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!

Circuits Op Amp R equilvalent help

  1. Mar 21, 2008 #1
    [SOLVED] Circuits Op Amp R equilvalent help!!!

    1. The problem statement, all variables and given/known data
    I'm working on Ideal op amps and I need to find the R equivalent of the system.
    I'm not sure if this term is used in other places but R equivalent = total Resistance of the circuit.
    http://img169.imageshack.us/img169/4386/reqrk6.png [Broken]

    2. The attempt at a solution
    I have to place a voltage at the circle at the top left and use the voltage to divide the current to get the Req, but I don't know how to find the current. I think I have to use a current divider at the 10 and 40 k ohms but I can't seem to get the answer of 14 k ohms as the answer in the back of the book. Please help!

    3. Relevant equations
    I'm also confused if this is the Negative Resistance mentioned in the book. But the given equation for Req = -(R1/R2)R
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Mar 21, 2008 #2
    First calculate the voltage on the positive input of the opamp. That is just a simple voltage divider.
    The opamp is used in a circuit with only negative feedback. What does this mean about the voltage on the negative input?

    Once you know the voltage on the inputs of the opamp its simple to compute the input current of the circuit.
  4. Mar 21, 2008 #3
    which resistance on the bottom is the one I need? I'm having problem on setting up that voltage divider at the positive input..
    is it 30/70 * v ?
  5. Mar 21, 2008 #4
    yes. you just have a current going from the input through the 40k then the 30k and then to ground. If the resistance of the input of an op-amp isn't given, you can assume it is infinite.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Circuits equilvalent help Date
What does W=VIt show? Tuesday at 9:20 AM
Find the charge of Capacitor 2 in the circuit Tuesday at 8:55 AM
Voltage in parallel V1=V2=V3 Mar 17, 2018
Kirchhoff's First Law to solve a circuit with 2 loops Mar 16, 2018
Equilvalent Resistance Problems Feb 1, 2004