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Regarding NMOS as a switch

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poojadholam

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as shown in the image. i want to use nmos as a switch and using that i want to get different combination of resistances. i will connect Vout to opamp's output(negative feedback).
i tried a lot but i am unable to switch on the NMOS. even i have tried for different W/L ratios.
Vin- : 1.65 V DC and 1 V AC
Gate volatges of NMOS : 0/1.65V DC
ref is connected to inv input of opamp.

Anybody can help me or give me a suggestion? Thank you in advance.
nmosswitch.jpg
 

Hi,

If it is unrelated to the VGS provided being too low... How about not connecting the NMOS in series but in parallel? And each NMOS drain to the junction of the first resistor and the four in a string? If that were a viable option, depends on signal origin, the NMOS could be substituted for PMOS and give a lower voltage drop, although not necessarily or by that much these days.
 

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