Re: Reduced Termination Loss by Output Impedance Synthesis

JohnRoberts

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[quote author="mediatechnology"]Wow. Amazing what unintended results you stumble upon whilst Googling:

Abstract: In high-speed transmission-line applications, it is important to match the output impedance of a line driver to the line. While this is achieved usually with a resistor, an active impedance synthesis has advantages. This application note describes how to use positive feedback around an op amp to create the desired output impedance. Equations and circuit examples are given for low-noise audio, and video op amps driving 50- to 600-ohm loads.

http://www.maxim-ic.com/appnotes.cfm/an_pk/3390[/quote]

Yup, that topology is pretty similar to classic synthesized current source (check old National applications series), but in that case you're trying to make 1M R look like >> instead of 75 to 600.

In this case the trade off is to use an smaller R (like r/10) and then multiply it back up to trade impedance accuracy for increased signal output. Perhaps a useful trade as PS rails keep dropping (3.3V and falling).

An amp designer I worked with used feedback manipulations to improve the damping factor of SR power amps (patented). He was able to actually deliver negative output impedance but I was afraid that would scare the boys and girls, so we were content to only claim 1000:1 damping factors, while routinely delivering 2-3K:1 at final production testing.

I used a variant on the synthesized current source topology in an old console summing bus structure to make 2K bus resistors look like a few hundred K and so reduce the bus noise gain for improved distortion/phase shift/ etc. I even did one console with a dBx VCA as the actual summing amp since it's noise gain was inconsequential.

JR
 

JohnRoberts

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[quote author="mediatechnology"]
An amp designer I worked with used feedback manipulations to improve the damping factor of SR power amps (patented). He was able to actually deliver negative output impedance but I was afraid that would scare the boys and girls, so we were content to only claim 1000:1 damping factors, while routinely delivering 2-3K:1 at final production testing.

Was the Crown Delta-Omega a similar approach?[/quote]

I'm not familiar with Delta-Omega but doubt it's the same or the PTO might have noticed...
http://www.audioannals.com/05796305-Sondermeyer-txt.htm

JR
 

bcarso

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Somewhere many threads ago I think I mentioned an audioXpress article where the author referred to a configuration of a '317 regulator as a constant power regulator. Since it had no obvious multiplications going on I was skeptical.

It turned out that it was merely an approximation to a power regulator for load impedances close to its operational output Z. What was surprising to me was over what a range such a circuit gives pretty decent regulation. I developed another discrete circuit a bit along those lines and sent to a colleague proposing to use it as a filament supply. I quipped that I knew it was of interest since, when he mentioned the idea to his friend James Boyk, the latter wasted no time saying it was unnecessary and frivolous (or words to that effect). :razz:
 
G

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Hmmm.... How anybody could ever patent combination of NFB by voltage and PFB by current? It was always used in walkmans and boomboxes... Speaking of amps, I used such aproach in late 1970'th, when my amp naturally spit out cone from speaker, as the result of very curved vinyl disc with Bach's toccata and fuga I used to try how deep the bass is...

Shameless patenters...

Shameless and brainless...
 

JohnRoberts

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Look ma, no quotes. (excuuuse me).
-----
The state of the competition was to use brute force (heavy duty relay, wire gauge, etc) with lesser results. I didn't find the damping factor improvement invention all that trivial. I agree after you learn how they work some of the better ones do seem obvious. If truly obvious (and useful) it would have been done and published before.

What was the damping factor in your Walkman?

I disagree with your ad hominum comments. Jack is/was a brilliant engineer and an honorable man.

Have a nice day

JR
 

clintrubber

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[quote author="NewYorkDave"]See Audio IC Op-Amp Applications, Third Edition, pp. 204-205 (figure on 207) for a slightly different approach to the same problem.[/quote]
Was looking for that book/section, and interestingly/FWIW, the section you mentioned also had the approval of Doug Self:

http://www.dself.dsl.pipex.com/ampins/library/library.htm

A useful guide to opamps. There is a nice little nugget on stages with defined output impedance on p207. There are some useful datasheets at the back.
 

Michael A

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Didn't Bogen use a combo of neg voltage feedback and pos current feedback in their DB series amps back in the mid 50's. I sort of recall reading an article about them in Radio and TV News or somesuch. I thought they could go from neg output impedance to quite a few ohms.

Maybe, maybe not,

Michael
 

clintrubber

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While looking for something else I ran into this:

Isolation transformer passes millihertz signals

It's nothing new w.r.t. what's already been said here, so TX-wise merely a consequence of the reduced output-Z.
But still, links are free, so why not:

http://www.edn.com/archives/1994/080494/16di3.htm

16di3fg1.gif
 

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