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How does negative feedback turn a near-perfect amplifier into precise inverting, non-inverting and summing circuits?

Operational amplifiers: the ideal op-amp properties, the inverting, non-inverting, summing and difference amplifiers, the voltage follower, and the virtual earth.

An Eduqas A-Level Electronics answer on operational amplifiers: the ideal op-amp properties, the inverting and non-inverting amplifier gains, the summing and difference amplifiers, the voltage follower as a buffer, and the virtual-earth concept that makes the analysis simple.

Generated by Claude Opus 4.814 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

Eduqas wants you to state the ideal op-amp properties, derive and use the inverting and non-inverting amplifier gains, analyse the summing and difference amplifiers, use the voltage follower as a buffer, and explain the virtual earth. The operational amplifier is the workhorse of analogue electronics.

The answer

The ideal operational amplifier

The inverting amplifier and virtual earth

The non-inverting amplifier and voltage follower

Summing and difference amplifiers

Examples in context

Op-amp circuits condition almost every analogue signal: an inverting or non-inverting amplifier boosts a small sensor voltage, a voltage follower buffers a potential-divider output so the next stage cannot load it, a summing amplifier mixes audio channels or forms a digital-to-analogue converter, and a difference amplifier extracts a tiny signal from common-mode noise in instrumentation. The same building blocks reappear in the active-filter and instrumentation topics.

Try this

Q1. An inverting amplifier has Rin=4.7 kΩR_\text{in} = 4.7\ \text{k}\Omega and Rf=47 kΩR_f = 47\ \text{k}\Omega. Find the gain. [2 marks]

  • Cue. Av=−474.7=−10A_v = -\frac{47}{4.7} = -10.

Q2. State the gain of a voltage follower. [1 mark]

  • Cue. +1+1 (unity gain).

Q3. State one ideal property of an op-amp that means no current flows into its inputs. [1 mark]

  • Cue. Infinite (very high) input resistance.

Exam-style practice questions

Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Eduqas 20205 marksAn inverting amplifier uses a 10 kΩ10\ \text{k}\Omega input resistor and a 220 kΩ220\ \text{k}\Omega feedback resistor. Calculate the voltage gain, and find the output voltage for an input of −50 mV-50\ \text{mV}.
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Voltage gain (up to 3 marks): for an inverting amplifier Av=−RfRin=−22010=−22A_v = -\dfrac{R_f}{R_\text{in}} = -\dfrac{220}{10} = -22.

Output voltage (up to 2 marks): Vout=Av×Vin=−22×(−50×10−3)=+1.1 VV_\text{out} = A_v \times V_\text{in} = -22 \times (-50 \times 10^{-3}) = +1.1\ \text{V}.

Markers reward the gain −22-22 from −RfRin-\frac{R_f}{R_\text{in}} and the output +1.1 V+1.1\ \text{V} (the inversion turns the negative input into a positive output).

Eduqas 20225 marksExplain what is meant by the virtual earth in an inverting amplifier, and state two properties of an ideal operational amplifier that make it possible.
Show worked answer →

Virtual earth (up to 3 marks): in an inverting amplifier the non-inverting input is tied to 0 V0\ \text{V}, and negative feedback forces the inverting input to almost the same potential, so the inverting input sits at about 0 V0\ \text{V} without being directly connected to ground. It is a "virtual" earth because it is held at earth potential by the feedback, not by a wire. This makes the current through the input resistor equal the current through the feedback resistor, giving the gain.

Ideal properties (up to 2 marks): infinite open-loop gain (so the input difference is driven to essentially zero) and infinite input resistance (so no current flows into the input pins), which together force the virtual earth.

Markers reward the feedback holding the inverting input at 0 V0\ \text{V}, the "no current into the input" point, and two ideal properties (infinite open-loop gain and infinite input resistance).

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