Dear all :
I use NMOS do a MOS varactor , in simulation I tied D/S/B together
and do a one terminal and gate is another terminal,
this method is correct ??
it is impossible for you to connect your bulk to D and S if you didn't have a seperate well for nmos. you can connect the bulk to ground and use the D and S as one terminal and gate as another terminal.
Actually the bulk of the NMOS will be connected to the whole bulk of the substrate (which is grounded)...unless as mentioned above the NMOS is in a seperate well.
So if one terminal of the cap you are using will be always connected to ground. If that's ok with you the go on. If not, you can try using a PMOS instead, as PMOS can be easly put in a seperate N-well.