NPN Transistor Questions

---

Very Short Answers Questions [1 Mark Questions]

Ques. When an NPN transistor is used as an amplifier

1. Holes move from emitter to base
2. Electrons move from the base to the collector
3. Electrons move from the collector to the base
4. Holes move from base to emitter

Ans. The correct answer is b. Electrons move from the base to the collector

Explanation: When the NPN transistor is used as an amplifier, the majority charge carriers electrons of the N-type emitter move from emitter to base and then from base to collector.

Ques. Which region of the transistor is highly doped?

1. Base
2. Emitter
3. Collector
4. Both Emitter and Collector

Ans. The correct answer is b. Emitter

Explanation: The emitter of a transistor is of moderate size and highly doped. The collector is moderately doped and bigger than the emitter. The base is very thin and lightly doped.

Ques. What are the majority charge carriers in NPN transistors?

1. Electrons
2. Holes

Ans. The correct answer is a. Electrons

Explanation: The majority charge carriers in NPN transistors are electrons.

Ques. What are the operative modes of a transistor?

Ans. The operative modes of a transistor are

• Cut-off mode
• Saturation mode
• Active mode

Ques. Which junction is forward-biased when a transistor is used as an amplifier?

1. Collector-Base
2. Emitter-Collector
3. Emitter-Base
4. No junction is forward-biased

Ans. The correct answer is c. Emitter-Base

Explanation: For the use of a transistor as an amplifier, the emitter-base junction must be forward-biased and the base-collector region must be reverse-biased. This is referred to as an active state.

---

Short Answers Questions [2 Marks Questions]

Ques. What is a transistor?

Ans. A transistor is a semiconductor device that is used to amplify or switch electrical signals and power. It is a fundamental component of modern electronics. It is typically made of semiconductor material and has at least three terminals for connecting to an electronic circuit.

Ques. What is an NPN transistor?

Ans. NPN transistors are three-layer bipolar transistors that are used for signal amplification. This device is controlled by current. The full form of the NPN transistor is a negative-positive-negative transistor.

Ques. What is a Field Effect Transistor?

Ans. A field effect transistor is a type of transistor that controls the flow of current in a semiconductor by using an electric field. 

There are two types of FETs: Junction-gate FETs and metal-oxide semiconductor FETs.

There are three terminals on FETs: Gate, Drain, and Source.

Ques. What is a PNP transistor?

Ans. A PNP transistor is a bipolar junction transistor made by sandwiching an N-type doped semiconductor between two identical P-type doped semiconductors.

Read More:

---

Long Answers Questions [3 Marks Questions]

Ques. What is the difference between an NPN transistor and a PNP transistor?

Ans. The differences between an NPN transistor and a PNP transistor are

Ques. What are the applications of NPN transistors?

Ans. The following are the applications of NPN transistors

• NPN transistors are most commonly used in switching applications.
• They are also used in amplifying circuits.
• Darlington pair circuits use NPN transistors to amplify weak signals.
• NPN transistors are used in applications where sinking current is required.
• NPN transistors, like 'push-pull' amplifier circuits, are used in several classic amplifier circuits.
• Temperature sensors use NPN transistors.
• They are commonly used in high-frequency applications.

Ques. Why is n-p-n preferred over p-n-p?

Ans. In an n-p-n transistor, electrons are the majority charge carriers, whereas holes are the majority charge carriers in a p-n-p transistor. 

• Electrons have more mobility than holes because their effective mass is greater than electrons' effective mass. 
• Thus, n-p-n is preferable because electrons have more mobility than holes, resulting in high energy mobility. 
• The positive supply line of a p-n-p transistor forms a common point of input and output current, resulting in a positive ground current. 
• This is inconvenient for design and maintenance.
• The negative supply line of an n-p-n transistor forms a common source of input and output current, resulting in a negative ground current. This is suitable for design and maintenance.

---

Very Long Answers Questions [5 Marks Questions]

Ques. In an NPN transistor, 108 electrons enter the emitter in 10^-8 s. If 1% of electrons are lost in the base, what fraction of current enters the collector? Also, find the current amplification factor.

