Why does my non-inverting feedback Op Amp have loading effect?

Discussion in 'General Electronics Chat' started by Dong-gyu Jang, Dec 11, 2015.

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  1. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
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    Hello.

    In regards to my previous question, I've been digging it and found some weird things to me.

    Please see the pictures below.

    50 Ohm load_1024x596.jpg

    500 Ohm load_1024x587.jpg

    You can clearly see that voltage over the load is depending on the load resistance (loading effect). I've learn that the output impedance of the negative -feedback Op Amp is a fraction of ohm so loading effect is negligible. However, the simulation shows different results to what text book says. Besides, the real circuit also shows similar tendency in amplitude. In real experiment, it is also observed that the pulse duration is extended about twice if the load resistance is above several hundreds ohms.

    In simulation, I've replaced Op Amp to arbitrary ones and some of samples give similar results. I've not tested whole stock of Op Amp but It looks to me that high speed Op Amp has observable loading effect.

    Could you give me some explanation of such a loading effect for Op Amp? Is almost zero output impedance for negative feedback Op Amp not universal? If so, how I can distinguish Op Amp which has such a effect?


    Mod edit:
    Please cut the useless part of the file and compress the file resolution to 800x600 or 1024x768.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The 50 Ohms is way to low for the opamp.
    It will go into current limiting.

    OPA637_output.png

    See the attached datasheet for more info.

    Bertus
     
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  3. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    100
    4
    Thanks for your comments.

    So, too much low load with which current drawn from Op Amp is limited such that voltage developed across the load is low.

    Is it no matter whether Op Amp is open-loop or closed-loop? Is such a current limitation for certain range of load values common for Op Amps?

    I'm still confused about the data sheet which doesn't tell full test circuit parameters like input voltage. Even without input voltage, output voltage is always above 11.5 V? I'm still confused about how to interpret data sheet..
     
    Last edited: Dec 11, 2015
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Yes, almost all opamps will have current limiting for protection.
    There are special opamps for high current available.
    See the attached spreadsheet. (remove the .txt and open it in microsoft excel or open office calc).

    Bertus
     
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  5. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    100
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    Oh, current protection! I got the feeling now! I should have thought this basic design rule.

    I really appreciate your direct help. Thanks.

    Could I have one more question? So beside the current limiting regime, can I think that output impedance of feedback loop Op Amp essentially zero as text book claims? If it is true, my circuit design will be simplified.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When you look at the datasheet of the OPA637, the open loop output impedance is about 55 Ohms.

    Bertus
     
  7. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    100
    4
    Hello.

    Yes, open loop output impedance is specified. However, some articles says output impedance of close-loop Op Amp is very close to zero. I would like to confirm this one more time for safe design.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    3,229
    The apparent output impedance is basically the open loop output impedance divided by the loop gain.
    Since the loop gain is very high for usual closed-loop gains, then the output impedance will be low.
    But for that to be true you have to keep the output current below its rated value as given in the data sheet.
     
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  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    @Dong-gyu Jang

    A couple of things:

    1. Learn how to post reasonably sized attachments. I cannot wait long enough to download your attachments on my 250k-baud DSL line...
    2. This is a continuation of this thread. Why did you split it and start a new thread when we were just getting to the crux of you problem. Now all of the context of what you are doing is lost...
     
  10. ScottWang

    Moderator

    Aug 23, 2012
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