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Homework Help: Non-Inverting Op-Amp

  1. Aug 10, 2012 #1
    I'm supposed to be analysing this circuit to go with my lab report. However I haven't seen a circuit like this. First off I'm not sure what the symbol is on the top left resistor (15kΩ//1.2kΩ) means.

    Secondly I'm used to doing inverting op amps and they generally don't have many/ any resistors attached to the side that doesn't have the voltage source. I'm curcious if and what effect the other resistors will have. I'm assuming there's some kind of feedback there, however I'm not sure how to connect the voltage source and the feedback.

    Any help would be greatly appreciated. The circuit diagram is below.

  2. jcsd
  3. Aug 10, 2012 #2


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    Staff: Mentor

    The circuit is complete as shown. You don't add any more resistors. The 10k resistor is the feedback resistor. You connect your source to input S1 and take output at Out1.

    15k // 1.2k is a pair of resistors in parallel, and is a method of providing a resistance of a special intermediate value when it's needed, that's all.
  4. Aug 10, 2012 #3


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    Homework Helper
    Gold Member

    Well, what did you actually do in your lab (I assume that you did the experiment if you are writing a report on it)? Did you maybe build 2 different versions of this circuit - one with a 15kΩ resistor and one with a 1.2kΩ resister?

    As always (when dealing with op amps), start with the Gain equation:

    [tex]V_{\text{out}}=A(V_{+} - V_{-})[/tex]

    Use Kirchoff's circuit laws to express [itex]V_{+}[/itex] and [itex]V_{-}[/itex] in terms of the resistors in your circuits, the output potential ([itex]V_{ \text{out} }[/itex]) ,and the input potential ([itex]S_{1}[/itex]?).
  5. Aug 10, 2012 #4
    Thanks to both of you for responding. The lab was just connecting a board with jumpers and using an oscilloscope to evaluate the gain and the phase shift.

    So from what I've gathered:

    The odd symbols for the resistors was two resistors in parallel. And thanks especially for the tip to start with A(V--V+), our lecturers and tutors just run ahead and do each circuit in the quickest way possible, so I'm left trying to think of the best way to look at a problem. I'm not great with analysis when there's heaps of different ways of doing it. I get overloaded everytime I look at a problem trying to think of the best way to do it haha.

    Thanks again!
  6. Aug 10, 2012 #5
    This is an amplifier with negative feedback so you can assume that V at the + input equals V at the - input.
    If you know the voltage S1 can you see how to work out the output voltage to make V+ =V- ?.
    Small tip .....The 10k from S1 to ground is not important in the calculation, do you know why?
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