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.

 

[JTEX]

No idea. Is the vari-cap grounded in the radio? Then maybe, but you'd need an op amp with a wide bandwidth (tens of MHz), so it can still have significant gain (10x or so) at AM radio frequencies (up to almost 2MHz).

 

[Roberman]

I was chased out of RadioShack for asking about a variable capacitor in 1995 as a 13 year old. The employee correctly accused me of attempting to build an illegal cable box. Good times.

 

[pricklyrobot]

Radio Shack + Radio Shack employee who actually knows something about electronics =

They were always just trying to sell me an extended warranty for my rechargeable batteries, or towards the end there, a shitty cell phone plan.

 

[lowpitch]

I'm simulating this in LTSpice as part of a larger circuit and while AC analysis shows the wanted behavior, transient analysis does not.

Relevant part of the circuit, excuse the messy drawing:


The capacitance multiplier around C54 is supposed to replace C27 (not connected here). At pres1 = 1 it should be equivalent to 47p, at pres1 = 0 it's supposed to be equivalent to 4.7n.

AC analysis with the multiplier removed and C27 4.7n connected to ground:


Compared to C27 disconnected and pres1 at 0:


And the same with both C27 and the capacitance multiplier removed:


And only C27 removed, but pres1 at 1:


That's identical for all intents and purposes and shows the capacitance multiplier working as expected. But transient analysis doesn't agree with this at all. I'm using a .wav of a DI guitar track at the input, simulating the entire circuit and writing the output to another .wav file.

With both C27 and capacitance multiplier removed:
View attachment withoutcap.mp3

Compared to C27 removed at pres1 at 1:
View attachment multipliermin.mp3

Yup, pretty close.

But here's C27 4.7n connected to ground, multiplier removed:
View attachment withcap.mp3

Compared to C27 removed and pres1 at 0 for an equivalent of 4.7n going to ground via the capacitance multiplier:
View attachment multipliermax.mp3

Not at all as expected. There's next to no change with the capacitance multiplier at min or at max in transient analysis, even though AC analysis suggests otherwise.

I haven't breadboarded any of this yet so I don't know whether the simulation is close to reality or not but will report back when I have done so. Just wanted to share this for now because I found it weird.

 

[JTEX]

I'm afraid I can't help, I don't use LTSpice.

 

[anevilspar7an]

This sounds like a perfect use case as a range potentiometer in a wah pedal to replace the rotary switch we normally use.