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Homework Help: Help with this OPAMP CKT

  1. Oct 2, 2007 #1
    Problem I:

    Can someone help decribe what this circuit is meant to do ie. its basic operation? I am not quite sure what is going on.

    I have attached the input/output waves and the actual circuit diagram.

    The smaller signal (green) is the input (from the left) and the larger signal (yellow) is the output of the OPAMP.

    If you can just let out as much as your thoughst as possible because I might not be able to respond quickly. 13 hours time difference. Thank you in advance.

    Attached Files:

  2. jcsd
  3. Oct 2, 2007 #2
    Problem II:

    This is what I believe to be an integrator but it is not working the way it is on the board in the lab. The pictured ckt is exactly as it is on the board but is there anything I must add to correctly model this ckt so that it behaves in spice the same way it is suppose to when I use the scope in the lab.

    Attached is the input (yellow) and the output (green). In spice I always get a -4V saturation. Should I put a resister across the feedback cap?

    Attached Files:

  4. Oct 3, 2007 #3


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    Science Advisor
    Gold Member

    This looks a little like a low pass active filter. The way I think it should be wired (not sure) would be to eliminate C111, run C112 down to the negative input, and R126 to the junction of R125-R178. The output would be biased at zero volts since the positive input is grounded.

  5. Oct 3, 2007 #4
    Thank you for you reply.

    The thing is the wiring is the same on the circuit board as you see in the picture. I have to just understand how this designed circuit works. Therefore, I cannot just rewire it. I put it in Spice and cannot get anything other saturation at the output. If I can understand and getting it working in Spice then that would be great bit for now I am just confused about what this thing is doing.
  6. Oct 3, 2007 #5


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    Staff: Mentor

    Thread moved to the Homework Help forums. Welcome to the PF, higha level. The circuit in Problem I is pretty goofy -- why are they paralleling-up resistors in the schematic? Just to make it more work to solve for the transfer function? The positive feedback term is pretty unusual too. Do you have any ideas yet as to what the circuit's transfer function is like?

    On Problem II, Can you be more explicit about the power supplies? They look reversed...?
  7. Oct 3, 2007 #6
    Thank you for the reply and thank you for the welcome. Side note, I found your Bad Circuits thread very useful.

    I did not post this in the homework thread because I didn't think it was homework. I am doing an internship and these circuits are part of a much bigger! schematic that I have to understand. I think they parrallel the resistors because maybe they did not have those resister values when they built the actual circuit.

    I was thinking that the first Problem circuit was an Oscillator because it looks very similar to Wein Bridge Oscillator on first glance. Then I was thinking bandpass filter when looking at the configuration and can't seem to undertsand this positive feedback or what is going on? Any small hints so that I can start reading would be of great help.

    All opamps have a 4.2 (VCC)/-4.2 V(VEE) voltage supply. I apologize for not including the in the schematic drawing.
  8. Oct 3, 2007 #7
    Similar to Problem II

    I have an additional circuit that serves the same purpose somewhere as the 2nd circuit that I posted. They do the same thing but are configured differently. At first glance I said it was an integrator, taking the average of the input signal. What you are seeing is that sine wave from the first Problem feeding this circuit. At the output of the first Opamp the signal is roughly the same is the previous Problem II but shift 90 degrees. The output is similar a DC constant voltage. No matter what I do I get saturation. And for some reason I am getting an offset of approx. 3.3mV everytime at the noninverting into. I know Integrators do not like that but on the real thing there is no offset (or very small) and gives me a nice DC ouput. Why? How is this thing averaging. Is it averaging? :confused:


    Attached Files:

  9. Oct 4, 2007 #8

    In case anyone wanted to know, I now understand the first problem. It is a bandpass filter centered at 2kHz (which makes sense to me) and has 20dB of gain. Without the positive feedback it would be a bandpass filter centered at 2kHz but it is not a very good filter and has about -4 to -5 db of gain.

    If you Spice and look at the frequency response you will see. I then understand some of the circuits around it now.

    Thank you for stirring my thoughts.

    The problems I have are still Spice related with Problem II and the added similar problem.
  10. Jan 1, 2008 #9
    Wein-Bridge Bandpass Filter

    Your Circuit I is indeed a bandpass filter, based on the Wein bridge... simply an oscillator where the positive feedback is reduced to below the point of oscillation, which can be a fussy adjustment if you want exactly the right Q or gain. The parallel resistors are to get the exact amount of feedback and the exact frequency, I guess - getting values not available from the E24 range, or maybe to allow easy changes later.

    Anyway, this is a pretty good filter circuit, yet almost never mentioned (I was beginning to think nobody else used it after going through many text books and google search results before finding this example). As with most single-amplifier highish-Q filters, it is very sensitive to source impedance, but is tolerant of opamp imperfections (better than MFB and S-K bandpass filters, IMO).

  11. Jan 3, 2008 #10
    Thank you

    Thank you very much for your reply. It's add no one else new what it was or cared to share till now. But I just put it in SPICE, ran a frequency response sim and figured it out. Thank you again.

    The second circuit I also understand completely. Just had to concentrate more because no one was really providing much ideas.
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