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# The capacitance of the varicap

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The varicap is a special diode that which changes its capacity according to DC voltage applied to it in reverse manner, that is when high DC => low capacitance and vice versa, I think this is OK.
Now, looking to Fig.varicap (FROM: http://hem.passagen.se/communication/bug_rf.html), the varicap is BB149A, this varicap (according to its datasheets) has acapacitance of more than 20pF for DC votage less than 1VDC.
Lets assume for a moment that its capacitance is 20pF. As seen in the Figure, this BB149A is connected to other capacitors,i.e. 3.9pF & 15pF and the three elements with L1 combine an LC circuit.
My question is: What is the equivalent capacitance of the three
mentiond above with explination about series or parallel.
THANKS.

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I'm not going to tell you the answer but I'll tell you how to figure it out.

Capacitors in series:
Ctotal = 1 / (1/C1 + 1/C2+ 1/C3)

Capacitors in  parallel:
Ctotal = C1 + C2 + C3

If you need more help then don't be afraid to ask.

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