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Comment by elevaet | Hacker News Reader

Resonant filters, like the classic Moog ladder filter, are another great example of recursive/feedback circuit designs.

As someone coming from a computing background, it always seems magical how the recursion in these types of analogue circuits happens instantaneously.

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> how the recursion in these types of analogue circuits happens instantaneously.

Yeah, theoretically instantaneous, in practice not. (There's a measure for that but I can't remember how's it called)

Even funnier that the feedback actually increases the bandwidth of the system, compared to the open loop amplifier.

>There's a measure for that but I can't remember how's it called Slew rate for opamps I believe

Also worth noting that the "recursion" without negative feedback will hit a limit when the op-amp is saturated. Similar to a "stack overflow" I guess. Things mostly won't break but you'll get a flat output at the positive or negative rails of the op-amp - which causes some of the pain of analog signal processing.

>Even funnier that the feedback actually increases the bandwidth of the system, compared to the open loop amplifier.

The reason for this is because of how bandwidth is defined, which is basically the point at which the amplifier gain starts dropping the higher the frequency you go.

Adding feedback is effectively clipping the low frequency/ high gain area with the net effect that the gain versus frequency plot is a lot flatter (but now, instead of being able to get say 10,000x gain at 1Hz, you're clipping it to 100x, since you're unable to get 10,000x at 1Mhz)

No matter how hard I try, I can't get this stupid snake to stop eating itself!

It's not recursive. To be recursive you'd have to be able to unwind some sort of memory to get all previous states.

I don't think that's required for recursion. You only need access to as many previous states as you need for the result, but not all. We often talk about tail recursion optimisation where the stack never grows.

the unwinding property comes from the stack; see tail recursion

You can implement binary search recursively, which doesn’t care about the stack just the final iteration. I don’t see where the stack unwinding is necessarily for “call the same function inside itself unless you’re done” except in cases where the stack is used to compute a final answer that’s based on the prior iterations. Maybe it’s fair to say those would best be implemented iteratively, but it’s unnecessary that they be.

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