Positive and Negative Feedback, Concepts and Differences for SS 3

Class: SS 3
Term: First Term
Week: 5
Age: 16 years
Duration: 45 minutes
Subject: Radio, TV and Electronics
Topic: FEEDBACK.
Subject Matter: Concept of feedback; Negative and positive feedbacks and their differences; Advantages of a negative feedback; Effects of a positive feedback on amplifiers, bandwidth noise, gain and distortion
Previous Lesson: RLC Circuits, Reactance and Series and Parallel Calculations

Specific Objectives

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

Cognitive Domain

• Define feedback in electronic circuits.
• Explain the concept of negative feedback.
• Explain the concept of positive feedback.
• Differentiate between negative and positive feedback.
• List the advantages of negative feedback.
• Describe the effects of positive feedback on amplifier gain, bandwidth, noise, and distortion.

Affective Domain

• Appreciate the importance of feedback in designing stable and efficient electronic circuits.
• Value the role of feedback in controlling amplifier characteristics.

Psychomotor Domain

• Identify feedback paths in simple circuit diagrams.
• Sketch basic block diagrams illustrating positive and negative feedback.

Reference Materials

The following resources were used in planning this lesson:

• 2025 Revised 9 Years Basic Education Curriculum
• Relevant State Unified Scheme of Work
• Radio, TV and Electronics textbook for SS 3
• The HeadTeacher Scheme of work

Instructional Materials

The teacher will teach this lesson with the aid of:

• Whiteboard and markers
• Charts showing block diagrams of feedback systems
• Circuit diagrams illustrating negative and positive feedback
• Projector (if available) to display circuit diagrams
• Examples of electronic devices that use feedback (e.g., an amplifier circuit board)

Rationale for the Lesson

This lesson provides students with a foundational understanding of feedback, a crucial concept in electronics. Understanding feedback is essential for analysing and designing stable amplifiers, oscillators, and control systems. It helps students grasp how circuit performance can be controlled and improved or degraded.

Prerequisite/Previous Knowledge

Students should have a basic understanding of electronic components such as resistors, capacitors, and transistors, as well as the working principles of simple amplifiers.

Lesson Content/Board Summary

FEEDBACK

Concept of Feedback

Feedback in electronics refers to the process of returning a fraction of the output signal from a system or circuit back to its input. This returned signal can either add to or subtract from the input signal, thereby modifying the system’s overall behaviour.

Negative Feedback

Negative feedback occurs when the feedback signal is out of phase (180 degrees out of phase) with the input signal. When combined at the input, the feedback signal subtracts from the input signal, reducing the overall gain of the system. It is widely used to improve the stability and performance of electronic circuits.

Positive Feedback

Positive feedback occurs when the feedback signal is in phase with the input signal. When combined at the input, the feedback signal adds to the input signal, increasing the overall gain of the system. While it can lead to instability, it is intentionally used in circuits like oscillators to generate signals.

Differences Between Negative and Positive Feedback

1. Phase Relationship: In negative feedback, the feedback signal is 180° out of phase with the input signal. In positive feedback, the feedback signal is in phase with the input signal.
2. Effect on Gain: Negative feedback reduces the overall gain of the amplifier. Positive feedback increases the overall gain, often leading to instability or oscillation.
3. Stability: Negative feedback improves the stability of a circuit, reducing distortion and noise. Positive feedback tends to make a circuit unstable, leading to oscillation.
4. Applications: Negative feedback is used in amplifiers for stability, bandwidth extension, and distortion reduction. Positive feedback is primarily used in oscillators to generate continuous waveforms.

Advantages of Negative Feedback

1. Increased Stability: Negative feedback makes the amplifier’s gain less dependent on variations in component values, temperature, or supply voltage.
2. Reduced Distortion: It significantly reduces non-linear distortion, making the output signal a more accurate replica of the input.
3. Increased Bandwidth: Negative feedback extends the frequency range over which the amplifier operates effectively.
4. Reduced Noise: It helps to reduce unwanted noise generated within the amplifier circuit.
5. Improved Input and Output Impedance: Depending on the feedback configuration, it can increase input impedance and decrease output impedance, or vice-versa, which is beneficial for impedance matching.

Effects of Positive Feedback

Positive feedback, when applied to amplifiers, has several distinct effects:

1. On Amplifiers (Gain): Positive feedback increases the overall gain of the amplifier. If the loop gain (product of amplifier gain and feedback factor) becomes unity or greater, the amplifier will become unstable and oscillate, producing an output signal without an input signal.
2. On Bandwidth: Positive feedback generally reduces the bandwidth of an amplifier. As gain increases, the frequency response narrows, making the amplifier more selective to certain frequencies.
3. On Noise: Positive feedback tends to amplify any existing noise within the circuit, making the system more susceptible to noise interference.
4. On Distortion: Positive feedback increases distortion. Any non-linearity in the amplifier is amplified along with the desired signal, leading to a distorted output waveform.

Teaching Methods/Instructional Techniques

Discussion, Explanation, Demonstration, Question and Answer, Observation, Guided Practice

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Activating prior knowledge

Teacher’s Activity: The teacher greets the students and asks them to recall what they know about amplifiers and how they work. The teacher then introduces the topic of feedback as a way to control amplifier behaviour.

Pupils’ Activity: Students respond to questions about amplifiers and listen to the introduction of feedback.

