Variable capacitor

[JTEX]

If you've used (or thought about using) something like a rotary switch to switch between different capacitor values in a circuit, you might want to consider a continuously variable "active cap" instead. It can be made with a simple pot and a couple of op amps that form a capacitance multiplier. I almost forgot that these things existed!

The circuit on the B side of the switch simulates a cap that does the same thing as the A side (a cap grounded at the bottom), except you can vary it between 100 and 1000pF with the values shown. C2 to, say, 10nF and then you can go between 10 and 100nF.

 

[jwin615]

Nice.
Can this be utilized only when the varicap is to ground or can it be part of a larger network.
Say, in a opamp feedback network.

 

[JTEX]

I read somewhere that there's a floating version, but haven't investigated further.

 

[Aentons]

Is this better than panning between two different value caps?

 

[JTEX]

^ Depends on the circuit. Panning adds a significant series resistance when not fully at one end or the other. The circuit above acts like a pure cap with no series resistance.

 

[Aleph Null]

This might be just the thing I need to continuously change the knee frequency on a high-shelf filter...

 

[leadfoot]

Nice.
Can this be utilized only when the varicap is to ground or can it be part of a larger network.
Say, in a opamp feedback network.

I did a little bit of digging and stopped when it basically brought up conference papers on active component designs (which don't get me wrong, I love to nerd out and read these). There exists floating versions but it is basically two modified grounded versions back to back and cross coupled through what was ground and are prone to oscillation. So, four op amps, needing bipolar power (unless you want the active capacitor to clip your signal), and a chance at oscillation at high gain seems like a fun experiment but not a lot of usefulness.

 

[Bricksnbeatles]

Would this work in place of a standard vari-cap in something like an AM receiver? I have a sound installation thing I’ve been wanting to make for years with the intent of controlling the tuning of some AM receivers with LDRs. Was helping a friend install their MFA thesis exhibition today, and was talking about that idea with them, so now it’s on my mind again.