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Simple Op-Amp Inverting Amplifier

  1. Feb 24, 2014 #1
    1. The problem statement, all variables and given/known data

    I need to find an expression for the gain in this circuit. I know it is an interting amplifier, because with the models we were given to look at, this one fits the inverting amlpifier. I've also constructed a simulation in a simple Java applet that I have.

    My problem is, I need to do this analytically. It is very late right now and I know I'm just having a brain fart. This is what the simplest inverting amplifier looks like:


    It's fairly obvious that, given a little thinking, the gain on this circuit:

    [itex]\frac{V_{in}}{R_{in}} = \frac{-V_{out}}{R_{f}} \rightarrow V_{out} = \frac{R_{f}}{R_{in}} V_{in}[/itex]

    The problem is, with my circuit, Rf isn't just one resistor. It's 3, in a very strange configuration, and I cannot for the life of me figure out how to find the total resistance in that part of the circuit. This is my circuit:


    I tried looking at it as sort of a voltage divider, but that third resistor connected to ground is really screwing with my head! Help!

    2. Relevant equations

    V = IR is basically all I think you'd need.

    3. The attempt at a solution

    See above description.
  2. jcsd
  3. Feb 24, 2014 #2
    R_3 is connected to the ground and R_2 is connected to a point that's at ground potential because of the negative feedback. These are parallel for the purpose of calculating the potential at the point where R2, R3 and R4 meet.
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