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Value of RF (feedback resistor formula)

Discussion in 'Electronics Homework Help' started by qwerty22, Nov 10, 2013.

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  1. qwerty22

    qwerty22

    1
    0
    Nov 10, 2013
    I need help as I have the answer but I don't know the formula

    its an 741, with vi 1Vpp, input resistor is 10k, Value of output signal 10Vpp

    I have to work out RF which is 100k, I did the usual **** 1vpp = 10k
    10 vpp=10k x 10= 100k
    But not the proper formula so I though this:

    AV ( idk what it is) = 10v/1v = RF/10k =10

    so RF = 10k*10 = 100k

    Is that the formula? i need to show proper formula as working out, which i don't know,

    however the answer is correct...

    I can't seem to find the formula online anywhere btw.
     
  2. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    464
    105
    Aug 27, 2013
  3. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    Instead of trying to find the correct formula, why not derive it yourself? Just realize two facts:

    1. When the 1 Vp-p input is at +1V the 10 Vp-p output is at -10V.
    2. The '-' input to the opamp is a virtual ground. The '+' is tied to real ground and negative feedback keeps the '-' input at the same voltage as the '+' input (within a few microvolts).

    [​IMG]

    So what you have there is a simple voltage divider where 11 volts is divided between Rin and Rf. Solve that and you have the correct formula for finding Rf.
     

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