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

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.

JoeJester

Joined Apr 26, 2005
4,390
You can edit your photos and resize them for direct input to your postings .... I'll let you do the second photo.

sitg_latin_cousin

Joined Jul 30, 2017
6
Thank you, didn't know that. Here's the second one:

Jony130

Joined Feb 17, 2009
5,222
What value Rs will have if Vp = -3V and Idss = 4mA and for Vp = -6V; Idss = 8mA ?

sitg_latin_cousin

Joined Jul 30, 2017
6
What value Rs will have if Vp = -3V and Idss = 4mA and for Vp = -6V; Idss = 8mA ?
Depends on what Id I choose. The thing is, I have to choose one that does not make Vds less than 4V, and I can't do that with any method.

Jony130

Joined Feb 17, 2009
5,222
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.

Last edited:

sitg_latin_cousin

Joined Jul 30, 2017
6
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.
Yep, I gave up. Anyway I had the final this morning, so it doesn't matter anymore =)

Jony130

Joined Feb 17, 2009
5,222

sitg_latin_cousin

Joined Jul 30, 2017
6
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.

Jony130

Joined Feb 17, 2009
5,222
Ok I see so, no similar question on your actual exam? And good luck with exam.

sitg_latin_cousin

Joined Jul 30, 2017
6
Ok I see so, no similar question on your actual exam? And good luck with exam.
Luckily, not really. Although it had some other problems that were not on my list of exams' problems, so it didn't matter anyway. And thanks! It's two days until it's graded!