The big idea: An operational amplifier is a high-gain voltage amplifier with a non-inverting input (+), an inverting input (−) and a single output.
It amplifies the difference between its two inputs — not the signal on either one — and it is one of the basic building blocks of analogue circuits.
A millivolt signal in, volts out — and what happens when there is no feedback around it.
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| Property | What it means | What a designer gets from it |
|---|---|---|
| Differential inputs | It amplifies the difference between + and −, so anything appearing on BOTH equally is subtracted away | Mains hum picked up along a long sensor cable arrives on both wires and is rejected — which is why sensor leads are run as pairs |
| Very high gain | Around 100,000 with nothing around it, so a millivolt of difference drives the output to the supply rail | It is far too much to use directly, which is why a real amplifier always has feedback around it |
| Feedback sets the gain | Two resistors return part of the output to the inverting input, and their ratio fixes the gain | A stable, predictable gain chosen by the designer rather than by the chip's tolerance |
| Single-ended output | One output voltage, referred to the supply's common rail | It can drive a comparator, an analogue-to-digital converter, or the next stage directly |
The sentence that carries the marks: The op-amp amplifies the difference between its inputs, and the two feedback resistors — not the chip — set the gain.
Saying both halves shows you understand why a component with a gain of 100,000 is useful at all.
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| Application | The problem it solves |
|---|---|
| Amplifying a sensor signal | A microphone gives millivolts, a thermocouple tens of microvolts, a strain gauge less — and a converter wants volts. The op-amp fills the gap without changing the shape of the signal |
| Comparing two voltages | With no feedback, its huge gain makes it swing fully high or low the instant one input passes the other — which is exactly a comparator, the decision stage of an analogue system |
| Filtering | With capacitors in the feedback path it removes mains hum or fast noise from a slow signal before anything else sees it |
| IoT home appliances | A smart thermostat, a connected doorbell, a fitness band and a kitchen scale each amplify a tiny sensor signal before a microcontroller reads it — the op-amp is the stage nobody outside sees |
Why it matters more now, not less: Every connected product measures something, and the thing it measures always starts as a tiny analogue signal.
So the more digital a product becomes, the more it depends on one small analogue stage at the front — which is the point worth making in an answer about IoT devices.
How this is tested — describing common applications of op-amps, including amplifying sensor signals. It comes up two ways:
Paper 1 — multiple choice
- Identify the inputs and output of an op-amp.
- Choose the stage an op-amp performs in a described system.
Paper 2 — analysing a product
- Describe the purpose of the op-amp in a named product.
- Explain why a sensor signal must be amplified before it is used.
The trap: Saying "it makes the signal bigger" and stopping. Say what it amplifies — the DIFFERENCE between its inputs — and what sets the gain.
A connected kitchen scale weighs to the nearest gram and sends the reading to a phone. Describe the role of the op-amp in it.
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