How to measure the capacitance of the NMOS used as a varactor

[gkini19]

Hi All,



I am using the circuit shown below. I want to have a tuning range of 2-4 GHz for LC VCO. I want to use the varactor in this configuration.

My question:

1) How can I find the capacitance of the transistors used as a varactor? which analysis should I use and which plot will be useful?

2) To have a tuning range of 2 GHz how many stages of such Varactors should I have? is there a method or a way to choose this?

Thanks all in advance

 

[dick_freebird]

I prefer to do it like a C-V probe pull would - ramp Vg{dbs} at fixed dV/dt and capture Igg. You've got I and dV/dt so you know C and can plot or fit vs V.

Soon you will also care about Rgg but that's for someone more RF-y to chime in on.