I have a series resonant circuit where I will have a lot of capacitors. I want to select a capacitor value for resonance for a particular frequency and want to turn on that capacitor using a MOSFET. To do that I am using 2 N channel power Mosfet in series. I don't want to control the current flow through the circuit just want to switch the circuit on and off. What I understood that means I have to switch the MOSFETs in saturation and cutof mode to achieve that. I know for cutoff Vgs< Vth( which for my MOSFET is 3V). This I can do. But in Saturation mode it should be Vds> Vgs-Vth. I am not sure how to achieve this. My supply voltage is AC with around +3v to -3V. So there will be times when Vds wont be greater than Vgs-Vth and the MOSFET will go into linear mode. How do I overcome this.
Firstly I believe you don't mean "saturation" but the linear part of a MOSFET's characteristic: -
Secondly, you don't need two series connected MOSFETs; a single N channel MOSFET with a grounded source will do the job of switching in a capacitor and, to prevent the MOSFET's parasitic capacitance having too much of a detuning effect when "off", bias the drain to some voltage (like 12 volts to 24 volts) via a 100 kohms resistor.
Tried and tested idea.