1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Textbook equation magic

  1. Mar 27, 2014 #1
    1. The problem statement, all variables and given/known data
    Supposed to find Ro for a CS MOSFET amp given V'A=10V/μm, L=0.4μm, and ID=100μA.

    2. Relevant equations


    3. The attempt at a solution

    I say that ro=10μ*0.4μ/100μ=
    = 4*10^-8

    Book says "10x0.4/.1=40kΩ"

    .. what exactly went into that? for one thing (10*0.4)/.1 does not = 40,000.

    also if you multiply something in micrometers by something in micrometers, then divide by something in microamps, you should end up with a very small number, I think.

    I'm sure 40kΩ makes sense somehow but they're doing a bad job of explaining it.
  2. jcsd
  3. Mar 27, 2014 #2
    Nevermind, I got it. V'A was in units of V/um, so really 10*(V/um)

    [(10*(V/um))(0.4*um)]/(100*uA)=(10*V*0.4)/(100*uA)=40,000 ohm

    ^ because the um cancels out in numerator, ^ becomes just 4 V
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted