Amplifier
New to topics? Read the documentation here!
Main implementations: the same as electronic switches: vacuum tubes in the past, and transistors in the second half of the 20th century.
How to make an LM386 audio amplifier circuit by Afrotechmods (2017)
Source. Builds the circuit on a breadboard from minimal components, including one discrete transistor. Then plays music from phone through headset cables into a speaker.