# Variable capacitor circuits

Discussion in 'General Electronics Chat' started by walters, Jan 13, 2006.

1. ### waltersBanned

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What circuits or a network act like a variable capacitor?

I know some circuits produces like if its a variable capacitor
where can i find these circuits and whats the theory behind them?

2. ### akgNew Member

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do u mean a varactor??

3. ### mstechcaNew Member

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Why do you care?
Connect a diode to a 1nF capacitor. Make sure it is reverse biased.
the point where you made the connection is the voltage control point. You can create a voltage divider network with two resistors and the output of the divider goes to the voltage control point. You have just made yourself a way of controlling capacitance by adjusting voltage.

You can use different diodes, or even transistors to obtain different ranges.

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5. ### audioguruWell-Known MemberMost Helpful Member

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Never mind the few pF change of the capacitance of a varactor diode which is used at high RF frequencies. You need to vary the capacitance of a capacitor of about 0.22uF for an audio filter.

A 0.22uF capacitor's effective value in a filter circuit can be changed with PWM. With 100% duty-cycle it is 0.22uF. With 10% duty-cycle it is 0.022uF. With 1% duty-cycle it is ..., you figure it out. Of course, the frequency of the PWM is very high so can't be heard. :lol:

6. ### waltersBanned

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Thanks guys for the help

I know i seen a circuit that used a Fixed capacitor like a .01uf and could vary the "capacitance" up and down it was variable but it was like a filter eq circuit but it wasn't it was a "variable capacitance circuit" but could "sweep" the capacitance

7. ### RusslkNew Member

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In the old days, there was a circuit called "reactance tube". It was used to simulate an inductor because that was easier than a capacitor, but either could be done. Today you would use an FET instead of a tube. If you Google "reactance tube circuit" you will no doubt find one.

8. ### audioguruWell-Known MemberMost Helpful Member

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A solid-state "gyrator" circuit has its inductance simulated in the circuit.

9. ### mstechcaNew Member

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Why do you care?
Walters didn't specify a capacitance range.

10. ### waltersBanned

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guitar signal audio ranges

11. ### waltersBanned

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One of them are varcap diode.
If any diode biased in revers direction,it act like capacitor, and it's capacity varies with reverse voltage.

12. ### audioguruWell-Known MemberMost Helpful Member

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Walters,
You didn't hear me.
A varactor is a varicap diode and is only a few pF and changes a few pF. They are used to tune high frequency VHF and UHF radio circuits. You need to change a capacitance 1000 times more.

You also didn't hear me describe how PWM can vary the capacitance over a wide range of any capacitor. I've used switched-capacitor lowpass filter ICs for years. They switch internal capacitors similar to PWM and work perfectly.

13. ### waltersBanned

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sorry

I need to change the capacitance by alot how can i do this?

If i have a fixed .01uf how can i vary the cap .01uf alot?

If i have a Fixed .47uf how can i vary the cap. .47uf alot?

14. ### audioguruWell-Known MemberMost Helpful Member

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Again you didn't hear me. PWM can vary the capacitance of any capacitor a lot.

You said correctly in another thread that changing the resistance or changing the capacitance of an RC network is the same. So why change the capacitance when changing the resistance is easier?

15. ### waltersBanned

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Because the capacitors changes the phase shift
the resistor changes the frequency

I'm trying to seperate the frequency from the phase shift

How can i hook up a PWM circuit to a cap/resistor phase shift network ?
so i can vary the phase shift of the cap?

16. ### audioguruWell-Known MemberMost Helpful Member

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No!
Changing the capacitor and changing the resistor in a RC circuit does exactly the same thing.

17. ### waltersBanned

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Changing the capacitor and changing the resistor in a RC circuit does exactly the same thing.

No its doesn't the Capacitor Value changes the frequency Range a .01uf can only phase shift so many frequencys range so u have to change the .01uf to another value like a .47uf to change the frequency range the resistor can't go past the .01uf frequency range without changing the .01uf to another value like a .47uf to get a different frequency range with different phase shifts

100k with a .01uf is different than a 100k with a .47uf phase shift network whats the difference?

By change the capacitors value whats the difference ? what is the caps value going to do ?

18. ### audioguruWell-Known MemberMost Helpful Member

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Walters,
A 0.01uF capacitor and a 100k resistor RC circuit produces a phase shift of 45 degrees at 160Hz.
So does a 0.1uF capacitor and a 10k resistor.
So does a 1uF capacitor and a 1k resistor.
So does a 10uF capacitor and a 100 ohm resistor.
So does ..., you figure it out.
At about 16Hz and lower and 1.6KHz and higher, their phase-shift will be 0 degrees or 90 degrees.

Change either the capacitor or the resistor to change the frequency range. Capacitors are usually switched to change the frequency range because they aren't variable like a potentiometer variable resistor that is used for the fine tuning.

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20. ### audioguruWell-Known MemberMost Helpful Member

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Its value is fixed so it sets the allpass filter's frequency range.

The one that contols all FETs controls the amount of phase-shift at the filter's tuned frequency, and changes the frequency range of the tuned frequency.

[qiote]the variable resistor is the FET[/quote]
Correct.

Go to Google and search for Allpass Filter. Links explain what they are and how they produce phase-shift over a fairly narrow range of frequencies. One link has a circuit with 8 allpass filters in series, each tuned to a different frequency, to produce a 90 degree phase-shift from 20Hz to 20kHz without a change in amplitude. It would be a nightmare to tune its phase variably, so they recommend using DSP to do it.
Here is a single allpass filter from the link:

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21. ### waltersBanned

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This is that we have to do is this

.05uf with a 22K in parallel the frequency range is ?

.047uf with a 10K in parallel the frequency range is?

.01uf with a 100uf in parallel the frequency range is?