Learning Point: Amplifier function recall

Step 2: Concept of Feedback

Time: 8 minutes

Teaching Skill: Explanation/Definition

Teacher’s Activity: The teacher defines feedback in electronics, explaining that it involves returning part of the output signal to the input. The teacher uses a simple block diagram to illustrate the basic feedback loop.

Pupils’ Activity: Students listen attentively, ask questions for clarification, and observe the block diagram.

Learning Point: Definition of feedback

Step 3: Negative Feedback

Time: 7 minutes

Teaching Skill: Explanation/Illustration

Teacher’s Activity: The teacher explains negative feedback, emphasising that the feedback signal is out of phase with the input. The teacher uses circuit diagrams to illustrate how negative feedback is implemented and discusses its primary effect of reducing gain.

Pupils’ Activity: Students observe the circuit diagrams and listen to the explanation of negative feedback.

Learning Point: Understanding negative feedback

Step 4: Positive Feedback

Time: 7 minutes

Teaching Skill: Explanation/Illustration

Teacher’s Activity: The teacher explains positive feedback, highlighting that the feedback signal is in phase with the input. The teacher uses circuit diagrams to show positive feedback and discusses its effect of increasing gain, often leading to oscillation.

Pupils’ Activity: Students observe the circuit diagrams and listen to the explanation of positive feedback.

Learning Point: Understanding positive feedback

Step 5: Differences Between Negative and Positive Feedback

Time: 5 minutes

Teaching Skill: Comparison/Discussion

Teacher’s Activity: The teacher guides students in a discussion to compare and contrast negative and positive feedback based on phase relationship, effect on gain, stability, and applications. The teacher may draw a comparison table on the board.

Pupils’ Activity: Students actively participate in the discussion, identifying key differences between the two types of feedback.

Learning Point: Feedback type differentiation

Step 6: Advantages of Negative Feedback and Effects of Positive Feedback

Time: 8 minutes

Teaching Skill: Explanation/Demonstration

Teacher’s Activity: The teacher explains the advantages of negative feedback (stability, reduced distortion, increased bandwidth, reduced noise). The teacher then demonstrates or explains the effects of positive feedback on amplifier gain, bandwidth, noise, and distortion using relevant examples or simulations if available.

Pupils’ Activity: Students observe the teacher’s explanations and demonstrations, taking notes on the advantages and effects.

Learning Point: Feedback advantages and effects

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 feedback in an electronic circuit?
2. Differentiate between negative and positive feedback.
3. State two advantages of using negative feedback in an amplifier.
4. Describe one effect of positive feedback on amplifier bandwidth.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Feedback concepts 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 feedback concepts, types, differences, advantages, and effects into their notebooks.

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

Learning Point: Feedback notes recorded

Step 9: Conclusion

Time: 1 minute

Teaching Skill: Consolidation

Teacher’s Activity: The teacher briefly summarises the main points of the lesson, reiterating the importance of understanding feedback for electronic circuit design and analysis.

Pupils’ Activity: Students listen and prepare for the next lesson.

Learning Point: Lesson concepts consolidated

Continuous Assessment/Further Study

Type: Homework

Instruction: Answer the following questions in your notebook.

1. Explain why negative feedback is preferred in most amplifier designs.
2. Describe a practical application where positive feedback is intentionally used.
3. Research and explain how feedback affects the input and output impedance of an amplifier.

Lesson Keywords

• Feedback – The process of returning a fraction of the output signal to the input of a system.
• Negative Feedback – Feedback where the output signal is 180 degrees out of phase with the input, reducing gain and improving stability.
• Positive Feedback – Feedback where the output signal is in phase with the input, increasing gain and potentially causing oscillation.
• Gain – The ratio of output signal to input signal in an amplifier.
• Bandwidth – The range of frequencies over which an amplifier operates effectively.
• Distortion – Unwanted changes in the waveform of a signal.
• Noise – Unwanted random electrical signals that interfere with the desired signal.

Differentiation

For weaker learners: Provide simplified diagrams and focus primarily on defining and identifying negative and positive feedback and their basic effects on gain and stability. Use more direct questions during evaluation.
For faster learners: Encourage them to research specific feedback configurations (e.g., voltage series, current shunt) and their impact on input/output impedance. Ask them to explain how an oscillator works using positive feedback.

Suggested Lesson Videos

Search YouTube for “Positive and Negative Feedback in Amplifiers SS3” or “Effects of Feedback on Amplifier Performance”.

Teacher Guide for Using This Lesson Plan

Before the lesson, ensure you have clear, large-scale diagrams of feedback circuits, both block diagrams and simple component-level illustrations, to aid visual learning. Begin by reviewing basic amplifier concepts to ensure students have the necessary foundation. When explaining negative and positive feedback, use analogies to make the concepts concrete. For instance, negative feedback can be compared to a thermostat controlling temperature, while positive feedback can be likened to a microphone too close to a speaker causing a screech. Guide students through the comparison of the two feedback types, highlighting the practical implications of each. During Step 6, demonstrate the effects of positive feedback on parameters like gain and distortion using clear examples. Students should be instructed to copy the Board Summary notes during Step 8. Constantly check for understanding through questions and encourage students to ask questions. Provide additional support to weaker learners by simplifying explanations and offering more visual aids, while challenging faster learners with deeper analytical questions or research tasks.