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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.

the basic circuit should be like this

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    \$\begingroup\$ Things get much simpler when you apply a DC Bias voltage such that the Vds of the MOSFETs is always positive, then you don't need two NMOS in anti-series, you only need one NMOS. Apply a 3 V (or more) DC bias to the drain of the top NMOS through a resistor. Make the resistor high value so that it doesn't interfere with the LC tank. \$\endgroup\$ – Bimpelrekkie Dec 10 '19 at 14:15
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    \$\begingroup\$ You shouldn't be worrying about saturation mode or linear mode. You want to use the NMOS as a switch and that means applying as much Vgs as possible which will put the NMOS in linear mode. \$\endgroup\$ – Bimpelrekkie Dec 10 '19 at 14:20
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Firstly I believe you don't mean "saturation" but the linear part of a MOSFET's characteristic: -

enter image description here

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.

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  • \$\begingroup\$ Thats also my question. to operate the MOSFET as just a switch so when i turn it on it should give the lowest resistance path for the current. For that shouldn't i operate it in saturation region? If i just use one MOSFET during the negative half cycle the ground potential would be higher so won't the body diode of the MOSFET allow the current to flow from ground to circuit... that will turn on all my capaciotrs \$\endgroup\$ – zak3877 Dec 10 '19 at 14:46
  • \$\begingroup\$ @zak3877 look at the graph - look where MOSFET saturation is. Don't confuse MOSFET saturation with BJT saturation. BJT saturation is equivalent to MOSFET linear region. Don't ask this question again! The bias resistor holds the drain voltage highly positive and always reverse biases the body diode even when moderate AC voltages are present. \$\endgroup\$ – Andy aka Dec 10 '19 at 14:59

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