Classes of Amplifiers, Bipolar, JFET and MOSFET for SS 3

Class: SS 3
Term: First Term
Week: 2
Age: 16 years
Duration: 45 minutes
Subject: Basic Electronics
Curriculum Theme: Electronic Devices and Circuits
Previous Lesson: Concept and Principles of Amplifiers
Topic: AMPLIFIERS
Subject Matter: Classes of amplifier; bipolar, JFET, MOSFET, etc

Specific Objectives

By the end of the lesson, pupils/students should be able to:

Cognitive Domain

• Define an amplifier.
• Identify the main classes of amplifiers based on their operating characteristics.
• Describe the operating principle of Class A, Class B, Class AB, Class C, and Class D amplifiers.
• State the characteristics of Bipolar Junction Transistor (BJT) amplifiers.
• State the characteristics of Junction Field-Effect Transistor (JFET) amplifiers.
• State the characteristics of Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) amplifiers.

Affective Domain

• Appreciate the importance of different amplifier classes in various electronic applications.
• Show interest in learning more about transistor-based circuits.

Psychomotor Domain

• Draw simple input and output waveforms for different amplifier classes.
• Distinguish between BJT, JFET, and MOSFET amplifier symbols in circuit diagrams.

Reference Materials

The following resources were used in planning this lesson:

• 2025 Revised 9 Years Basic Education Curriculum
• Relevant State Unified Scheme of Work
• Basic Electronics textbooks for Senior Secondary School
• The HeadTeacher Scheme of work

Instructional Materials

The teacher will teach this lesson with the aid of:

• Pictures and diagrams of various amplifier circuits.
• Charts showing input and output waveforms for different amplifier classes.
• Diagrams illustrating the construction and symbols of Bipolar, JFET, and MOSFET transistors.
• Whiteboard and markers.

Rationale for the Lesson

This lesson is important for students to grasp the fundamental principles of electronic amplification, which is a core concept in modern electronics. Understanding the different classes of amplifiers and the types of transistors used helps students appreciate their diverse applications in audio systems, radio communication, and control systems. This knowledge forms a basis for further studies in electronic circuit design and analysis.

Prerequisite/Previous Knowledge

Students should have basic knowledge of transistors (BJT, JFET, MOSFET), their terminals, and how they operate as switches. They should also understand basic AC and DC circuit concepts.

Lesson Content/Board Summary

AMPLIFIERS

Definition of an Amplifier

An amplifier is an electronic device that increases the power, current, or voltage of an input signal. It takes a weak input signal and produces a stronger, amplified output signal, typically maintaining the original signal’s waveform.

Classes of Amplifiers

Amplifiers are classified based on their operating characteristics, specifically the portion of the input signal cycle during which the amplifying device conducts current. The main classes include:

1. Class A Amplifier:

• Conducts current for the entire 360° of the input signal cycle.
• The transistor is always biased in the active region.
• Offers excellent linearity and low distortion.
• Has low efficiency (typically less than 50%) because it consumes power even when there is no input signal.
• Used in low-power applications where fidelity is critical.

• Conducts current for only 180° (half) of the input signal cycle.
• Typically uses a push-pull configuration with two transistors, each amplifying one half of the waveform.
• Has higher efficiency than Class A (up to 78.5%).
• Suffers from crossover distortion at the point where the signal crosses the zero-voltage axis, as one transistor turns off and the other turns on.
• Used in battery-powered devices and general audio applications.

• Conducts current for slightly more than 180° but less than 360° of the input signal cycle.
• A compromise between Class A and Class B, designed to reduce crossover distortion.
• Each transistor conducts for a small portion of the opposite half-cycle, ensuring a smooth transition.
• Offers good linearity and better efficiency than Class A (around 50-70%).
• Widely used in high-fidelity audio amplifiers.

• Conducts current for significantly less than 180° of the input signal cycle (typically 90-150°).
• The transistor is biased such that it is off for more than half of the input cycle.
• Has very high efficiency (up to 90-100%).
• Produces high distortion due to the highly non-linear operation.
• Primarily used in radio frequency (RF) amplifiers where the output is tuned to a specific frequency (e.g., in transmitters).

