# 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
5,435
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
5,435
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
5,435

#### 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
5,435
Ok I see so, no similar question on your actual exam? And good luck with exam.