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Homework Help: Op amp and 3-db frequency problem

  1. Jan 18, 2015 #1
    1. The problem statement, all variables and given/known data
    An inverting amplifier with the DC gain of -9 (V/V) is built with an op amp having its
    DC gain of 105
    and corner frequency of 30 rad/s.
    ▪ Determine the 3-dB frequency of the inverting amplifier.
    ▪ Sketch the magnitude Bode plot of the voltage transfer function of the inverting amplifier.

    2. Relevant equations

    3. The attempt at a solution

    So I am dealing with this homework and I have absolutely no idea how to approach this problem.
    Just to clear things out I do not want someone to do this for me as you'll all guess. But I am kinda asking for help on how to approach and start this problem. Also I am not sure what the question is. I know that that 3-db frequency is the half power point but because this is a DC gain I would think that I am dealing with a DC circuit and therefore I would not expect any frequency in this problem (but I am probably wrong). I am also having a hard time understanding why there are 2 given gains, the -9 V/V and the other 105
    What are the difference in those gains. So far we've learned that an op amp can be represented with a simple circuit consisting of 2 resistors and Voltage-Controlled-Voltage source (VCVS).

    So basically my question is how do I approach this and am I suppose to simplify the op amp with a simple amplifier circuit with 2 resistors and VCVS?

    Thanks in advance.
  2. jcsd
  3. Jan 18, 2015 #2


    User Avatar

    Staff: Mentor

    The high gain is the open-loop low-frequency gain. The -9V/V is the closed loop low-frequency gain with the negative feedback. Show us the Bode plot for the open loop gain of an opamp (find it via Google Images or scan your textbook). Then show us how that open-loop Bode plot of gain is altered when you close the loop with negative feedback...
  4. Jan 18, 2015 #3
    This is the only thing that was given in the problem. There is no image or anything else. But shouldn't I first determine the 3 db frequency before sketching up the Bode plot?

    Also I am not quite sure how to draw the circuit. I've completed one sketch but I am not sure if it is the right thing to do.
    I will link the sketch that I've drawn.

    Attached Files:

    Last edited: Jan 18, 2015
  5. Jan 18, 2015 #4


    User Avatar

    Staff: Mentor

    I was just asking for some visual support for this problem. It helps a lot to see the Bode plots of opamps with and without feedback when working this type of problem. As I said, use Google Images or some other way to find some Bode plots that you can use to help you illustrate and work this problem.... :-)
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