4. Varactors! https://en.wikipedia.org/wiki/Varicap https://en.wikipedia.org/wiki/Varicap Reverse biasing a diode at different levels changes the junction capa

4. Varactors! https://en.wikipedia.org/wiki/Varicap https://en.wikipedia.org/wiki/Varicap Reverse biasing a diode at different levels changes the junction capacitance. Also used in radio, for things like variable filters. edit: oh, it's topped pinned comment!