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Homework Help: Op amp in circuit: find load current

  1. Feb 25, 2013 #1
    1. The problem statement, all variables and given/known data

    http://imageshack.us/a/img27/7043/homeworktest2prob5.jpg [Broken]

    Find the load current IL (in microamperes) in the circuit.

    2. Relevant equations

    V = IR

    KCL, KVL,

    voltage division, current division,

    Op-Amp Output V = (Gain)(V+ - V-)

    (Gain) = -Rf / Rin

    3. The attempt at a solution

    I'm not sure how to figure out the voltages at the nodes before the inputs to the op-amp

    I think the gain is just -5k / 10k

    I'm pretty sure all I would need to figure out is the input node voltage or whatever the Vin is for each input.

    But I'm guessing:

    V out = (5V - 3V)(-5kΩ/10kΩ) = -1V

    IL = -1V/4000Ω

    = -250μA

    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Feb 25, 2013 #2


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    Perhaps check here...

    http://www.ustudy.in/node/2939 [Broken]

    Your circuit doesn't have an R3 so substitute R3=0 into the transfer equation.
    Last edited by a moderator: May 6, 2017
  4. Feb 25, 2013 #3


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    Staff: Mentor

    Color_of_Cyan, do you know the basic characteristics/properties of an ideal op-amp? Can you describe them?

    If you know these basic things and the rule about the potential difference between the op-amp's inputs when there's feedback from the output to the negative input then you will be able to derive the output voltage fairly easily from the diagram.
  5. Feb 25, 2013 #4
    CWatters, woops it did have a resistor there, I fixed it now though.

    Thanks... although that's kind of weird. Is there any way to derive that? Or are you supposed to know for each type of op amp?

    I still got V out = -1V with that by the way.

    And the current through the 4k out is still -1A / 4000 ohms right?

    gneill I really don't; my professor went through it pretty fast. I looked something up though and says that the + and - inside is connected by a resistor with a big value and the gain is supposed to be high.

    The output is supposed to also be between here -5V to 5V.

    That's pretty much all I know
  6. Feb 25, 2013 #5


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    If the gain of the opamp is very high then the +ve and -ve inputs must have very similar voltages or the output will saturate (be driven to the supply rail). For example if the opamp had a gain of 100,000 and the max output swing was say +/- 5V then the inputs cannot differ by more than 5/100,000 = 50uV or it will saturate.

    In your circuit you can easily work out the voltage on the +ve input using the potential divider rule. Then using the above you know that the -ve input will be at a very similar voltage. Then you can look at the currents through the 5k and 10k to work out the output voltage.

    Aside: On many opamp circuits (eg simple inverter) the +ve pin is 0V (or earth). Therefore if the amplifier is designed to operate in it's normal linear mode the -ve input must also be very close to 0V. This is known as a "virtual earth" because although it's not connected to 0V(earth) the -ve pin must be very close to it.
  7. Feb 25, 2013 #6


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    For example..

    +ve pin...
    3 * 3/9 = 1V

    therefore -ve pin must also be 1V or very close to it!

    Current through 10K resistor is

    (Vin-1)/10K = 4/10k

    Current through 5K resistor is


    These currents are the same as "none" goes in the -ve Pin (high impedance inputs).

    Solve to give Vout = -1V
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