The LM339 is a voltage comparator, but it can be used as a low frequency amplifier, as shown in the circuit below. The circuit is from a component manual and is used for the other types of the same series.

The LM339 is a voltage comparator, but it can be used as a low frequency amplifier, as shown in the circuit below. The circuit is from a component manual and is used for the other types of the same series.