I'm looking for a way to make a varicap. in the 1-4pF range I want it ideally setup so that at 2.5V the capacitance is 4pF and at 1V, the capacitance is 1pF or vice-versa. The point is, I want to be able to use a DAC to tune between 1 and 4pF. I couldn't find commercially made varicap diodes in this range so I thought of rolling my own.

I looked at the PN3563 datasheet since thats the transistor I want to configure as a varicap. (see red circled section). The datasheet provided no other information nor graphs.My circuit carrier frequency will run at about 433Mhz. I can't satisfy the output capacitance test conditions in the sheet because my frequency is way beyond 1Mhz and I won't be reaching 10V.

Is there a way mathematically that I can determine the proper capacitance values from this transistor based on my conditions?

Some test conditions I use are:

Carrier Frequency=433Mhz, V=2.5V

and

Carrier Frequency=433Mhz, V=0V

I wanted to tie base and emitter together at first, but if it doesn't give me the proper capacitor range, then I'll tie base and collector together instead.

 

There is one here goes from 0.56pF to 7pF 9V to 1V,

http://uk.farnell.com/infineon/bb85702vh7902xtsa1/rf-diode-0-52pf-0-02a-sc-79/dp/2725709RL

 

See

 

I'm confused by your equations. Does Vc mean voltage? and How was the chart constructed on the right? and are the numbers at the bottom of the chart voltage values? I'm confused.

 

Dependency capacity is on the right shown On diagram from voltage (pF at V).
I am change constant offset on collector (DC). And shall add a little more the AC voltage in addition (DC+AC).
Possible so assign the voltage (AC) that current will numerically is a capacity.

 

But I'm trying to figure out the equation. what if I changed the transistor to 2N3904 instead?

 

With this scheme can count any NPN transistor.