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Inverting Op Amp Circuit

  1. Jul 1, 2015 #1
    1. The problem statement, all variables and given/known data

    Write output voltage in terms of input voltages for the circuit drawn.

    I drew the circuit and then feed back loop.

    circuit-9_4 (1).png
    2. Relevant equations

    Ohms Law V=IR
    Basic Properties of Op Amps (ideal), voltages are assumed equal at both inputs...

    3. The attempt at a solution

    The difficulty is that there should be two currents that need defining, but I'm not sure how to eliminate both of them. I wrote two equations for the current through the feedback loop and then equated to write Vout in temrs of the resistances and Va.

    [tex]I_a = \frac{V_{out}-V_a}{R_2} [/tex]
    [tex]I_a = \frac{V_a-V_{out}}{R_1}[/tex]
    [tex]V_{out}=\frac{V_a-V_{in}}{R_1}R_2 +V_a[/tex]

    I can't figure out how to write Va in terms of Vin and the resistances at this point. The only other relationships between Va and other quantities come from the fact that [tex]V_b=I_bR_3[/tex] which means that [tex]V_a=I_bR_3[/tex] also, but I still need to get rid of the current Ib now, so that doesn't help much. I can't figure out what I'm missing. Any help would be great.
  2. jcsd
  3. Jul 1, 2015 #2
    There's a mistake in the feed back loop in my drawing, the ground at the end of R2 should not be there, that should be Vout.
  4. Jul 1, 2015 #3


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    Staff: Mentor

    You've listed one useful property for the op-amp in a negative feedback situation. What other property might be useful?
  5. Jul 2, 2015 #4


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    Have a look on the second equation. Obviously, it is wrong (Vout mixed-up with Vin).
    More than that, you can forget the influence of R3 (because an ideal amplifier with infinite input resistance is assumed). Hence, you can set Va=Vb=0.
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