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Well the basic need of Negative feedback is that to provide stability to the system.
Stability is a well defined term in circuit theory. "stability ... in terms of gain" is rather referring to common language usage of the term and sounds somewhat screwed if you're actually talking about "stable gain".
Usually, the output voltage of an amplifier won't exceed the supply coltage, with or without negative feedback.
Perhaps one of you should clarify the misuse of "stability" in the lead paragraph of https://en.wikipedia.org/wiki/Negative_feedback while you're at it.
If a system has overall a high degree of negative feedback, then the system will tend to be stable.
And unclear, too. E.g., what's meaned with overall?This sentence (second line of the referenced wiki contribution) is rather questionable and misleading.
What did I forget?
Open loop gain is a system level term. It shouldn't be hold against the fact, that an amplifier can already implements internal feedback, although it complicates the analysis.Which open loop gain amps are you referring to?
Negative feeedback has many advantages (e. g. it stabilizes the dc operating point) - however, it reduces system stability because in real systems it always moves into positive feedback for rising frequencies.
Can you clarify what "it moves into positive feedback" means?
So "gain stable" can have different meanings in both time response control systems and linear oscillator control circuits and may be somewhat an abbreviated term depending overall gain or loop gain margin is being considered.
I recall that we used this to define the servo phase margin tested in every 14" disk drive in production at Burroughs in Winnipeg, Mb, Canada using a Bode Plotter in the early 80's. Anything less than this margin was rejected, corrected with various components or worst case, another new massive baseplate and servo mechanical parts were put in. Consider if you had more than one overshoot pulse > 10% and servo said it had arrived "ONTRACK". If data position was ready to write to a sector that was just coming around on that track AND lack of loop gain margin continued to move the heads off-track with ringing {in this negative feedback control system}, it is possible that this data written offtrack might never be recovered, since it occurs over a fraction of a sector. This design used a large >1Hp linear voice-coil motor with >3 " stroke like a big woofer to drive a stack of heads up to 30 inches/sec and onto a track with 0.0005" error within 30 milliseconds. It was impossible to achieve the "optimal" value of Zeta or dampening factor of 0.7 but a gain margin > 15dB was possible, but any less gain margin was "unstable".
When we think of "stable gain in amplifiers" with closed loop gain , we tend to have plenty more than 15 dB margin so it is the stability of gain that is more critical, rather than gain margin of the AGC loop.
But in 2nd and higher order Control Systems, we are much more concerned about the a "stable gain margin" measured by the lack of ringing or minimal overshoot to a step input or namely the "step response", than the stability of the actual forward gain or feedback gain.
This should be clearly avoided. Besides requiring stability, applications have specific performance criteria, e.g. overshoot, gain peaking, settling time for linear amplifiers, or integral of absolute error value for control systems, and different performance numbers.But this would lead to a situation in which each application requires its own definition for stability.
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