biased, and Collector-Base junction reverse-biased
B
Both junctions forward-biased
C
Both junctions reverse-biased
D
Emitter-Base reverse-biased, Collector-Base forward-biased
โ Correct Answer: A - Emitter-Base junction forward-biased, and Collector-Base junction reverse-biased
๐ Step-by-Step Solution & Conceptual Rationale:
In Active mode: EB junction is forward-biased to inject carriers into the base, and CB junction is reverse-biased to sweep carriers into the collector. Saturation mode requires both forward-biased. Cutoff mode requires both reverse-biased.
Sample Question 3
BJT & Biasing
Easy • Electronics Engineering
In a BJT operating in the Cutoff region, the transistor behaves as a(n):
A
Closed switch (short circuit)
B
Open switch (off state) with collector current near zero (IC = ICEO)
C
Linear amplifier with unity gain
D
Constant current source
โ Correct Answer: B - Open switch (off state) with collector current near zero (IC = ICEO)
๐ Step-by-Step Solution & Conceptual Rationale:
When both EB and CB junctions are reverse-biased, majority carrier injection ceases completely. The transistor acts as an open circuit with only microamps/nanoamps of reverse leakage current flowing, representing digital logic '0'.
Sample Question 4
BJT & Biasing
Easy • Electronics Engineering
In a BJT operating in the Saturation region, the collector-emitter saturation voltage (VCE_sat) for a Silicon transistor is approximately:
A
0.2 V
B
0.7 V
C
5.0 V
D
Zero strictly
โ Correct Answer: A - 0.2 V
๐ Step-by-Step Solution & Conceptual Rationale:
In saturation, both junctions are forward-biased (VBE โ 0.7 V, VBC โ 0.5 V). Collector-emitter voltage drops to VCE_sat = VBE - VBC โ 0.7 - 0.5 = 0.2 V. The transistor acts as a closed switch carrying maximum collector current limited only by the external load.
Sample Question 5
BJT & Biasing
Easy • Electronics Engineering
The common-base DC current gain (alpha) and common-emitter DC current gain (beta) of a BJT are related by:
A
beta = alpha / (1 - alpha)
B
alpha = beta / (1 - beta)
C
beta = alpha / (1 + alpha)
D
beta = 1 / alpha
โ Correct Answer: A - beta = alpha / (1 - alpha)
๐ Step-by-Step Solution & Conceptual Rationale:
Since IE = IB + IC, dividing by IC gives 1/alpha = 1/beta + 1, which rearranges to beta = alpha / (1 - alpha), and inversely alpha = beta / (1 + beta). For example, if alpha = 0.99, beta = 0.99 / (1 - 0.99) = 99.
Sample Question 6
BJT & Biasing
Medium • Electronics Engineering
The Early Effect (base-width modulation) in a BJT refers to the phenomenon where:
A
Increasing reverse bias across the Collector-Base junction widens the depletion layer into the lightly doped base, reducing effective neutral base width
B
Emitter current drops at early morning temperatures
C
Base resistance doubles with frequency
D
Transistor breaks down prematurely
โ Correct Answer: A - Increasing reverse bias across the Collector-Base junction widens the depletion layer into the lightly doped base, reducing effective neutral base width
๐ Step-by-Step Solution & Conceptual Rationale:
As reverse voltage VCB increases, the CB space-charge region penetrates deeper into the thin base. This decreases the effective neutral base width Wb, reducing recombination, slightly increasing alpha and beta, and giving the output characteristics a finite upward slope corresponding to the Early Voltage (VA).
Sample Question 7
BJT & Biasing
Hard • Electronics Engineering
Punch-through (or reach-through) breakdown in a BJT occurs when:
A
Collector-base reverse voltage increases to the point where the CB depletion layer expands across the entire base width and touches the EB depletion layer
B
Base current exceeds collector current
C
Emitter lead is detached
D
Thermal runaway burns the package
โ Correct Answer: A - Collector-base reverse voltage increases to the point where the CB depletion layer expands across the entire base width and touches the EB depletion layer
๐ Step-by-Step Solution & Conceptual Rationale:
When VCB is excessively high, the CB depletion region completely consumes the neutral base width, contacting the emitter space-charge boundary. The barrier collapses, and direct collector-emitter punch-through current surges, destroying transistor action.
Sample Question 8
BJT & Biasing
Medium • Electronics Engineering
Thermal runaway in a BJT is triggered because:
A
An increase in junction temperature causes collector leakage current (ICBO) to increase, which increases total collector current IC and power dissipation, causing further temperature rise cumulatively
B
Base current drops with temperature
C
Emitter resistance increases
D
VCE rises to infinity
โ Correct Answer: A - An increase in junction temperature causes collector leakage current (ICBO) to increase, which increases total collector current IC and power dissipation, causing further temperature rise cumulatively
๐ Step-by-Step Solution & Conceptual Rationale:
Collector current IC = beta*IB + (1 + beta)*ICBO. As temperature rises, ICBO increases exponentially. The higher IC increases collector power dissipation (P = IC * VCE), raising junction temperature further in an unstable positive-feedback loop that destroys the transistor.
Sample Question 9
BJT & Biasing
Hard • Electronics Engineering
To prevent thermal runaway in a common-emitter BJT amplifier, the circuit design must satisfy which stability condition?
A
VCE < VCC / 2
B
d(Pc)/d(Tj) < 1 / theta (rate of heat generation less than rate of heat dissipation through thermal resistance theta)
C
Beta must be greater than 200
D
Collector resistance must equal zero
โ Correct Answer: B - d(Pc)/d(Tj) < 1 / theta (rate of heat generation less than rate of heat dissipation through thermal resistance theta)
๐ Step-by-Step Solution & Conceptual Rationale:
Thermal stability requires that the rate at which heat is generated at the collector junction d(Pc)/dTj be strictly less than the rate at which heat is dissipated to the ambient environment (1 / theta_th). Maintaining VCE < VCC / 2 also naturally prevents thermal runaway.
Sample Question 10
BJT & Biasing
Easy • Electronics Engineering
Which BJT biasing configuration provides the HIGHEST stability factor against variations in temperature and transistor beta?
A
Fixed-bias (base bias) circuit
B
Voltage Divider Bias (Self-Bias / Potential Divider) with an emitter resistor RE
C
Collector-to-base feedback bias without emitter resistor
D
Direct coupling without resistors
โ Correct Answer: B - Voltage Divider Bias (Self-Bias / Potential Divider) with an emitter resistor RE
๐ Step-by-Step Solution & Conceptual Rationale:
Voltage divider bias with an unbypassed/bypassed emitter resistor provides exceptional negative feedback. If IC tries to rise due to temp or high beta, the voltage drop across RE (VE = IE*RE) rises, reducing VBE (VBE = VB - VE), which chokes base current IB and restores IC to its stable Q-point.
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