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!

Homework Help: Op-amp resister circuit analysis

  1. Mar 9, 2015 #1
    1. The problem statement, all variables and given/known data
    Given the ideal op-amp, determine the resistor R2 and the current I2 where Vin=500mV, Vout = 2.5 V, and R1=5kV.
    2. Relevant equations
    No current passes through op-amps, the op-amp tries to make V+ and V- equal.
    Ohm's law, voltage divider equation

    3. The attempt at a solution
    Since V+ is 500mV, I assumed that the node between R1 and R2 must also be 500mV, as it is connected to V-. So 500mV = 2.5V(R2/(5000Ω+R2)). I got R2 to be 1,250Ω.
    Then I found I2=Vout/Req = 2.5V/(1250Ω + 5000Ω) = .4mA.
    However, I am unsure because I am not sure if I did the part with the op-amp right; namely, the node between the resistors being set to 500mV.
  2. jcsd
  3. Mar 9, 2015 #2


    User Avatar
    Gold Member

    You are 100% correct sir.

    V+ = V- in ideal opamps...

    Nice job.
  4. Mar 9, 2015 #3


    User Avatar

    Yes - and for the future you can use the general closed-loop non-inverting gain formula (assuming an ideal opamp): Acl=1+R1/R2.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted