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Old 9th October 2013, 10:21 AM   #1
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Default How to check circuit for oscillation?

Silly question perhaps, and a little embarrassing that i have to ask it.

Do i leave input floating with no signal, OR do i ground the input?
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Old 9th October 2013, 10:28 AM   #2
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What kind of circuit are you trying to check ?

If it is a power amplifier then you can simply use an LED // DIODE in series with a small cap to detect if the output is oscillating.

Low power circuits - you really need a 'scope.
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Old 9th October 2013, 11:37 AM   #3
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I have it connected to a scope, and I'm running a freq response plot using a scope.

This is a fairly simple opamp circuit, which i want to check for stability.

My problem is am IC gets warmer than i like at higher freqs, so i suspected an oscillation. Mind you i have just added an input R to gnd, since i forgot that earlier, and i hope that solves it.

Strangely i had none of this with my breadboard prototype.....

So it could still be oscillation.
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Old 9th October 2013, 11:40 AM   #4
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Post a diagram of your circuit

Here are a couple of threads on opamp oscillation. The CFB thread is a bit different but it shows what oscillation looks like and how it can show up.

Current Feedback Opamps in simulation and practice.

Swapping Op-Amps... you have checked to see it's stable haven't you ?
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Old 9th October 2013, 11:50 AM   #5
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Drive the circuit with a square wave. If the output show ringing, it's on the verge of oscillation. It may also just break into oscillation.

If it's getting hotter than expected at high frequencies, it's probably just an effect of driving a capacitive load. Either external or internal.

Most of the time, too hot because of oscillation exists with no signal. Seeing as you're not seeing that... I can't rule out oscillation, but it doesn't look to be the case with the little information you've given.
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Old 9th October 2013, 12:29 PM   #6
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Sorry guys!

Im at work and its slack today, but even so....

Click the image to open in full size.

Thats my circuit (from my Filter Q thread...)

I have made up on stripboard, and switched to LME49740 ICs from the NE5532 I was using. This was mainly because I got them as samples and the Quads made the cct easier to implement in stripboard.

Mods to cct that arent shown in schematic:

1: I have added output resistors of 150 Ohm (as AndrewT or another helpful soul suggested)

2: I have just added 22k from sig to gnd also (just in case...)

3: I cut the common signal input to isolate the other feed to the second filter cct.

Im running +/- 12VDC rails, and I was driving with 10 Vpk-pk input, but I have since reduced it to 4V pk-pk.

I think running too close to the supply rails maybe?

Running at 4V pk-pk the devices are cooler, but not COLD.

Running into a scope im surprised they are warm at all.
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Old 9th October 2013, 12:37 PM   #7
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It all looks OK. The input does need ground referencing at all times (your 22k ?)

The NE5532 does draw around 10ma and will become a little warm, that's normal. They will never feel cold.
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Old 9th October 2013, 12:55 PM   #8
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Yes thats correct Mooly, I have 22k from sig to gnd.

Testing with a Square wave as advised was a bit weird.

No overshoot or anything like that, but instead at lower freqs I get a strange sawtooth type wave, moving to a couple of kHz and it becomes sinusoidal.......

Slewing? thats what it looks like. I cant see how that can be the case with LME49740.

Must double triple check layout and wiring, something isnt right here.
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Last edited by mondogenerator; 9th October 2013 at 12:57 PM. Reason: switch "sawtooth" for "sharktooth"
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Old 9th October 2013, 01:01 PM   #9
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Some parts of the circuit will behave as you describe. Top row for example has a 3k3 and 0.1uf cap to ground. That has a 480Hz -3db point for starters and you then cascade two active filters after that.
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Old 9th October 2013, 01:13 PM   #10
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Ok. Yes the RC should give that roll off.


Doh! Of course the square wave output isn't square due to attenuation of harmonics...

I'm so dumb.

Thanks for the kick back to the real world ��
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