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What's 50 fF?

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I'm working on an amplifier that needs 50 fF of input capacitance -- which is an easy number to write on paper but extremely difficult to achieve in practice. In the interest of not spoiling the input characteristics of my FET -- what's 50 fF look like? If I take a 2 mm metal sphere, that's (4*pi*e0)*(1 mm)=100 fF, but I don't know how to extrapolate a sphere to a wire bond or what have you. Just looking for your experience!

Alex Eisenhut:
It would like about half of one of these


I'm only kinda kidding, I don't have a good idea of what capacitance "means" outside of a physical part on a board, you know?

But this sounds like some Ka stuff. This is where you need Southwest Microwave connectors that need screwing down to the board because the quality of the solder joint from the SMA to a microstrip determines the return loss so just don't put solder.

Nothing achievable over any kind of size, that's for sure.

Is this going to be a bootstrap application?  (Sounds like it!)


It's not. It's a HEMT or JFET being wire bonded directly onto the island whose charge I'm trying to sense -- it should all fit inside a few square mm.

And that capacitor is a bit funny. I believe there's 0.1 pF between the two leads, it's just that I imagine the self capacitance of each is probably a bit bigger -- but I don't know.


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