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Op amp question

  1. Jun 21, 2010 #1
    What will happen when the input voltages applied to the op amp is higher than its supplied voltage? Will it damage the op amp?
     
  2. jcsd
  3. Jun 21, 2010 #2
  4. Jun 21, 2010 #3
    I'd say it depends more on the input current levels than the voltages. You should look at the input circuit of the opamp to determine whether it might be damaged. One factor is whether the input transistors are PNPs or NPNs. The base-emitter junctions of transistors when reverse biased typically have zener voltages in the range of 5 to 7 volts though I don't know what it is for opamp input transistors. If you have a PNP transistor at the input like an LM324 and you don't exceed that voltage, you may be okay. You may even be able to exceed that voltage if your current is low enough.

    On the other hand the LM741 uses NPNs at its input and just as with any NPN the voltage applied to the base bias resistor can be higher than Vcc at the Collector as long as the current rating is not exceeded.
     

    Attached Files:

  5. Jun 21, 2010 #4
    Op amps and comparators have two parameters supplied by the manufacturers

    Differential input Voltage, which is the max the difference between the inputs
    and the

    Max Input Voltage which is just what is says.

    Design must take account of both figures.

    eg for a 710 comaprator the max input voltage is +/- 7 volts and the max differential input is +/- 5 volts.
     
  6. Jun 21, 2010 #5

    sophiecentaur

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    With a high series input resistor there would be no problem.
     
  7. Jun 21, 2010 #6
    Doesn't this divide the input with the amp input resistance, thereby reducing the actual voltage at the input pin?
     
  8. Jun 21, 2010 #7

    sophiecentaur

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    Yes but op amps have a huge input resistance, in any case. Their input current is often just a few nano amps. The inverting input is often wired as a virtual earth so the voltage on the actual pin is around zero due to the feedback.
    I'm basically saying that op amps are normally embedded inside a circuit and the actual input pins are not usually exposed to damaging voltages because of the intervening resistors etc. But, yes, you can knacker a chip if you are careless in your design.
     
  9. Jun 21, 2010 #8
    I misunderstood your original question. I have had to use an opamp where the input voltage was higher than the supply voltage and I did that by limiting current to the input. Limiting the current does of course also limit the voltage to that pin. Opamps generally are flexible enough that the limited input voltage can be compensated for by the gain.
     
  10. Jun 22, 2010 #9

    sophiecentaur

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    Yes - in fact you often assume infinite voltage gain when doing your initial calculations and when the input signal voltage (to the whole circuit) is outside the range of the device, you use appropriate feedback / biasing with absolutely no ill effects.
     
  11. Jun 22, 2010 #10
    This discussion is all very well and dandy, but kris originally asked

    not input voltages applied to the circuit as a whole.

    I forgot to mention that a series resistor also divides the voltage with the feedback resistor, which can also cause issues if too high.
     
  12. Jun 23, 2010 #11
    so, attached here is the circuit i simulated.

    Vbat acts as the input to the four stages of the voltage level indicator.
    what will happen to the LM324 if for some reason the terminal voltage being measured
    exceeds 16.6V which is the supply voltage of the op amp?
    let's say a voltage spike of up to 30V?
     

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  13. Jun 23, 2010 #12
    The absolute max differential input voltage is specified as 32 volts so you should be OK up to this value, although the output will obviously limit.
     
  14. Jun 23, 2010 #13
    Yes at 30 V you should be okay, however if your voltage spike should go up to 35 or 40 V, then you may destroy the device. To prevent that you could add a high value resistor in series with each input you want to protect. The opamp input impedance is high enough that the resistor will have little effect in normal operation yet will protect the opamp in the event of a voltage spike.
     
  15. Jun 25, 2010 #14
    your posts have been a great help!
     
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