1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    gneill

    User Avatar

    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

    LvW

    User Avatar

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Inverting Op Amp Circuit
  1. Non inverting op-amp (Replies: 3)

  2. Inverting Op Amp (Replies: 5)

  3. Non-Inverting Op-Amp (Replies: 4)

Loading...