• Operates as a switching amplifier, where the transistors are either fully on or fully off.
• The input signal is converted into a series of pulses (Pulse Width Modulation – PWM).
• Has very high efficiency (often over 90%) because the transistors spend little time in the active region, minimizing power dissipation.
• Requires complex circuitry for modulation and demodulation.
• Commonly used in modern audio systems, car stereos, and active subwoofers due to their compact size and high efficiency.

Amplifier Types Based on Transistor Technology

Amplifiers can also be categorized by the type of transistor used as the active amplifying device:

Bipolar Junction Transistor (BJT) Amplifiers

BJTs are current-controlled devices, meaning a small change in base current causes a large change in collector current. They are commonly used in:

1. Common Emitter (CE) Amplifier: Provides high voltage and current gain, often used as a voltage amplifier.
2. Common Collector (CC) Amplifier (Emitter Follower): Provides high current gain but voltage gain close to unity, primarily used for impedance matching.
3. Common Base (CB) Amplifier: Provides high voltage gain but current gain close to unity, often used in high-frequency applications.

BJTs are known for their high gain and good frequency response, but they are sensitive to temperature changes.

Junction Field-Effect Transistor (JFET) Amplifiers

JFETs are voltage-controlled devices, where the voltage applied to the gate controls the current flow between the drain and source. Key characteristics include:

1. Very high input impedance, making them suitable for sensitive signal amplification without loading the source.
2. Lower gain compared to BJTs for a given operating point.
3. Less noisy than BJTs in certain applications.
4. Used in applications requiring high input impedance, such as pre-amplifiers and RF amplifiers.

Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) Amplifiers

MOSFETs are also voltage-controlled devices, similar to JFETs, but they have an insulated gate, resulting in even higher input impedance. They are widely used due to their versatility:

1. Extremely high input impedance (in the order of teraohms).
2. Can operate in both enhancement and depletion modes.
3. High power handling capability, making them suitable for power amplifiers (e.g., Class D audio amplifiers).
4. Used extensively in digital circuits and switching applications, as well as analog amplification.

MOSFETs are less susceptible to thermal runaway than BJTs and are a popular choice for modern amplifier designs.

Teaching Methods/Instructional Techniques

Discussion, Explanation, Demonstration, Question and Answer, Visual Aids, Guided Practice

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Activating Prior Knowledge

Teacher’s Activity: The teacher greets the students and reviews the concept of a transistor and its basic function (e.g., as a switch or simple amplifier). The teacher then introduces the topic of amplifiers, asking students if they know what an amplifier does in devices like radios or sound systems.

Pupils’ Activity: Students respond to questions about transistors and share their understanding of amplifiers.

Learning Point: Amplifier function recall

Step 2: Explanation of Amplifier Classes (Part 1)

Time: 8 minutes

Teaching Skill: Explanation/Demonstration

Teacher’s Activity: The teacher explains that amplifiers are grouped into classes based on how much of the input signal cycle the amplifying device conducts. Using charts and diagrams, the teacher defines and illustrates Class A and Class B amplifiers, highlighting their conduction angles, efficiency, and typical applications. The teacher also explains crossover distortion in Class B.

Pupils’ Activity: Students observe the diagrams, listen to explanations, and ask questions for clarification.

Learning Point: Class A and B characteristics

Step 3: Explanation of Amplifier Classes (Part 2)

Time: 7 minutes

Teaching Skill: Explanation/Comparison

Teacher’s Activity: The teacher continues by explaining Class AB, Class C, and Class D amplifiers. The teacher compares their conduction angles, efficiency, and distortion levels, emphasizing the trade-offs involved in each class. Diagrams of waveforms for each class are used to illustrate the concepts.

Pupils’ Activity: Students pay attention to the differences between the classes and note down key characteristics.

Learning Point: Class AB, C, D understanding

Step 4: Introduction to BJT Amplifiers

Time: 6 minutes

Teaching Skill: Explanation/Visualisation

Teacher’s Activity: The teacher introduces Bipolar Junction Transistors (BJTs) as a common device used in amplifiers. The teacher explains that BJTs are current-controlled devices and briefly describes their use in common emitter, common collector, and common base configurations, showing simple circuit symbols.

Pupils’ Activity: Students listen and observe the BJT symbols and circuit diagrams.

Learning Point: BJT amplifier characteristics

Step 5: Introduction to JFET Amplifiers

Time: 6 minutes

Teaching Skill: Explanation/Comparison

Teacher’s Activity: The teacher introduces Junction Field-Effect Transistors (JFETs) as another type of transistor used in amplifiers. The teacher explains that JFETs are voltage-controlled devices and highlights their key characteristic of high input impedance, comparing it to BJTs.

