The homework has been solved, but I am having a problem which I hope someone could help me understand because there might be a incorrect solution from my teacher. I pit the details in the picture. I did try to take KVL from V+ but my question is how did he end up taking Rs coltage instead of Rd. Or since he took coltage drop on Rs why didn’t he start KVL from V-.

 

Well, It is a sloppy solution indeed. And it only works because V+ is equal to |V-|.

As you know the drain current (Id) is equal to the source current (Is). And this current is controlled by Vgs voltage but the source terminal is not grounded or connect directly to V-. We have Rs resistor hence must be included in the equation, not Rd.

 

I did my solotion but I got different answer because I did quadratic formula to get Vgs. But when I compared solutions I saw the only difference is the V- at the end before setting the wuadratic formula I got 30vgs^-59vgs+35. Where in the solution manual 30vgs^2-59vgs+25. I still don’t understand why he did this and how can I know which solution is definitely right.

 

What if I change the circuit to this form:



Can you solve it now?