walters

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?

akg

do u mean a varactor??

mstechca

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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audioguru

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:

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Uncle $crooge

walters

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

Russlk

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.

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audioguru

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

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Uncle $crooge

mstechca

Walters didn't specify a capacitance range.

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walters

guitar signal audio ranges

walters

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.

audioguru

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.

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Uncle $crooge

walters

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?

audioguru

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?

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Uncle $crooge

walters

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?

audioguru

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

[quote]I'm trying to seperate the frequency from the phase shift]/quote]
Impossible and for no reason.

The PWM turns on and off a transmission gate. The gate is in series with the capacitor (or resistor) and applies it to the circuit for a selected duty-cycle.
The result is a small phase shift at lower frequencies and more phase shift at higher frequencies.
If the RC circuit is by itself then the result is a bass or treble tone contol.
If the RC circuit is in an opamp allpass filter then the result will sound like the original.
If the RC circuit is in an opamp allpass filter and is added or subtracted with the original signal then the result is a comb filter.

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