Exceeding Op-Amp Power Supply Rail

Discussion in 'General Electronics Chat' started by newbie217, Sep 24, 2009.

  1. newbie217

    Thread Starter Active Member

    Apr 12, 2009
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    0
    Hello,

    I was playing with an op-amp in lab and I think I exceeded the negative power supply voltage maximum. When I applied -6 V, as an example, I noticed the output started to oscillate like crazy.

    In regards to the op-amp circuitry, what is going on inside? As soon as I back off the voltage, the output stabilizes again. I just need a simple picture to help visualize what is going on. Suppose I kept driving it more and more negative, trying to reach destructive failure. What is usually the first thing to breakdown internally? What would I expect to see the scope?

    Thanks!
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Opamps have a positive supply pin and a negative supply pin. They do not have a 0V pin.
    Which opamp part number are you talking about?
    Which opamp circuit are you talking about? Post it for us to see what is wrong with it.
     
  3. newbie217

    Thread Starter Active Member

    Apr 12, 2009
    52
    0
    Any op amp. I just want to understand what is going on behind the scenes. For example, and op-amp is rated at absolute maximum of +5 and -5 V. I hook up the positive supply to +5 V and hook up the negative supply to -6 V. The output starts to become unstable. What is breaking down internally? In real world application, what is the most common means of failure?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    One good way to investigate that kind of phenomenon is to use a PSPICE simulator.

    Linear Technology has a very good and free PSPICE simulator, available for download at their website:
    http://www.linear.com
    Or just go here:
    http://www.linear.com/designtools/software/ltspice.jsp
    You don't have to register if you don't care to.

    In the supplied examples/instructional folder, you'll find LM741.asc, a simulation of a 741 opamp made of discrete components. It's already set up with a sinewave on the non-inverting input and a gain of 11. You can experiment with different input levels, and probe around the circuit to see what's happening with current, voltage and power.
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    Absolute Maximum Ratings are those values beyond which the life of a device may be impaired, and/or the device starts to malfunction like it did in your case. The behavior may differ. It does not exist a answer book telling exactly what will happen. You may zap a opamp input stage with ESD. Or kill the output stage because you draw to much current over a period. If a component is goner, you can not do much than replace it. You should never make design with values equal to Absolute Maximum Ratings.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    The failure mode for overvoltage on an IC is an avalanche breakdown effect which punches through the junctions of devices. This is a destructive effect. The Abs Max may be 30V, but the actual breakdown point for a specific part will be above that but by how much is never known until it dies.
     
  7. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    69
    Many CMOS devices can 'fire' due to incidental thyristor-like structures in the IC if a pin is forced outside the supply pins; the power pins become a near short circuit.

    If the supply has a lowish current limit, it may give the appearance of oscillation, but with a higher current supply the chip will just pop, functionally or literally...
     
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