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Determine input voltage given resistances

  1. Oct 3, 2015 #1
    1. The problem statement, all variables and given/known data

    Calculate the expected input voltage for each set of resistances below. For the CMOS gate, assume the input resist of the gate is infinite.

    2. Relevant equations
    i=v/R
    Voltage divider rule v*R1/(R1+R2)=v1

    3. The attempt at a solution
    1)For R1=1K and R2=infinity, would V simply be 0 since the resistance is infinity? I'm mostly confused with how the CMOS gate and the infinity resistance will effect the voltage.

    2)For R1=1k and R2=10K
    v1=5V*1K/(1K+10K)= 0.45V?

    I assume for all the rest, the values with infinity I'd calculate as in step 1, and the values with two specific numbers I'd calculate as I would in step 2?
     

    Attached Files:

  2. jcsd
  3. Oct 3, 2015 #2

    gneill

    User Avatar

    Staff: Mentor

    Check your voltage divider equation: The input voltage is developed across R2, not R1.

    Redraw the input circuit without R2 (an infinite resistance is equivalent to an open circuit):

    Fig1.png
    What's the input voltage?
     
  4. Oct 3, 2015 #3
    Oops,

    1)The input voltage would just be 5V then wouldn't it?

    2) v1=5V*10K/(1K+10K)=4.545V?
     
  5. Oct 3, 2015 #4

    gneill

    User Avatar

    Staff: Mentor

    Looking good!
     
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