Please help!!! Op-amps

Discussion in 'Homework Help' started by Dara84, Jan 10, 2012.

  1. Dara84

    Thread Starter New Member

    Jan 9, 2012
    1
    0
    Going back to college next week to do electronics exam after 6 year away and really need help with the question below. Any help would be great. Thanks

    In the case of a non inverting op am determine
    -the attenuation factor B
    -the closed loop input impedance
    -closed loop output impedance
    -closed loop voltage gain

    Given

    Input impedance (Zin) 4MΩ
    Output impednace (Zout) 60Ω
    Open loop gain (Aol) 200,000
    Input resistor 15KΩ
    Feedback resistor 180KΩ

    So i think the folrmulas

    attenuation factor = B = Ri / (Ri + Rf)
    closed loop input impedance Zin(NI) = (1 + Aol B) Zin
    closed loop output impedance Zout(NI) = Zout / (1 + Aol B)
    closed loop voltage gain = Acl(NI) = 1 + (Rf/Ri)

    But have am not sure what figures to sub in where
    I presume
    Ri = 4MΩ
    Rf = 180KΩ

    So
    B = 4,000,000 / (4,000,000 + 180,000) = 0.9569
    Acl(NI) = 1 + (180,000 / 4,000,000) = 1.045

    and that is about as far as i get... any help would be great. Thanks
     
  2. zaman999

    New Member

    Jan 29, 2011
    26
    0
    > Ri is the input resistance and not the input impedance as you assumed.
    > substitute the values for Ri and Rf in your formulas to get correct result for B and Acl.
     
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