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1)\$ V_S \$ potentially has two values. Which value of \$ V_S \$ should I choose?

2) Why?

3) What should I generalize from here about value of \$ V_S \$ that should be chosen while solving such problems?

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  • \$\begingroup\$ You need to choose Vs value which has a physical sense and ensures that the NMOS is conducting current (Vg - Vs > Vth). \$\endgroup\$
    – G36
    Nov 2 '18 at 14:02
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You made the assumption

Assuming saturation region operation

Which is an entirely reasonable assumption. You need to validate that your assumption was correct, and eliminate "impossible" answers. Going by the requirements for saturation, you need Vgs > Vt, and Vgs-Vt =< Vds.

You'll find that one of the answers does not satisfy both conditions for saturation.

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  • \$\begingroup\$ What in case both the values don't satisfy Vgs > Vt, and Vgs-Vt =< Vds? Then do we conclude that the transistor is not in saturation region and assume that the transistor is in triode region and check that? I mean is it possible in such cases for the transistor to not be in saturation region and be in triode region or cutoff region? \$\endgroup\$
    – Anwesa Roy
    Nov 2 '18 at 14:09
  • \$\begingroup\$ @AnwesaRoy Yes, if your answer does not validate your assumption, that's a good indication that either your math is wrong or you made a faulty assumption \$\endgroup\$
    – W5VO
    Nov 2 '18 at 14:45

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