BB 109 is a varicap diode and is used for modulation.

I'm going to use this for frequency modulation of voice signal . Since our voice signal is in range of millivolts. I need to know from which voltage they have response?

Like PN junction diode,Did they have cutoff or Breakdown voltage ?

From which voltage ,they have capacitance value ?


2 Answers 2


I think your questions are answered by this section of the BB109 datasheet: -

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Capacitance varies with the reverse voltage across the device so that at 1V, capacitance is about 40pF and at 10V the capacitance is about 13pF.

Applying a small signal from a microphone will produce modulation. Let's say your voltage is 1Vp-p centered at a dc level of 4V. Nominal capacitance is 25pF and half a volt higher it is about 23pF. Half a volt lower it is about 27pF. This means your 1Vp-p signal varies the capacitance by +/-2pF. This will, in the appropriate oscillator, move the frequency up and down. How much? Frequency change is the square root of capacitance change in LC oscillators and the capacitance change is +/- 8%. This translates to a frequency change of approximately 4% up and 4% down.

If your microphone signal is much smaller than an expected maximum of 1Vp-p then you'll need to apply an amplifier to bring the levels higher. You'll also need an LC oscillator - look up "Colpitts" on the internet to uncover what your options are. Alternatively, here is a link to another page on Stack exchange that has a circuit

Breakdown voltage of BB109 is 28V.


Yes, they have a maximum reverse voltage - 28V according to the BB109 Telefunken datasheet at this link. It also gives the other relevant specs such as a Vr-Capacitance graph.


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