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Homework Help: 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
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