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Using maths - Amplifiers

  1. Oct 19, 2005 #1
    hi all i got this operational amplifer question was wondering if anyone can help out

    1. a feedback amplifier has a closed loop gain of 100, and the negative feedback is 0.8% of the output signal. the opamp has an open loop gain which is considerable less then the ideal value of infinity.
    a) what is the open loop gain
    b) what would the closed loop gain be if the opamp had an infinite open loop gain

    ok for this question i assume this is an inverting amplifier so i know these formulas
    A (gain for closed loop) = Vout/ Vin = -RF / Rin
    A (gain for open loop) = Vout / (V1 - v2)
    i am guessing these steps hope it is right
    we know closed loop gain = 100 hence
    100 = -RF / RIN
    and i know -RF=0.8/100 * Vout
    i know RF=-100 and RIn=1
    100 = (0.8/100) * Vout
    Vout = 12500 V
    Vin = 125 V

    so open loop gain = 12500 / (125 - 12500)
    = -1.01 ???

    b) no idea about this, would it be 0?
  2. jcsd
  3. Oct 21, 2005 #2
    I could be wrong here, but if the O.L. gain of the Op-Amp is finite then I don't think that the formula


    applies anymore. You need to do some analysis of the amp without the assumption of an extremely large O.L. gain, that's how you arrive A=-Rf/Ri. Without that assumption, A=-Rf/Ri no longer holds good. Most electronics books probably give an analysis of the non-ideal inverting configuration.
    Last edited: Oct 21, 2005
  4. Oct 22, 2005 #3


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    Homework Helper

    Open loop gain of Op-Amps (in a feedback configuration) are constant but only at low frequencies. As you increase in frequency, its gain attenuates.. For example see Figure 5 on this reference. If you could look at a datasheet for the Op-Amp in this question, you can read off its open loop gain. From the information given in your question, I don't see how you can calculate it.
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