Large Variable Capacitor

[Vivake]

Does anyone know how to create a variable capacitor that can be actively controlled between say 1nF and 1uF?

The three types of variable capacitors (digital switched, mechanical rotary, and varicap diode) all seem to be limited to the pF range of capacitance. Any creative ideas to make much bigger variable capacitors?

 

[Laplace]

Capacitance is proportional to surface area, so in order to get more capacitance the device needs to be made larger. For a mechanical approach the size of the plates need to be made much larger. For digital switching with FET switches the largest component capacitor would need to be 0.5 uF. Not sure if varicaps would have the dynamic range necessary, but if they do then you would need a lot of them.

 

[Vivake]

Thanks for your response, Laplace. So far I haven't found mechanical capacitors with plates that are large enough. I think this is because they have air dielectrics to allow them to move freely. The digital switching approach is certainly a viable option.

Another thought I had was to cascade two gyrators with a capacitive load. If the gyrator constants could be varied, then the effective capacitance would be modulated.

 

[Laplace]

@Vivake: When you said "create" I thought that meant "build" your own large capacitor plates.

Normally gyrators are used to simulate inductors by using capacitors, but it is possible to simulate a variable capacitor with a gyrator using an inductor and variable resistor. For instance, see the following article: "The Technology of Wah Pedals" http://www.geofex.com/article_folders/wahpedl/wahped.htm

 

[Vivake]

Laplace - Nice reference, thanks!