I have this question, where the drain-source current and its bias gate voltage of a MOSFET is known where source is connected to ground.

schematic and my calculations

I find its drain current is less than saturation current by a huge margin, but not zero. So I have concluded it's in linear region. Now I am asked to find the drain voltage (Vds).

Using Id equation in linear region, I found two possible values for Vd. Problem is that both the values are positive and for both Vd<Vg-Vt is satisfied. How do I identify which is the right answer?

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    \$\begingroup\$ So I have this question where the drain-source current and it's bias Gate voltage of a MOSFET is known where source is connected to ground My text-to-schematic parser is broken (permanently I'm afraid), so you'll have to include a schematic. Without a schematic my brain simply refuses to answer this although I am sure my brain knows the answer as it has been dealing with MOSFET circuits for 30 years already. \$\endgroup\$ Commented Nov 6, 2020 at 8:43
  • \$\begingroup\$ Ah, let me see. Both are right, or your calculation is dodgy. Check out the chart in Jaeger Microelectronic Circuit Design 4th Ed (free eBook) Page181, Section 4.9.3 LOAD LINE ANALYSIS FOR THE Q-POINT. Happy reading. Cheers. \$\endgroup\$
    – tlfong01
    Commented Nov 6, 2020 at 9:01
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    \$\begingroup\$ Being able to compute a drain voltage depends on something being connected to the drain. What it is? Show a schematic, otherwise there's no way anybody can identify which is right answer. \$\endgroup\$
    – Neil_UK
    Commented Nov 6, 2020 at 9:53

1 Answer 1


Where you stopped showing your work is where you went wrong. Solving the quadratic equation, I didn't get the same results.

Here's Wolfram Alpha's interpretation.

The roots they find are +0.04V and +4.6V. With these numbers, it should be more obvious which one is incorrect as one of them violates the "Not in saturation" conclusion you found earlier.

  • \$\begingroup\$ I just put it in calculator. prolly, made a mistake entering the values. My bad. thanks for pointing it out. \$\endgroup\$ Commented Nov 6, 2020 at 14:36

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