Op Amp

https://en.wikipedia.org/wiki/Operational_amplifier_applications

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Comparator

Simplest op amp circuit

Non-Inverting amplifier

Amplifies the input voltage. Cannot go below unity.

For high power, output can go through a transistor.

Schmitt Trigger

Uses positive feedback.

Current Sensor

I had to do the math analysis to understand this. Configuration is a differential amplifier.

Real Op Amps

I have a 741 and the lowest it will go is to 1.9V, but it can go all the way up to V+. Which kind of sucks, because it can't easily drive N fets.

I tested this on a 2n7000 and the active region is really around a Gate-Source voltage of 1.2v, so the 741 doesn't go low enough.