Dangers of injected current into opamp output?

Discussion in 'General Electronics Chat' started by Veracohr, Jul 17, 2016.

  1. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    The NE5532 opamp has output short circuit protection with a typical maximum output current of 38mA. Could it be damaged if an external circuit forced more current in/out of the output?

    This AC buffer with input overvoltage protection keeps the input below the supply voltage, which is necessary, but forces current into/out of the opamp in excess of the specified output short-circuit current unless R3 is high enough (in simulation). I guess the opamp's short circuit protection can't compensate for the external current. Could this damage the opamp?

    Screen Shot 2016-07-17 at 11.45.46 AM.png
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Yes it could damage the output.
    So why not increase the value of R3?
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Why are you connecting input overvoltage to the op amp output instead of to ground?
     
  4. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    I'd like to keep noise down, but also if I increased it enough then I'd want to increase R2 to match. R3 of 165 ohms looks sufficient, but the next 1% resistor value above 20k is 20.5k, which means increasing R3 to 499 ohms, so that would be even more noise. Although I know that noise at that resistance is pretty low in the audio band, which this is intended for, I'd still like to keep it as low as possible.
     
  5. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    I was looking for a method which keeps the input voltage always under the supply voltage. The usual method of connecting the diodes to the supply lines still allows it to go slightly above. I found this page on Maxim's website which shows a method in Figure 4, where the idea is to keep the bias across the diodes at 0V. I don't want amplification, and I found that just connecting the diodes to the negative input terminal didn't keep the input from exceeding the supply, but I figured since the output should be equal to the input I could connect the diodes to the output. It seems to work even at peak input amplitudes that are 5V over the supply (I'm going for an abundance of caution).

    If you have a better idea I'd be happy to hear it. I want (ideally) a protection circuit that has no effect at less than 13V peak, but limits the opamp inputs to 15V peak in the audio band.
     
  6. benta

    Member

    Dec 7, 2015
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    I'm completely at a loss as to what this circuit does (or is supposed to do)...
     
  7. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    One standard way to protect the outputs is diodes to the supply voltages from the output. Reversed biased, of course,
     
  8. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    It's an AC-coupled buffer for audio signals with input overvoltage protection.

    The opamp output is inherently limited to about 13-13.5V with 15V supplies. It's the input I'm looking to protect here.
     
  9. AlbertHall

    Well-Known Member

    Jun 4, 2014
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  10. crutschow

    Expert

    Mar 14, 2008
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    The NE5532 op amp has an input noise level of 5 nV/√Hz which is equivalent to the noise level of a 1.5kΩ resistor.
    You already have a 20kΩ resistor R2, in the feedback loop creating about 18nV/√Hz of noise so if you are really that concerned about noise then you need to reduce that resistor value.

    Actually there's no need for R2 as that's only needed to reduce DC output offset due to the input bias current and that's not a problem for an AC amplifier which is AC coupled.

    Also R1 doesn't add to the noise if you have a low source impedance (V4).

    So I would remove R2 and increase R3 to a value necessary to keep the current below the maximum op amp output current.
    With that you will end up with less noise than the original circuit.
    What's the highest input voltage you expect?
     
  11. Veracohr

    Thread Starter Well-Known Member

    Jan 3, 2011
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    I didn't mention that this is the buffer part of a buffered, balanced input. I could have sworn that I had run some simulations that showed that DC offsets of the input signals degraded the CMRR of the difference amp, but now I see that's not the case. That's why I had R2 there. I'll give your suggestions a try, thanks.

    I don't expect input voltages high enough to need protection from, but that's the whole point isn't it? To protect against what I don't anticipate.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    No, the idea is to protect against the worst-case voltage you might expect.
    If you don't have some idea of the maximum input you might see then how can you design to protect the input from it? :confused:
    For example, ICs are designed to protect against the maximum static charge expected from the human body.
    Where is the signal source coming from?
    What do you think will generate the overvoltage spikes?
     
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