BJT MCQs | Electronics Engineering

1. A Bipolar Junction Transistor (BJT) consists of:

(A) Two PN junctions (B) Three PN junctions (C) One PN junction (D) Four PN junctions

Correct Answer: (A) Two PN junctions

Explanation:
A BJT consists of two PN junctions formed by sandwiching either a P-type semiconductor between two N-types (NPN transistor) or an N-type between two P-types (PNP transistor). These two junctions are the emitter-base junction and the collector-base junction.

2. The three terminals of a BJT are:

(A) Gate, Drain, Source (B) Anode, Cathode, Grid (C) Collector, Base, Emitter (D) Drain, Source, Substrate

Correct Answer: (C) Collector, Base, Emitter

Explanation:
The three terminals of a BJT are:
1. Emitter: Heavily doped region that emits charge carriers
2. Base: Thin and lightly doped middle region that controls carrier flow
3. Collector: Large, moderately doped region that collects carriers
These differ from FET terminals (Gate, Drain, Source).

3. In an NPN transistor, the majority charge carriers are:

(A) Electrons (B) Holes (C) Both electrons and holes (D) None of the above

Correct Answer: (A) Electrons

Explanation:
In an NPN transistor:
- Emitter (N-type): Majority carriers are electrons
- Base (P-type): Majority carriers are holes
- Collector (N-type): Majority carriers are electrons
Current flow is primarily due to electrons moving from emitter to collector through the base.

4. In a PNP transistor, the majority charge carriers are:

(A) Electrons (B) Holes (C) Both electrons and holes (D) None of the above

Correct Answer: (B) Holes

Explanation:
In a PNP transistor:
- Emitter (P-type): Majority carriers are holes
- Base (N-type): Majority carriers are electrons
- Collector (P-type): Majority carriers are holes
Current flow is primarily due to holes moving from emitter to collector through the base.

5. The collector current (I_C) in a BJT is primarily controlled by:

(A) Emitter current (I_E) (B) Collector-Base voltage (V_CB) (C) Base current (I_B) (D) Collector resistance

Correct Answer: (C) Base current (I_B)

Explanation:
In a BJT, the collector current is controlled by the base current according to the relationship I_C = βI_B, where β is the current gain. This current control mechanism makes BJTs useful as amplifiers. The base current is typically much smaller than the collector current (β >> 1).

6. The three operating regions of a BJT are:

(A) Active, Cutoff, Saturation (B) Reverse, Forward, Breakdown (C) Linear, Non-linear, Digital (D) Amplifier, Switch, Rectifier

Correct Answer: (A) Active, Cutoff, Saturation

Explanation:
BJTs operate in three distinct regions:
1. Active: Emitter-base junction forward biased, collector-base junction reverse biased (amplification)
2. Cutoff: Both junctions reverse biased (OFF state)
3. Saturation: Both junctions forward biased (ON state)
These determine whether the transistor amplifies or switches.

7. When a BJT is in cutoff, the transistor:

(A) Acts as an open switch (B) Is fully conducting (C) Amplifies signals (D) Works in the breakdown region

Correct Answer: (A) Acts as an open switch

Explanation:
In cutoff region:
- Both junctions are reverse biased
- No significant current flows (I_B ≈ 0, I_C ≈ 0)
- Behaves like an open switch between collector and emitter
- Used in digital circuits to represent logical '0' or OFF state

8. When a BJT is in saturation, the transistor:

(A) Acts as an open switch (B) Acts as a closed switch (C) Works as an amplifier (D) Has zero current

Correct Answer: (B) Acts as a closed switch

Explanation:
In saturation region:
- Both junctions are forward biased
- Maximum collector current flows (I_C ≈ V_CC/R_C)
- V_CE is very small (~0.2V for Si)
- Behaves like a closed switch between collector and emitter
- Used in digital circuits to represent logical '1' or ON state

9. The common-emitter current gain (β) is given by:

(A) β = I_B / I_C (B) β = I_C / I_E (C) β = I_C / I_B (D) β = I_E / I_B

Correct Answer: (C) β = I_C / I_B

Explanation:
β (h_FE) is the common-emitter current gain, defined as the ratio of collector current to base current. Typical values range from 20 to 1000 depending on transistor type. Related to α (common-base current gain) by β = α/(1-α). This parameter is crucial for amplifier design.

10. The voltage gain of a BJT amplifier in a common-emitter configuration is:

(A) Low (B) Unity (C) High (D) Zero

Correct Answer: (C) High

Explanation:
Common-emitter amplifiers provide:
- High voltage gain (typically 50-500)
- High current gain (β)
- 180° phase shift between input and output
- Moderate input and output impedance
The voltage gain A_V ≈ -g_mR_C, where g_m is transconductance.

11. A BJT can be used as a:

(A) Switch (B) Amplifier (C) Oscillator (D) All of the above

Correct Answer: (D) All of the above

Explanation:
BJTs are versatile devices used for:
1. Switching: Operated between cutoff and saturation (digital circuits)
2. Amplification: Operated in active region (audio/RF amplifiers)
3. Oscillators: Combined with feedback networks (LC, RC oscillators)
4. Other: Voltage regulators, current mirrors, etc.

12. A common-emitter amplifier provides:

(A) Voltage gain (B) Current gain (C) Power gain (D) All of the above

Correct Answer: (D) All of the above

Explanation:
Common-emitter configuration offers:
- Voltage gain: High (output at collector)
- Current gain: Moderate (β)
- Power gain: Product of voltage and current gains
- Phase inversion: 180° between input and output
This makes it the most widely used BJT amplifier configuration.

13. In a BJT switch, which region represents the "ON" state?

(A) Cutoff (B) Active (C) Saturation (D) Breakdown

Correct Answer: (C) Saturation

Explanation:
In switching applications:
- Saturation = ON state: Minimum V_CE, maximum I_C
- Cutoff = OFF state: Minimum I_C, maximum V_CE
The transistor switches between these two regions. Active region is avoided in switching to minimize power dissipation.

Bipolar Junction Transistor (BJT) MCQs

1. Bipolar Junction Transistor (BJT) – Basics

2. BJT Operating Regions & Characteristics

3. BJT Applications

Your Score: /13