Peak Detector

Circuit:

Any time $V_{in}>V_{out}$, the diode conducts. When $V_{in}<V_{out}$, the diode is off. $V_{out}$ stays constant. Basically, $V_{out}$ will stay at the highest $V_{in}$ value that it’s seen until it sees a higher value.

We can reset it by adding a switch:

There will be some leakage, so it won’t hold the value perfectly: