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Homework Help: Phototransistor modeled as a Thevenin equivalent circuit

  1. Mar 22, 2008 #1
    1. The problem statement, all variables and given/known data
    you have a phototransistor that you have modeled as a Thevenin equivalent circuit. It has a output impedence(Thevenin Impedence) of 10kohm and produces a peak voltage o2 2V at a frequency of 80 MHz. The input impedence of the oscilloscope as this simple combination of input impedences.
    a.draw the osilloscope/photodetector circuit
    b. calculate the half power frequency
    c. a 50 ohm resistor is added in parallel to the 1M ohm load. draw the new circuit.
    d. calculate the new break frequency.


    2. Relevant equations
    [tex]f_b=\frac{1}{2\Pi R C}[/tex]



    3. The attempt at a solution
    1.The photodetector in parallel with the capacitor and 1m ohm resistor
    2. I just took [tex]f_b=\frac{1}{2*\Pi*10^3 ohms*20^-9F}=81.40MHz[/tex]
    3.The photodetector in parallel with the capacitor,1m ohm resistor, and 50 ohm resistor
    4. not too sure
     
  2. jcsd
  3. Mar 23, 2008 #2
    Could you clear-up this typo?

    "The input impedence of the oscilloscope as this simple combination of input impedences."
     
  4. Mar 24, 2008 #3
    Sorry,
    "Model the oscilloscope as this simple combination of input impedences."
     
  5. Mar 24, 2008 #4
    Actually for b. I get 7.87 Hz.
     
  6. Mar 25, 2008 #5
    Sooo......is that right?
     
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