From: firstname.lastname@example.org (Ben Bradley)
Subject: Re: Historical question: negative feedback and the op amp
Date: 13 Nov 2002 15:13:35 -0800
NNTP-Posting-Date: 13 Nov 2002 23:13:35 GMT
Hi, Ban, you've made some 'interesting' claims, and it looks like I'm
far from the first to respond.
There must be some reason I haven't heard of the digital op amp.
Actually, in the USA, DOA has a previous meaning, "Dead On Arrival."
In sci.electronics.design, "Ban" wrote:
>I'm an analog designer such as you are and I know very well it will
>but not anymore to improve the circuit linearity.
Adding negative feedback to amplifiers always improves linearity,
and I've never heard anyone complain about it. I have no doubt it
would still be commonly done if improving linearity were the only
reason to do it.
>digital circuits work with saturated transistors, no feedback there.
As I recall, LS TTL transistors have Shottky diodes between base
and collector to keep them out of saturation when turned on, so that
they turn off faster. This is definitely feedback in a digital
>filter does not use feedback.
True as a standalone, theoretical entity, but see below about
digital systems, clocked with an...
>An amplification can be done with a comparator, triangle generator
How does the triangle generator generate something with a regular
period? I imagine it's based on something else with a regular period,
perhaps an oscillator...
> and 2 MOSFETs to higher voltage and a passive filter
>(D/A, no feedback either).
Many modern D/A's are sigma-delta, and so use, um,... but anyway,
even if it's not that kind, ANY D/A and the digital system to which
it's connected is surely clocked with an oscillator, which operates
>Somewhere a control circuit will have to be used with feedback to
>voltages constant but the feedback is not in the signal chain. And
>digital amp will have a offset stabilisation etc.
>Control systems per se work *by principle* with feedback. A balance
>But if a mathematician multiplies by 100 he cannot improve his result
>dividing it by 100 and compare to the input. He will just waste his
I got feedback on my mathematics every time I received a graded
test paper back from a mathematics teacher, and just about as often
from an electrical engineering teacher. I've even gotten feedback on
my mathematics from employers, fortunately it wasn't as negative as it
had been when I was in school. :)
And then there's Newton's Method for getting square roots and
In response to earlier posts, and more in line with the original
topic, feedback in automatic systems easily predates electronics and
electrical devices. I was just reading Bob Pease's column titled
"What's all this P-I-D stuff, anyhow?" where he mentions a flyball
governor (presumably for a steam engine) with an integral term in the
feedback loop. It's not exactly a PID controller, but it's two-thirds