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It is a positive feedback loop, not a negative feedback loop. See https://en.wikipedia.org/wiki/Positive_feedback.

It’s negative WRT the chip production rate.

No, still positive. Negative feedback is self-righting, while positive feedback is amplification (to speak in very broad terms.)

No! It's negative WRT chip production rate! I have a phd in controls and meant what I said; I assume roboticsresearcher also knows some freshman-level control theory ;-).

Stability ==== good in undergrad engineering, but not here. We DON'T want production rate to be stable when we have a global supply shortage! Here, a negative feedback loop is stabilizing the system in an undesirable equilibrium.

I.e., the function that's being controlled in "supply of chips", the stable state is "saturated supply", and the negative feedback loop that maintains that equilibrium is "starving chip fab suppliers".

(meta: people down-voting comments on control theory terminology by two different experts in this field at least makes me feel a bit better about the signal:noise ratio on the vote counts on my other comments in this thread ;))

It's not stabilizing if the economy is in a death spiral of failing interdependent companies who can't produce machines for each other.

In control terms it is. There’s a stable asymptote of ‘we made no machines today’ every day, until some external process breaks the dependency cycle.

Only partially facetious here: you have mistakenly decided that it it clamps at zero. If we consider the hoarding aspects, the feedback continues into net negative chip availability. Before long, there will be roving gangs of looters taking back the chips you thought you already had! ;-)

Yes, it's fair, I do assume that chip supply/demand isn't subject to the forces of roving gangs of looters :)

That's fair. I was looking at it wrong.

It does have negative external effects, as you observe, in the sense of a viscious circle. But even a viscious circle, mathematically, has positive feedback, resulting initially in a positive exponent growing against time, not a negative exponent damping out in time.

Doesn’t positive feedback require a positive loop gain? Right now, not enough chips are available to make new chips. So the loop gain is less than zero, damping the output recursively.

I'm also confused by the confusion here... probably naive pattern matching? Reminds me of TAing undergrad courses where you could get more than half the class to confuse "positive feedback" and "negative feedback" on a midterm by just giving examples where stability = bad :)

Grandparent gives the choices of ‘positive exponent growing against time’ or ‘negative exponent damping out against time’. Here we have a positive exponent damping (reducing) the output over time, because it’s value is less than one, resulting in a negative loop gain. The Wikipedia article linked up thread defines positive feedback as having positive loop gain, and negative feedback negative loop gain.

IIRC, generally a positive loop gain greater than one will lead to diverging behavior aka instability, whereas even a positive-sign to feedback, if loop gain is "less than one" will not. I might be brain-farting here, but I cannot be precise anyway, which IIRC gets into plotting poles in a complex plane. (Cue joke my applied math professor would tell, about why all the Polish people were asked to sit in the right-hand aisle of an aircraft.)

Self-reply: Not a derogatory reference. Just: "Poles in the left half-plane cause instability."

The problem is, if the feedback signal is too slow in getting back to the thing which measure error (i.e. too much phase lag), then negative feedback can turn into positive feedback and this leads to an instability.

One of the most complex pieces of the semiconductor fab is the building itself. Even with plans and permits in hand, it takes years to make one that can output at reason throughput and yield.

This report is from 1999 and it hasn't gotten easier.

https://www.imia.com/wp-content/uploads/2013/05/Construction...

"Typically the product life of a semiconductor chip (nine to 12 months) is less than the time required to construct the facility and install the equipment for manufacturing (24 to 36 months). As such, the construction/commissioning process is a rapid, constantly overlapping and complex set of events. In addition, construction of semiconductor facilities is very complex and costly (about USD 1.2 to 1.5 billion) due to the extraordinarily sophisticated processes and equipment required to manufacture semiconductor chips."

Yes, and same situation in every mining-related commodity market. Multiple time delays of order several years. Large up-front investments. Large uncertainties in payoff. Look at the multi-year price behaviors in those markets and see too if there is much stability.

"Typically the product life of a semiconductor chip (nine to 12 months) is less than the time required to construct the facility and install the equipment for manufacturing (24 to 36 months).

That's an absurd underestimate of market lifetime. I'd bet that fully 80% of the chips available in 1999 when that report was written are still in production today (or would be, if not for the crunch.)

Self-reply, too late to edit. Looks like I remembered the definition of loop gain incorrectly. This should say a loop gain less than 1, not negative

I have always understod "positive feedback" as "feedback that prompts the existing change to continue in the same direction, with equal or larger speed".

So positive feedback on a falling signal would tend to make it drop more. And positive feedback on a rising signal would cause it to rise more.

So, basically, for any signal S at time t, we would expect something like S(t+1) = S(t) + S'(t) * k, for some k > 0. And blatantly abusing derivatives for "should probably be a delta between S(t) and S(t-1)".

But, then, I am not a control theory specialist, I don't even play one on TV.

Negative Vs positive is about the eigenvalue of the feed back loop not about the weighting how it affects external signals

No, there is a shortage, leading to a shortage, leading to a shortage, ...

If it was negative feedback, the "error" (shortage in production) would lead to an error cancelling signal, and therefore an increase in production. Positive feedback has error leading to larger error, shortage leading to more shortage.

And it's positive WRT the shortage of chips (a small shortage leads to a larger shortage and so on).

Negative feedback means the feedback coefficient is negative, you are trying to redefine the usual meaning.

It is like saying that I tested positive for COVID-19 WRT my health because I don't have the virus, it is not what a positive test means and it will confuse anyone who knows the correct terminology.

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