Opamp instability

Discussion in 'General Electronics Chat' started by mentaaal, Oct 11, 2008.

  1. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Hey guys, I am trying to study my notes for Opamp circuits and am a bit confused, especially with instability. My lecturer explained that when "a 40db rolloff" occurs, like that with cascaded inverting opamp circuits, this leads to instability. Could someone please explain to me how this occurrs. From what i understand, due to the internal compensating capacitor in the opamp, the maximum phase shift of an inveting amplifier is 270 degrees. So if this output gets fed into the 2nd opamp, the phase shift out of the 2nd opamp could be around 360 degrees. But i dont understand how this is a problem leading to instability as the output of this opamp is not being fed back to the 1st one?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Here is a link to a brief article that discusses opamp stability.

    A fairly intuitive graphical method is to use the "rate-of-closure" to detemine whether the amplifier is conditionally or unconditionally stable.

    hgmjr
     
  3. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Ok i have made an effort trying to read the contents of that pdf link but its quite heavy going. I am now even more confused that i was before.

    Ok the quote above says that at the 3dB gain occurs at the intersection of Acl and A (closed loop gain and open loop gain), is this always the case? no matter what the closed loop gain is?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    The answer is a qualified yes. If a significant amount of capacitance is introduced into the opamp's negative feedback path, it is possible to roll off the closed loop gain earlier than it would roll off under the influence of the opamp's inherent open loop gain.

    hgmjr
     
  5. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
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    Ok fair enough thanks for that. Do you think you could give me a hint about this 40db rate of closure thing? Its driving me nuts!

    I just cant see how instability can arise because the opamps are not feeding back to the same opamp... the feedback paths are independent!

    I have no idea what this means??
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    Figure 6 illustrates the concept by showing the opamp's open loop gain overlayed with the opamp's closed loop response.

    You can see from the graph that as the example opamp's closed loop gain is increased the resulting bandwidth decreases and as the opamp's closed loop gain decreases the resulting bandwith increases. This behaviour is often discussed in the context of the constant gain-bandwith product.

    hgmjr
     
  7. AchMED

    Active Member

    Aug 5, 2008
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    You can find additional information here.

    Code ( (Unknown Language)):
    1. http://www.en-genius.net/site/zones/acquisitionZONE/technical_notes/acqt_092407
     
  8. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    451
    0
    That link looks very accessable to a student like me without much knowledge of opamps. i shall read it as soon as i get a chance thank you!
     
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