Case Study:

In a local electronics workshop in Mumbai, a technician was repairing an audio amplifier used in a home speaker system. The device used a common emitter transistor configuration to amplify weak audio signals received from a mobile phone.

The transistor amplifier works by taking a small input signal at the base and producing a larger output signal at the collector. The current gain (β) of the transistor plays a crucial role in amplification and is given by:

 β =  I_C/I_B

In this circuit, the base current (I₍B₎) was measured as 20 μA, and the transistor had a current gain of 100. Therefore, the collector current (I₍C₎) became:

I₍C₎ = β × I₍B₎ = 100 × 20 μA = 2000 μA = 2 mA

This amplified current was used to drive the speaker, producing audible sound.

However, the technician noticed distortion in output. On inspection, it was found that improper biasing of the transistor caused the signal to clip. After adjusting the biasing resistors, the output became smooth and distortion-free.

This case highlights how transistor amplifiers are essential in communication devices, audio systems, and broadcasting equipment. It also demonstrates the importance of proper circuit design, biasing, and gain control in ensuring efficient signal amplification.

Questions:

Section A - MCQs

1. What is the primary function of a transistor amplifier?

A. Store energy
B. Amplify weak signals
C. Reduce voltage
D. Generate current

2. If base current increases, what happens to collector current (assuming constant β)?

A. Decreases
B. Remains constant
C. Increases proportionally
D. Becomes zero

3. What caused distortion in the amplifier output?

A. High voltage
B. Improper biasing
C. Low resistance
D. High frequency

4. What is the collector current if β = 100 and base current = 10 μA?

A. 1 mA
B. 10 mA
C. 100 mA
D. 0.1 mA

Section B - Short Answer Questions

1. Calculate the collector current when β = 100 and base current = 20 μA.

2. Explain the importance of proper biasing in a transistor amplifier.

3. Why is a common emitter configuration widely used for amplification?

Section C - Long Answer Question

1. If the current gain of the transistor decreases due to temperature changes, how will it affect the amplifier’s performance? Analyze its impact on output signal strength and circuit stability.

Answer Key:

Section A - MCQs Answers:

1. B
2. C
3. B
4. A

Section B - Short Answers:

1. I₍C₎ = β × I₍B₎ = 100 × 20 μA = 2 mA

2. Proper biasing ensures the transistor operates in the active region, preventing distortion and enabling faithful amplification of signals.

3. It provides high current and voltage gain, making it suitable for amplifying weak signals efficiently.

Section C - Long Answer:

• Decrease in β reduces collector current
• Output signal becomes weaker
• Amplification reduces
• Circuit may become unstable
• Requires adjustment in biasing for proper operation
• Affects overall performance of the amplifier