A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Difference Amplifier Configuration and Output Voltage Calculation — mark scheme explained
The short answer
In AQA A-Level Physics, understanding the difference amplifier configuration is crucial for electronics. This topic covers how to calculate the output voltage of a difference amplifier using specific formulas. The key equations are: 1.
The question
Calculate the output voltage of a summing amplifier if V 1 = 4V, R 1 = 2kΩ, V 2 = 6V, and R 2 = 3kΩ.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For calculation questions, marks are typically awarded for correct substitution of values into the formula, showing intermediate steps, and providing the final answer with units. For explanation questions, marks are given for clarity, accuracy, and completeness of the response.
What the command words demand
- Calculate
- Perform a numerical calculation using the given data and formulas.
- Explain
- Provide a clear and concise explanation of how something works or why it is important.
- Determine
- Find the value of a specific quantity using the appropriate formula and given data.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 2-3 minutes per mark to ensure you have enough time to read the question carefully, perform calculations, and check your work.
- Identify the given values. - V 1 = 4V - R 1 = 2kΩ - V 2 = 6V - R 2 = 3kΩ - Feedback resistor R f = 1kΩ (the question omits R f ; we take the standard assumed value R f = 1kΩ).0 marks
- Use the summing amplifier formula. V out = - R f × (V 1 /R 1 + V 2 /R 2 )1 mark
- Calculate each input current. - V 1 /R 1 = 4V/2kΩ = 2mA - V 2 /R 2 = 6V/3kΩ = 2mA1 mark
- Sum the input currents. V 1 /R 1 + V 2 /R 2 = 2mA + 2mA = 4mA1 mark
- Multiply by -R f . V out = - 1kΩ × 4mA = - (1 × 10 3 Ω) × (4 × 10 -3 A) = -4 V1 mark
Final answer: Vout = -4 V (assuming Rf = 1 kΩ — question is missing Rf)
Work through every step correctly and you earn all 4 marks.
Another worked example
A difference amplifier has R f = 10kΩ, R 1 = 5kΩ, V 1 = 3V, and V 2 = 7V. Calculate the output voltage.
- Identify the given values. - R f = 10kΩ - R 1 = 5kΩ - V 1 = 3V - V 2 = 7V0 marks
- Use the difference amplifier formula. V out = (R f /R 1 ) × (V 2 - V 1 )1 mark
- Substitute the values into the formula. V out = (10kΩ/5kΩ) × (7V - 3V)1 mark
- Calculate the gain and the difference in voltages. - Gain = 10kΩ/5kΩ = 2 - Difference in voltages = 7V - 3V = 4V1 mark
- Multiply the gain by the difference in voltages. V out = 2 × 4V = 8V1 mark
Final answer: 8V
Work through every step correctly and you earn all 4 marks.
Common mistakes
Forgetting the negative sign in the summing amplifier formula.
Why it happens: Students often overlook the negative sign, leading to incorrect calculations.
Fix: Always include the negative sign when using the summing amplifier formula.
Using inconsistent units for voltages and resistances.
Why it happens: Students may mix volts with millivolts or ohms with kiloohms, leading to incorrect results.
Fix: Ensure all values are in consistent units before performing calculations.
Confusing the roles of R f and R 1 in the difference amplifier formula.
Why it happens: Students may mix up which resistance is the feedback resistance and which is the input resistance.
Fix: Clearly identify R f as the feedback resistance and R 1 as the input resistance.
Forgetting to calculate the difference between V 2 and V 1 in the difference amplifier formula.
Why it happens: Students may add the voltages instead of subtracting them, leading to incorrect results.
Fix: Always subtract V 1 from V 2 when using the difference amplifier formula.
Not matching input resistances in a difference amplifier for accurate measurements.
Why it happens: Students may not realize that mismatched resistances can lead to errors in the output voltage.
Fix: Ensure that the input resistances are matched to avoid measurement errors.
Using the wrong formula for a given configuration (summing vs. difference amplifier).
Why it happens: Students may confuse the formulas for different configurations, leading to incorrect calculations.
Fix: Clearly identify whether the circuit is a summing or difference amplifier and use the appropriate formula.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Summing Amplifier Output | Calculate the output voltage of a summing amplifier from input voltages and resistances. | 4 |
| Difference Amplifier Output | Calculate the output voltage of a difference amplifier from its resistor and input values. | 4 |
| Summing Amplifier Output | Calculate the output voltage of a summing amplifier using its input voltages and resistor values. | 4 |
| Difference Amplifier Output | Use the difference amplifier gain formula to calculate output voltage from two inputs. | 4 |
| Total across these question types | 16 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.