Electronic – How is a varactor diode used in a circuit when it only has 2 terminals

capacitor

I've been reading about varicaps, and I'd like to try to use one in an op-amp integrator circuit to be able to adjust the gain.

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My confusion though is that every varactor diode datasheet I've read indicates that only 2 pins are used, like a normal diode. The capacitance is supposed to change with reverse voltage across the diode, but how then is the capacitor supposed to be used in useful way?

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Only 2 pins used in both package types!

Doesn't make any sense to me, since obviously you can't keep the diode at a constant reverse voltage AND use it in any type of circuit where an alternating waveform is applied. I was expecting them to have a 3rd pin where an external control voltage was applied.

What am I missing?

Best Answer

you can't keep the diode at a constant reverse voltage AND use it in any type of circuit where an alternating waveform is applied.

Sure you can as the millions of varactor tuners produced will testify.

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Of course, the AC signal must be relatively small. From Wikipedia.

When designing tuning circuits with varicaps it is usually good practice to maintain the AC component of voltage across the varicap at a minimal level, usually less than 100mV peak to peak, to prevent this changing the capacitance of the diode too much and thus distorting the signal and adding harmonics to it.

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