Ans. Given

• Number of electrons enter the emitter, n = 10⁸
• Time taken by the electrons to enter the emitter, t = 10^-8 s

Current flowing through the emitter is given by

I_E = ne/t

Where e is the charge of an electron i.e. 1.6 x 10^-19 C

On substituting the values, we get

⇒ I_E = (10⁸ x 1.6 x 10^-19)/10^-8

⇒ I_E = 172.8 x 10^-11 A

Given that the 1% of electrons are lost in the base i.e.

1% of I_E is lost in the base, therefore the base current is given by

I_B = I_E/100 = 0.01I_E

The above statement clearly specifies that 99% of the emitter current enters the collector i.e.

Collector current, IC = 99IE/100

⇒ I_C = 0.99IE

Now the current amplification factor is given by

ꞵ = I_C/I_B

⇒ ꞵ = (0.99IE)/(0.01IE)

⇒ ꞵ = 99

Hence the current amplification factor is 99.

Ques. Consider an NPN transistor amplifier in a common emitter configuration. The current gain of the transistor is 100. If the collector current changes by 1 mA, what will be the change in emitter current?

Ans. Given

• The current gain of the transistor, ꞵ = 100
• Change in the collector current, ΔI_C = 1 mA = 1 x 10^-3 A

The current gain in a transistor is given by

ꞵ = ΔI_C/ΔI_B

Where ΔIB is the change in the base current.

⇒ ΔI_B = ΔI_C/ꞵ

On substituting the values, we get

⇒ ΔI_B = (1 x 10^-3)/100

⇒ ΔI_B = 10^-5 A = 0.01 mA

Also, we have

ΔI_E = ΔI_B + ΔI_C

On substituting the values of ΔIB and ΔIC we get

ΔI_E = 0.01 + 1 = 1.01 mA

Hence the change in the emitter current will be 1.01 mA.

Ques. An NPN transistor is connected in a common emitter configuration in a given amplifier. A load resistance of 800 Ω is connected to the collector circuit and the voltage drop across it is 0.8 V. If the current amplification factor is 0.96 and the input resistance of the circuit is 192 Ω, what will be the voltage gain and power gain of the amplifier?

Ans. Given

• The current amplification factor in the common base configuration, ɑ = 0.96
• Load resistance, R_L = 800 Ω
• Input resistance, R_i = 192 Ω

The relationship between the current amplification factor in common base configuration and common emitter configuration is given by

β = ɑ/(1-ɑ)

⇒ β = 0.96/(1 - 0.96) = 24

Voltage gain for common emitter configuration is given by

A_V = β x R_L/R_i

On substituting the values, we get

A_V = 24 x (800/192) = 100

Power gain for common emitter configuration is given by

P_V = β x A_V

⇒ P_V = 24 x 100 = 2400

Voltage gain for common base configuration

A_V = ɑ x (R_L/R_i)

⇒ A_V = 0.96x (800/192) = 4

Power gain for common base configuration is given by

P_V = ɑ x A_V

⇒ P_V = 0.96 x 4 = 3.84

---

Previous Year Questions

1. Extraction of metal from the ore cassiterite involves...[JEE Advanced 2011]
2. Commonly used vectors for human genome sequencing are...[NEET UG 2014]
3. Interfascicular cambium and cork cambium are formed due to​..
4. Pneumotaxic centre is present in​...[UP CPMT 2007]
5. Reaction of HBr with propene in the presence of peroxide gives….[NEET UG 2004]
6. Assuming the expression for the pressure exerted by the gas on the walls of the container, it can be shown that pressure is...[MHT CET 2016]
7. Which among the following is the strongest acid?...[TS EAMCET 2017]
8. Isopropyl alcohol on oxidation forms​..
9. A vector is not changed if​..
10. Which of the following arrangements does not represent the correct order of the property stated against it?...[JEE Main 2013]

For Latest Updates on Upcoming Board Exams, Click Here: https://t.me/class_10_12_board_updates

---

Check-Out: