Deep dive: the instability of op-amps
9 comments
·October 26, 2024dragontamer
FunkyDuckling
I agree.
Phase Margin (How far away you are from 180 Phase Shift) is a critical parameter used whenever designing any kind of feedback loop and testing for stability.
This is very to measure at the 0dB gain he pointed out, but lacked the phase diagram to show this shift.
arijo
This is only true for LTI (linear time-invariant systems).
The nonlinear system response to a sine signal is in general distorted (not just a change in phase and amplitude).
buildsjets
But the ringing, sustained oscillations, and excessive gain are sometimes desired characteristics of a particular op-amp implementation.
analog31
Sure, the Wien bridge oscillator. But then, oddly enough, even sustained oscillations need to be controlled in their amplitude, and the Wien Bridge has a secondary feedback loop for that purpose -- the temperature dependent resistance of a light bulb.
ericwood
Those op amp characteristics aren't really coming into play for that delay sound, it's just vanilla digital delay. Most of the "vibe" of the DD-3 is coming from the companding and filtering scheme it uses to work around the limitations of the digital pieces.
You will hear the effects of this in many hard clipping distortion circuits, though, where the amplifier gain factor will far exceed the voltage rails and be pushed into undefined clipping territory. Behaviors in this range can be an important part of the sound, e.g. the Proco Rat and the infamous LM308 op amp with its slow slew rate. Some like the TL072 exhibit a really nasty phase inversion that results in a pretty horrific (usually undesired) distortion.
It's a balancing act, though; search "op amp motorboating" in any DIY stompbox forum and you'll find thread after thread of people trying to keep op amp gain stages from oscillating. I know more than a few noisier artists who enjoy when designs can be tortured into doing that, though :)
082349872349872
to what degree does the Gartner hype cycle resemble the Gibbs phenomenon?
marcosdumay
To the degree that society is a complex system with feedback.
But notice that the Gartner hype cycle is full of unjustifiable hidden assumptions (like the fact that the thing being hyped is useful at all) so it has no predictive power at all. It only happens that some times people act like that.
Also, there's no guarantee that the society's response to a change will be stable at all.
sobriquet9
The tape delay methaphor confused me. Tape recorders do not record DC or other frequencies much lower than, say, 20 Hz. So that circuit would run into one of the rails just as quickly as the previous cirquit without DC feedback.
It's almost criminal that all of these bode plots are missing their phase diagrams.
Phase diagrams + OpAmp phase shift specs / phase margin are what you need to predict instability.
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EDIT: IMO it's also a lot easier to explain in the frequency domain. At 180-degree phase shift, all your negative feedback turns into positive feedback, causing instability. You need your amplifier to stay as far away from 180-degree phase shift as possible.
I get that what the author was trying to get to with the 'Tape Delay OpAmp' example. But it should be double downed upon and the starting point of the discussion rather than something brought up later IMO.