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Old 18th May 2003, 09:50 AM   #1
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Default What is oscillation?

Hi

As a newbe trying to get my head around all this I keep reading everywhere that opamps can oscillate and that this must be stopped at all cost. What exactly does it mean when an opamp oscillates? what causes it? and how do you stop it happening?


Any help appreciated


jnxw2
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Old 18th May 2003, 10:50 AM   #2
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oscillation can occur with any kind of amplifier. When the output is fed back to the input, the opamp can amplifie the signal further and further under certain conditions. So when the amplification is higher or equal to the attenuation of the feedback loop and when the total phase shift is 0 (or 360), the amplifier will oscillate at a certain frequency for which the conditions are like described.

Oscillation can be solved on different ways, depending on the circuit. The clue is just to reduce amplification or to change the phase shift (i.e. this can be done with simple RC-filters)

I hope this will be clear enough to you, maybe some other guys can give you more information (my English isn't that good)

Best regards,

HB.
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Old 18th May 2003, 11:12 AM   #3
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Let me try to put it simply:

Oscillations are seen when systems are unstable. So instead of seeing the output follow the input, there would typically be large deviations from this. If you have access to "The Art of Electronics" by Horrowitz and Hill, you should be able to get the hang of most things electronic in no time

Hugo talks about feedback. Most op-amps have a gain of tens of thousands if not more. Feedback techniques are used to throttle that gain in the simplest form which is inverting amplifiers. At higher frequencies, you typically get phase lag. When you have enough phase lag (you have a non-inverting amplifier with positive feedback instead of an inverting amplifier with negative feedback), the output signal ends up being additive to the input signal and reinforces it. The output signal will then go up as far as it can go. As it stops, it will come back the same way and you have your oscillations.

Petter
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Old 18th May 2003, 11:25 AM   #4
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ahh im starting to understand.
When you say phase lag do you mean between the input and output of the amp? If so are some opamps more prone to oscillation than others? And is there a way of knowing at what frequency it will oscillate?

thanks

jnxw2
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Old 18th May 2003, 11:42 AM   #5
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It's (conceptually at least) the same sort of thing as that familiar howling sound from a public address system when the volume is wound up too far. The microphone can hear the loudspeakers too much, and of course it sends this sound out the speakers again, then it can hear this etc etc.
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Old 18th May 2003, 12:08 PM   #6
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Generally speaking, oscillation only occurs as a serious problem in global feedback amplifiers.

These are amps where the output is fed back to the input in reverse phase. This subtracts from the input, and reduces the gain to a specified, set value.

If the feedback should have its phase changed with respect to the input, so it's not at 180 degrees, eventually, at some very high frequency, this phase shift will turn the negative feedback into positive feedback.

Then, with the output fed back and now increasing the input, we have uncontrollable gain, and hence, oscillation.

Cheers,

Hugh
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Old 18th May 2003, 12:31 PM   #7
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There is a thread about baxandall and it is full of that kind of information.

Feedback is a type of correction system. It use the output voltage and the signal input voltage from those it figure out the ratio and send to a second input a correction signal to maintain the output at the correct level. In fact in an amplifier with feedback the gain of the amp is always a lot to high and the feeback is the main feature to keep the overall gain at the correct level.

It like driving a car you must look at the road estimate your position and speed and correct parameters and your always making correction it is a continious process.

Back to the amp, now what will happen if the frequency get too high, first threre will be phase shift in the amp: the output will rise wen the input fall and the output will fall wen the input rise. That is typical to all amp wen you reach a certain frequecy and this is why the feedback cannot make the correction. The "timing" betwen feedback signal an amplifier reaction is reverse.

This is the oscillation process.
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Old 18th May 2003, 02:54 PM   #8
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Default oscillation

Yet another 2ct worth...

Oscillation in its purest form is an output signal without an input signal. Every amp has some noise, which gets amplified. If the output signal gets back to the input in the right phase, it increases the output signal agian, and again, and again.
If the conditions for the feedback of the output to input are only "correct" at a certain freq, you get a sine oscillator.

This is not necessarily a BAD THING. Oscillators use this feature, and amps with a freq selective filter in the feedback path can be build to produce very pure tones, usefull for testing.


Jan Didden
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Old 18th May 2003, 05:43 PM   #9
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Great I think im getting the hang of that but I read an article about how cables can become resonant at certain frequencies, has that got anything to do with oscillation or is that something different altogether?

thanks

jnxw2
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Old 18th May 2003, 10:08 PM   #10
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Default Re: oscillation

Quote:
Originally posted by janneman
Yet another 2ct worth...

Oscillation in its purest form is an output signal without an input signal. Every amp has some noise, which gets amplified. If the output signal gets back to the input in the right phase, it increases the output signal agian, and again, and again.
If the conditions for the feedback of the output to input are only "correct" at a certain freq, you get a sine oscillator.

This is not necessarily a BAD THING. Oscillators use this feature, and amps with a freq selective filter in the feedback path can be build to produce very pure tones, usefull for testing.


Jan Didden

This was the reason why my old RF prof used to joke that it was darned hard to fail at analog design--if you can't make an amplifier stable you can always just call yourself an oscillator designer instead.

Erik
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