Another NMOS-transistor problem

  1. [SOLVED] Another NMOS-transistor problem

    I'm sorry for asking again so soon, but these transistors really give my head a spin.

    1. The problem statement, all variables and given/known data
    The problem is pretty much summed up by the following photograph:

    3. The attempt at a solution
    To be quite honest I'm really stumped here. I can go over what I know (Or at least what I think I know):

    Since everything displayed here is in series, I'm presuming the current over each element is the same. The voltage over R1 should be 10 - v1, basically meaning if I could find v1 I should be able to find the current R1 and thus the current in every other element connected aswell, which would make the problem easy.

    Vgs for the first transistor is 5 volts, and vgs=vds for the second transistor. R1 and R2 should have the same voltage drop across them.
    All in all, 15 volts dissipate over this circuits as vdd = 10 volts and vss = -5.

    After this, it completely stops. I've got the correct answer, which is
    v1 = 6 v
    v2 = 2 v

    So by my logic, the current over the first resistance is (10-6)/1000 = 4 mA.
    This is where I get confused, because if v2 = 2v, doesn't that mean the voltage drop across the first transistor is 4v aswell?
    But if it is, this indicates that 16 mA runs through it, which isn't really possible if only 4 mA runs through the first resistance.
    I tried checking what the voltage vds over the first transistor had to have been in order to allow for 4mA to pass through, and as far as I can remember the answer I got was sqrt(2)+1. Which is a fairly ugly number so I'm presuming this is wrong.

    Anyone got a hint that can push me in the correct direction? It would be greatly appreciated. I'm dying to understand these blasted transistors.
  2. jcsd
  3. dlgoff

    dlgoff 3,158
    Science Advisor
    Gold Member
    2014 Award

    "Vgs for the first transistor is 5 volts..." Vgg=5 volts. Wouldn't Vgs=Vgg-V2?
  4. Aha, I see now, of course. Thanks!
    I managed to solve it now by using four different equations, was quite a hustle but I've never figured it out if not for this!

    Thanks again!
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?