Need help with simple FET circuit. Formulas I have to use included.

Thread Starter

sitg_latin_cousin

Joined Jul 30, 2017
6
So I took a pic because it's just easier than typing it out.

http://i.imgur.com/5vLpo93.jpg

One solution that was the most promising one was considering a triangle of slope m=1/Rs, height Idmax=(Vdd-Vds)/(Rs+Rd) and side of Vgs-Vth. Pic here of the math (triangle in red for better understanding)...

http://i.imgur.com/kJXtH3B.jpg

It gives complex Rs.

I have tried EVERYTHING. I took derivatives of gm with respect to Rs, Id and whatever combination of the formulas above. I also tried combining formulas and see what comes out. For example, I put Id from the first formula written (the one with Idss and Vp, replacing Vgs with Vth-Id*Rs) equal to (Vdd-Vds)/(Rs+Rd). Nothing, it came out Rs=0, which is absurd, because that would mean Vgs=-6v, so Id=0.

I honestly have no idea what to do next. I've been at this for over 4 hours.
 

Jony130

Joined Feb 17, 2009
4,980
To be honest I also don't have a clue how to solve it. I don't see how it is possible to get Vds_min = 4V with such a high voltage at the gate Vg = 6V.
 
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Thread Starter

sitg_latin_cousin

Joined Jul 30, 2017
6
Have you tried to ask yor professor about this problem?
No, because I got it from another student that had a list of finals from previous years. In my university, professors don't like when you practice for exams with previous exams. It's not exactly ilegal, but it's not a nice impression to the professor to ask him to teach you exercises that are supposed to be on the exam.
 
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