Voltage output of a Cascade Amplifier

Discussion in 'Homework Help' started by Jess_88, Apr 30, 2011.

  1. Jess_88

    Thread Starter Member

    Apr 29, 2011
    174
    1
    Hi guys :)

    I was wondering if I could get some help getting started with this question.

    Given Rf = ∞, determine Vo.
    [​IMG]

    Given Rf = ∞, I am thinking that there is no current flowing through the resister and therefore no voltage. So would I neglect it from my calculations???

    I'm also not sure how the input to the second out amp effects the circuit.... I'v only ever seen it connected to a negative input.

    I have started my analysis, finding the current through the 5kΩ and The 15kΩ resistor.

    I1 = 10mV/20kΩ

    Again... I'm not sure if I'm my method if correct at all... due to Rf.

    Thanks so much guys
     
    Last edited: Apr 30, 2011
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    782
    Yes - since Rf=∞ you simply neglect it in the calculations.

    You have two stages - the first is an inverting amplifier, the second a non-inverting amplifier. The overall gain magnitude is the product of the two gains - keeping in mind the inverting mode of the first stage. Will the overall gain have a positive or negative sign?

    You don't need to go from first principles to find the gain. See sections 2.2 & 2.3 in the link.

    http://en.wikipedia.org/wiki/Operational_amplifier_applications
     
    Last edited: Apr 30, 2011
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  3. Jess_88

    Thread Starter Member

    Apr 29, 2011
    174
    1
    ah ok.

    so for the first inverting amp
    v1 = -10mV(15/5) = -30mV

    and for the non-inverting amp
    v2 = -30(1 + 6/2)

    do I have to take into account the resistance at the output?
    or is v2 = vo?

    thanks so much for the quick reply :)
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    The amplifiers are probably meant to be considered as ideal & would notionally have zero output resistance. So the load resistance would have no impact on the overall gain - which you seem to have correctly resolved.
     
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  5. Jess_88

    Thread Starter Member

    Apr 29, 2011
    174
    1
    thanks so much for the help :)
     
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