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comparator circuits

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4 detailed 50-minute lessons with teaching scripts, worked examples, parent guides, and assessment criteria.

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Lesson Overview

Total Lessons: 4
Tier: Foundation and Higher
Duration: 50 minutes per lesson (200 minutes total)
Exam Boards: AQA, Edexcel, OCR, Eduqas, CCEA

Learning Objectives

Prerequisites

Materials & Equipment

Lesson 1: Introduction: comparator circuits

Duration: 50 minutes

Starter Activity (5 minutes)

Quick Recall

Write down everything you already know about comparator circuits. Then check against the key terms: key terms from comparator circuits. Use a mini-whiteboard or paper.

Main Content (35 minutes)

Parent/Teacher Guide:
Before lesson: Read the script below. Pre-teach key vocab: key terms from comparator circuits.
If stuck: Re-read the revision notes (link above), then break the content into smaller steps.
Extension: See the Stretch & Challenge ideas in Lesson 4.
Teaching Script (35 mins):
Mins 0-5 - Hook: "Today: comparator circuits. By the end you will be able to answer exam questions on it unaided. It connects to the rest of Electronics because the ideas here recur across the spec."
Mins 5-20 - Direct Instruction: Work through the core ideas below one at a time; after each, ask your student to explain it back in their own words.
Mins 20-30 - Guided Practice: Model the worked example together, then let your student attempt the first practice question with guidance.
Mins 30-35 - Independent Practice: 2-3 practice questions from Lesson 3 below, with immediate feedback.
First Look

Start with the revision notes summary, then attempt: explain the key ideas of comparator circuits

Plenary (5 minutes)

Check Out

Your student states one thing they learned and one question they still have about comparator circuits.

Lesson 2: Core Concepts: comparator circuits

Duration: 50 minutes

Starter Activity (5 minutes)

Review Previous Lesson

Quick recap: write 3 key points from Lesson 1 on comparator circuits. Check them against the notes below.

Main Content (35 minutes)

Key Fact: A comparator compares two input voltages and switches its output to indicate which is higher; no negative feedback is used.
Key Fact: When V+ > V−, the output goes to positive saturation; when V+ < V−, the output goes to negative saturation.
Key Fact: A simple comparator has a single threshold voltage; the output switches when the input crosses this level.
Key Fact: Noise on the input signal can cause a simple comparator to chatter (switch rapidly) near the threshold.
Key Fact: A Schmitt trigger uses positive feedback to create two different threshold voltages: upper (V_UT) and lower (V_LT).
Key Fact: Hysteresis is the difference between the upper and lower thresholds; it prevents output chatter caused by input noise.

Practice (10 minutes)

Q: explain the key ideas of comparator circuits

Answer:

Plenary (5 minutes)

Explain Back

Your student teaches the key points back to you without looking. Fill any gaps immediately.

Lesson 3: Application: comparator circuits

Duration: 50 minutes

Starter Activity (5 minutes)

Quick Recall

Recall the key terms: key terms from comparator circuits. Define each in one sentence.

Main Content (35 minutes)

Parent/Teacher Guide: Let your student attempt each question alone first, then compare with the model answer. Award method marks for correct working even if the final answer is wrong.

Work through the practice questions on the revision notes page for this topic.

Plenary (5 minutes)

Error Review

Review any questions answered incorrectly. Identify whether the error was knowledge, method, or reading the question.

Lesson 4: Exam Practice: comparator circuits

Duration: 50 minutes

Starter Activity (5 minutes)

Command Words

Review what these command words require: state (one point), describe (say what happens), explain (say why), compare (both sides), evaluate (judgement).

Main Content (35 minutes)

Extended Answer

Extended question: Full-Mark Response Explain the operation of a Schmitt trigger comparator circuit and describe how hysteresis eliminates output chatter. <div class="

A Schmitt trigger is a comparator circuit that uses positive feedback to create two distinct threshold voltages: an upper threshold (V_UT) and a lower threshold (V_LT). When the input rises above V_UT, the output switches to its high state and remains high until the input falls below V_LT. Similarly, when the input falls below V_LT, the output switches low and remains low until the input rises above V_UT again. This dual-threshold behaviour creates hysteresis (V_H = V_UT − V_LT), which means the circuit has memory of its previous output state. Hysteresis eliminates output chatter because small noise fluctuations around a single threshold cannot cause repeated switching — the input must move fully through the hysteresis window to change the output state. Positive feedback also accelerates the switching transition, producing clean, fast output edges. The hysteresis width can be adjusted by choosing appropriate feedback resistor values: a wider hysteresis gives greater noise immunity at the cost of reduced sensitivity.

Exam Tips: When drawing a Schmitt trigger, clearly show the positive feedback resistor from output to non-inverting input. | Label both threshold voltages (V_UT and V_LT) on the transfer characteristic and state the hysteresis value. | Explain hysteresis using the analogy: the output 'remembers' its previous state until the opposite threshold is crossed. | For calculations, always state the hysteresis formula: V_H = V_UT − V_LT. | Distinguish between a simple comparator (one threshold) and a Schmitt trigger (two thresholds with hysteresis).
Common Errors: ✗ Confusing positive feedback in a Schmitt trigger with negative feedback. ✓ A Schmitt trigger uses positive feedback to create hysteresis; negative feedback would eliminate the switching behaviour. ✗ Assuming a comparator has a single switching threshold like a Schmitt trigger. ✓ A simple comparator has one threshold; a Schmitt trigger has two thresholds (V_UT and V_LT) with hysteresis. ✗ Forgetting that comparator outputs saturate at the supply rail voltages. ✓ The output saturates near +Vsat or −Vsat (the supply rails), not at the input voltage level. ✗ Thinking hysteresis reduces sensitivity is always a disadvantage. ✓ Greater hysteresis improves noise immunity; the trade-off is reduced
Stretch & Challenge (Grade 8-9):
  • Synoptic links: explain how comparator circuits connects to another Electronics topic you have studied
  • Real-world: research one real-world use or example of comparator circuits
  • Critical: "What are the limitations of the models used in comparator circuits?"

Plenary (5 minutes)

Assessment Criteria
  • Got it: Confident explanation + correct worked examples
  • Getting there: Main points OK, needs support with detail
  • Not yet: Confused on key concepts - re-run Lesson 2

Homework & Consolidation

Recommended Resources

🎓 Smart Lesson (Guided)