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Message started by Marios on Jul 9th, 2020, 1:37am

Title: Varactor equivalent capacitance
Post by Marios on Jul 9th, 2020, 1:37am

Dear all,

I have a question with regards to varactors:

† Assume we have an ideal varactor whose C-V relationship is a straight line, for example C = a*v(t). Now assume that our varactor is driven by a pure sinusoid such that v(t) = cos(ω*t).
† If I wanted to replace my varactor by a constant capacitor that would produce the same effect what would the constant capacitorís equivalent value have to be?

(a)      The varactorís average capacitance value over a period
(b)      The varactorís RMS capacitance value
(c)      It depends; If we are after equivalent capacitor that would require the same reactive power we use the RMS value but if we are after an equivalent capacitance for tuning (using an inductor lets say) we use the average value

There are two papers that analyse this but each one of them takes its own approach; one of them takes the rms value while the other assumes that the varactor's capacitance can be approximated by a Fourier series. To this end I would assume that the equivalent value would be the 1st Fourier term, ie the average value of the varactor. This issue stems in LC-VCOs where the signal swing is large enough as to cause the varactor's value to depart significantly. The two papers are:

https://ieeexplore.ieee.org/document/1202006
https://ieeexplore.ieee.org/document/1214725

Any answer is appreciated.

Regards
Marios


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