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Opa2134 Stability Dependent on Supply Voltage?
Opa2134 Stability Dependent on Supply Voltage?
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Old 19th April 2020, 05:39 AM   #1
audiowize is offline audiowize  United States
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Default Opa2134 Stability Dependent on Supply Voltage?

I have an individual trying to tell me that an Opa2134 will oscillate of used on 12V rails. I can't for the life of me fathom how one could come to such an arbitrary conclusion, and the circuit in question is about as basic as it gets (the first gainstage in a car amp). There are appropriate decoupling capacitors in place as well.

Is there any basis for this? I installed it and scoped the outputs of the opamp and there's no oscillation. I scoped the output of the amp and there's no oscillation. Idle current from my bench supply is super low (80mA from a 12V bench supply), but this individual is convinced that I'm missing the big picture.
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Old 19th April 2020, 07:10 AM   #2
jan.didden is offline jan.didden  Europe
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You are correct. There's no basis for that. Oscillation has to do with returning some part of the output to the input in the wrong phase. That can happen through the power supply, but that would mean a spectacularly incompetent design. And it certainly is not related to a specific voltage.
Maybe he has seen it and concluded it is the fault of the opamp ...

Tell him to find another hobby ;-)

Jan
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Old 19th April 2020, 07:59 AM   #3
Mark Whitney is offline Mark Whitney  Netherlands
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Not supply voltage but single rail is more challenging. Plenty to be found online, lookup sloa030 and sboa059 from TI.

Here is another one.Avoiding Op-Amp Instability Problems In Single-Supply Applications | Analog Devices
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Last edited by Mark Whitney; 19th April 2020 at 08:08 AM.
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Old 19th April 2020, 12:48 PM   #4
Mark Tillotson is offline Mark Tillotson
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Follow the advice in the datasheet about bypass capacitors to the letter(*). Modern fast opamps can oscillate in the VHF or UHV bands if you don't pay attention to correct decoupling and this usually manifests itself as a reduction in performance that's mysterious and hard to diagnose without good test equipment.


So the individual might have been right about oscillations, but not about the actual cause.


This is why opamp-rolling without understanding decoupling issues is silly.


(*) I quote: "Connect low-ESR,10nF ceramic bypass capacitors between each supply pin and ground, placed as close to the device as possible. A single bypass capacitor from V+ to ground is applicable for single-supply applications."
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Old 19th April 2020, 05:08 PM   #5
audiowize is offline audiowize  United States
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The ceramic bypass caps are in place and within a few mm of the opamp.
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Old 19th April 2020, 06:36 PM   #6
jan.didden is offline jan.didden  Europe
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Did you actually have oscillations to begin with? And now?

Jan
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Old 19th April 2020, 07:18 PM   #7
audiowize is offline audiowize  United States
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Nope, nothing. I would expect a ~200W class B amp to draw more than 1W at idle if things were oscillating, and I see nothing on my scope at the opamp outputs or the amp outputs, but my scope only goes to 100mHz.
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Old 19th April 2020, 08:00 PM   #8
jan.didden is offline jan.didden  Europe
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You probably mean 100MHz ;-)
But nothing on the scope is a good sign! So you may not have any problem at all.

Is it really a class B amp, not AB? Do you know the quiescent current (bias current)?

Jan
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Old 19th April 2020, 08:02 PM   #9
audiowize is offline audiowize  United States
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Yeah, 100milliHertz would be an interesting scope.

I would call the amp class B, though it would make a little less than 1W class A I suppose. For a car amp, it's not unusual to see designs where as little extra heat is dissipated as possible at idle.

I will have to measure the bias current when I get some extra free time.
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