Chips

Op-amp & comparator

An amplifier chip with two inputs and one output. It amplifies the tiny difference between its inputs enormously. With feedback resistors it becomes a precise amplifier; with no feedback it becomes a comparator that answers "which input is higher?".

Think of it as: A very strong servant that copies whatever you point at, as hard as needed, until its two inputs match.

Pins

+ (non-inverting input): : When this rises, the output rises.
− (inverting input): : When this rises, the output falls. Feedback from the output usually comes back here.
OUT: : Swings between the supply rails (nearly all the way on "rail-to-rail" parts).
V+ / V−: : Power pins, often left off schematics but always connected.

Op-amps come as chips with one, two or four amplifiers inside. The LM358 (two op-amps in 8 pins) is a classic. The dot or notch marks pin 1; pins count anticlockwise from there.

What to look for inside

For 8 seconds the op-amp is wired as a buffer (output fed back to the − input), then for 8 seconds as a comparator with no feedback.
Inside, the first stage measures the difference between the inputs (ε) and the next stage multiplies it about 100 000 times.
With feedback, the output moves until the − input matches the + input, so ε stays near zero and the output copies the input.
Without feedback, even 1 mV of difference drives the output hard to one rail or the other: a clean yes/no answer.

The numbers that matter

Spec	Typical
Supply range Check it works at your supply. Not every op-amp runs from 3.3 V.	1.8 V to 36 V
Rail-to-rail in/out Older parts cannot get within about 1.5 V of the rails.	yes / no
Gain-bandwidth (GBW) Gain × frequency is limited: 1 MHz GBW at gain 10 only reaches 100 kHz.	1 MHz to 100 MHz
Input offset V_os A small built-in error that gets amplified along with the signal.	10 µV to 5 mV

Where you’ll use it

Buffering a weak signal, such as a high-resistance voltage divider, so the next stage does not load it.
Amplifying sensor signals before the ADC.
Comparators: battery-low alarms, thermostats, turning an analogue signal into a digital one.
Active filters.

Common mistakes

Expecting the output to reach 0 V or the full supply on a non-rail-to-rail part.
Leaving the inputs of an unused op-amp floating. Tie them to a fixed voltage so it does not oscillate.
Using an op-amp as a comparator for fast signals. Real comparator chips switch much faster.

Lessons that use it

Lesson 3.3: Op-Amp Basics: Buffer, Comparator & Amplifier

Go deeper

U_{out} = A_{OL}\,(U_+ - U_-)

Open loop: the huge gain A_OL (≈ 10⁵) is why it acts as a comparator.

A_v = 1 + \tfrac{R_f}{R_g}

Non-inverting amplifier: feedback resistors set the gain precisely.

A_v = -\tfrac{R_f}{R_{in}}

Inverting amplifier: the output is flipped upside down.