Pupils’ Activity: Students listen, compare JFETs with BJTs, and note down the main features.

Learning Point: JFET amplifier features

Step 6: Introduction to MOSFET Amplifiers

Time: 5 minutes

Teaching Skill: Explanation/Application

Teacher’s Activity: The teacher introduces Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs), explaining their very high input impedance due to the insulated gate. The teacher discusses their versatility and common applications, especially in power amplifiers like Class D.

Pupils’ Activity: Students listen and understand the advantages and uses of MOSFETs in amplifiers.

Learning Point: MOSFET amplifier applications

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. What is an amplifier?
2. Mention two classes of amplifiers based on their conduction angle.
3. Differentiate between a Class A and a Class B amplifier.
4. State one key characteristic of a JFET amplifier.
5. Why are MOSFETs often preferred in power amplifier designs?

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Amplifier classes and types assessed

Step 8: Note-Taking

Time: 4 minutes

Teaching Skill: Guided Writing

Teacher’s Activity: The teacher guides pupils/students to copy the essential Board Summary notes on amplifier classes and transistor types into their notebooks.

Pupils’ Activity: Pupils/students copy the notes carefully into their notebooks.

Learning Point: Amplifier concepts recorded

Step 9: Conclusion

Time: 2 minutes

Teaching Skill: Summarization

Teacher’s Activity: The teacher briefly summarizes the key points of the lesson, reiterating the importance of understanding different amplifier classes and the role of BJT, JFET, and MOSFET transistors in their design. The teacher encourages students to observe amplifiers in electronic devices around them.

Pupils’ Activity: Students listen and reflect on the lesson.

Learning Point: Lesson concepts reinforced

Continuous Assessment/Further Study

Type: Homework

Instruction: Answer the following questions in your notebook:

1. Draw the typical input and output waveforms for a Class C amplifier and explain why it is suitable for RF applications.
2. Research and write a short paragraph on one real-world application for each of the following amplifier types: BJT, JFET, and MOSFET.
3. Explain the term “crossover distortion” and how Class AB amplifiers minimize it.

Lesson Keywords

• Amplifier – An electronic device that increases the power, current, or voltage of an input signal.
• Class A Amplifier – Conducts current for the entire 360° of the input signal.
• Class B Amplifier – Conducts current for 180° (half) of the input signal cycle.
• Class AB Amplifier – Conducts current for slightly more than 180° but less than 360° of the input signal cycle.
• Class C Amplifier – Conducts current for significantly less than 180° of the input signal cycle.
• Class D Amplifier – Operates as a switching amplifier, converting signals into pulses.
• BJT – Bipolar Junction Transistor, a current-controlled amplifying device.
• JFET – Junction Field-Effect Transistor, a voltage-controlled amplifying device with high input impedance.
• MOSFET – Metal-Oxide-Semiconductor Field-Effect Transistor, a voltage-controlled device with extremely high input impedance.
• Crossover Distortion – Distortion occurring in Class B amplifiers when switching between transistors.

Differentiation

For weaker learners, the teacher can provide simplified diagrams and focus on identifying the main classes and their basic characteristics (conduction angle, efficiency). For faster learners, the teacher can challenge them to explain the biasing arrangements for each class or research specific amplifier topologies (e.g., common source, common gate).

Suggested Lesson Videos

For further understanding, students can search for “Classes of Amplifiers BJT JFET MOSFET” on YouTube.

Teacher Guide for Using This Lesson Plan

Teachers should prepare visual aids such as charts and diagrams of amplifier classes and transistor types before the lesson. Begin by quickly reviewing basic transistor operation to ensure students have the necessary foundation. When discussing amplifier classes, emphasize the conduction angle, efficiency, and typical applications for each. Use the diagrams to clearly illustrate the input and output waveforms. For BJT, JFET, and MOSFET amplifiers, highlight their control mechanism (current vs. voltage) and key characteristics like input impedance. Encourage students to ask questions throughout the lesson. Ensure students copy the Board Summary notes accurately during Step 8. Check for understanding through the evaluation questions and provide clear, concise answers. Support weaker learners by simplifying explanations and providing direct examples, while challenging faster learners with more in-depth questions or research tasks.