What op-amp circuits are in WJEC A-Level Electronics?

The operational amplifier section covers the ideal op-amp and its properties, the comparator (open-loop) with a reference voltage and the Schmitt trigger with hysteresis, the inverting and non-inverting amplifiers built with negative feedback, the virtual earth, the voltage follower used as a buffer, and the summing (mixer), difference and instrumentation amplifiers. You must use the gain equations and explain virtual earth, hysteresis, buffering and common-mode rejection.

What is a virtual earth in an op-amp circuit?

In an inverting amplifier the non-inverting input is grounded, and the enormous open-loop gain with negative feedback forces the two inputs to almost the same voltage. The inverting input is therefore held at almost $0\,\text{V}$ even though it is not wired directly to ground, which is called a virtual earth. It is the key to the summing amplifier, because several inputs can feed that node independently.

What is the difference between an inverting and a non-inverting amplifier?

An inverting amplifier feeds the signal through an input resistor to the inverting input, has gain $A_v = -\frac{R_f}{R_{in}}$, creates a virtual earth, and inverts the signal. A non-inverting amplifier feeds the signal to the non-inverting input, has gain $A_v = 1 + \frac{R_f}{R_{in}}$, keeps the output in phase, and has a high input resistance. Both use negative feedback so the gain depends only on the resistors.

Why use an instrumentation amplifier with a sensor bridge?

A Wheatstone-bridge sensor gives a tiny differential voltage on a larger common voltage, from relatively high-impedance outputs. A plain difference amplifier loads the bridge and is hard to match for good common-mode rejection. An instrumentation amplifier adds buffer stages for a very high input resistance, so it does not unbalance the bridge, and provides a high differential gain with strong common-mode rejection, amplifying the small difference while rejecting noise.

WalesElectronics

WJEC A-Level Electronics Operational Amplifiers: the comparator, inverting, non-inverting, summing and instrumentation amplifiers explained

A deep-dive WJEC A-Level Electronics guide to Operational Amplifiers. Covers the ideal op-amp and open-loop comparator, the Schmitt trigger and hysteresis, negative feedback with the inverting and non-inverting amplifiers and the virtual earth, the voltage follower, and the summing, difference and instrumentation amplifiers, with the gain equations and exam patterns WJEC repeats.

Generated by Claude Opus 4.815 min readWJECUpdated 2026-06-16

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

Jump to a section

What the op-amp section demands
Op-amp properties and the comparator
Inverting and non-inverting amplifiers
Summing, difference and instrumentation amplifiers
How the op-amp section is examined
The topics, dot point by dot point
For the official specification

What the op-amp section demands

The operational amplifier is the most versatile analogue building block, and the WJEC specification covers it from the ideal model through to instrumentation amplifiers. The topic is gain-equation heavy and rewards clear explanations of feedback, virtual earth, hysteresis and common-mode rejection.

This guide walks through the section in specification order, then sets out the exam patterns WJEC repeats. Each topic has a matching dot-point page with worked exam questions; this overview ties them together.

Op-amp properties and the comparator

An ideal op-amp has infinite open-loop gain, infinite input resistance, zero output resistance and infinite bandwidth. With no feedback it runs open-loop, and its huge gain drives the output to one rail depending on which input is higher, the basis of the comparator. A comparator switches at a single reference, so a noisy slow input makes it chatter; a Schmitt trigger adds positive feedback to give two thresholds (hysteresis), rejecting noise smaller than the gap.

Inverting and non-inverting amplifiers

Negative feedback feeds the output back to the inverting input, fixing the gain by the resistor ratio. An inverting amplifier has gain $A_v = -\frac{R_f}{R_{in}}$ and a virtual earth at its inverting input. A non-inverting amplifier has gain $A_v = 1 + \frac{R_f}{R_{in}}$ and keeps the output in phase. A voltage follower has unity gain but very high input and very low output resistance, so it buffers a source from a load.

Summing, difference and instrumentation amplifiers

A summing amplifier feeds several inputs into the virtual earth, giving the inverted weighted sum $V_{out} = -R_f(\frac{V_1}{R_1} + \dots)$ ; the inputs stay independent. A difference amplifier amplifies the difference of two inputs and rejects common-mode signals. An instrumentation amplifier adds buffer stages for very high input resistance and strong common-mode rejection, ideal for a small differential sensor signal.

How the op-amp section is examined

Expect comparator output and Schmitt-trigger noise-immunity questions, inverting and non-inverting gain calculations with the virtual-earth explanation, summing-amplifier output calculations, and the case for an instrumentation amplifier with a bridge sensor. Method marks are reliable if you quote the right gain equation and explain feedback clearly.

The topics, dot point by dot point

Each topic has a dot-point answer page with worked exam questions and cross-links. Browse them from this overview and the subject hub.

For the official specification

WJEC Eduqas publishes the full specification, past papers and mark schemes at eduqas.co.uk. Always revise from the current specification and the board's own past papers, because question style is board-specific.

Sources & how we know this

WJEC Eduqas GCE A-level Electronics specification — WJEC Eduqas (2017)