Date   

Re: How to explain how negative feedback lowers noise?

Tom Lee
 

Your comment about cathode resistors for bias stability will force me to do a little more background checking, but I don't think indirectly-heated cathodes were in production before 1929, so there could not have been cathode resistors for at least two years after Black. I did a quick perusal through a few schematics from that era, and I don't see cathode resistors of any kind showing up until around 1930 or 1931, and unbypassed ones don't show up for a couple of years after that. Blumlein doesn't invent the cathode follower until he starts working on radar on the eve of WWII. So my initial quick pass suggests that Black's invention does precede other forms of electronic negative feedback constructions. If you have earlier examples, I would be grateful to know about them. I'll look through other references in the meantime.

Tom

Sent from an iThing; please excuse the terseness and typos

On Mar 25, 2021, at 23:14, "Ed Breya via groups.io" <edbreya=yahoo.com@groups.io> wrote:

I don't know about this history, but am surprised that this didn't happen until 1929, well into the electronics era. Surely the concept of negative feedback in control systems has existed in nature, throughout human history, and in industry - at least since the steam era - look at the fly-ball governor, for example. Maybe in electronics, it wasn't so obvious, although it already existed in some forms, say for instance, with degenerative feedback in a vacuum tube, stabilizing the bias with the cathode resistor. People were driving horses, trains, planes, and automobiles successfully before 1929, using that PID controller in the skull.

Ed





Re: How to explain how negative feedback lowers noise?

Tom Lee
 

The reason it didn't happen for electronics until 1927 is that the preoccupation until that point was getting more gain per tube. Positive feedback (Armstrong's regenerative amplifier) was the magic elixir that had enabled the age of electronics to begin, around WWI. Early tubes struggled to achieve voltage gains of five. The first generation of EEs was thus trained to think of getting enough gain as the main problem.

It wasn't until AT&T ran into troubles with transcontinental telephony that someone had to invent a fix for a different problem: distortion. A cascade of hundreds of repeaters demanded individual amplifiers of unprecedented linearity. This necessity was the mother of negative feedback.

Black's invention required quite a change in thinking. Now he was recommending "throwing away" precious gain in exchange for reduced distortion. This went against the training of a generation of EEs. Given that, I'm not surprised negative feedback took that long to get formalized.

Although you are absolutely right that a basic intuitive notion of negative feedback was appreciated long before electronics came along, Black was the first in history to understand explicitly that excess gain could be used as currency to pay for reductions in distortion. It is as subtle a notion as it is powerful.

The very first mathematical treatment of negative feedback was by Maxwell himself, but his analysis was limited to understanding why speed governors for steam engines could go unstable. Until technology ran into the need for exquisite precision in control, there was no need for Black to come along.

Cheers
Tom

Sent from an iThing; please excuse the terseness and typos

On Mar 25, 2021, at 23:14, "Ed Breya via groups.io" <edbreya=yahoo.com@groups.io> wrote

I don't know about this history, but am surprised that this didn't happen until 1929, well into the electronics era. Surely the concept of negative feedback in control systems has existed in nature, throughout human history, and in industry - at least since the steam era - look at the fly-ball governor, for example. Maybe in electronics, it wasn't so obvious, although it already existed in some forms, say for instance, with degenerative feedback in a vacuum tube, stabilizing the bias with the cathode resistor. People were driving horses, trains, planes, and automobiles successfully before 1929, using that PID controller in the skull.

Ed





Re: 2440 and 2465B battery/ram replacement

Szabolcs Szigeti
 

Hi Mark,

I can't help in replacing the RAMs, but I have some experience with the
2440/2432 etc DSOs. Don't be afraid to replace the RAM without saving it's
content, it is much easier to get calibrated than the 2465.
The first time I calibrated a 2440 I used a DC supply, and an old, low end
function generator's square wave output. Since then I have much improved
instruments, but it was possible to get it calibrated with minimum
equipment.
Especially, that if it is has not been calibrated for long you will
probably need to do an internal adjustment and readjust the screen geometry
and the CCD timing anyway (for that you'll need some kind of a signal
generator) which requires clearing the nvram as a first step, so there is
absolutely no need to save the ram or worry about losing its content. There
is absolutely no point in saving and reloading the old data.

Szabolcs


Mark Vincent <orangeglowaudio@gmail.com> ezt írta (időpont: 2021. márc.
26., P, 2:45):

I have both of these and would like to get these replaced with the new
style of chip that does not need the back-up battery. The (real) 2465B is
still good. I do not know how long it will keep the data before becoming
dead. The 2440 needs replacing. I know these original devices will go bad
soon. I do not have or access to a programmer. I know I have the newer A5
board in the B with S/N 063838 (1994). I am in the mid-Atlantic area of the
US. The caps and resistors in both have already been done. Mr. Yachad's
2465B site was used. Thank you Mr Yachad. I do not know what I would send.
The whole piece, board, what. Is there anyone willing to do this and give
me a price on these? I do not know if calibration is needed after the chip
replacement. I know enough people in this group are good to expert at using
and knowing the 2465B features. I got this one because of the praise it
gets.

The 2440 I have used some. The B is recent and I need to learn the
features. This is new to me. I am used to the 7000 series and older. The
2440 has some FAIL modes. I think the CCDs are good. It likely needs
calibration to clear the fail. This one still shows signals like should. I
know I need to learn what all this one can do when it is working right.

I have adjusted the power supplies to within a couple of mV to exact
of the adjustments on each scope. I do this on other models as well.

If anyone wants to contact in private, you can use the email. I will
give my telephone number in a private email if any want it. That is a much
faster way to get me and discuss anything necessary.

Mark






Re: How to explain how negative feedback lowers noise?

Ed Breya
 

I don't know about this history, but am surprised that this didn't happen until 1929, well into the electronics era. Surely the concept of negative feedback in control systems has existed in nature, throughout human history, and in industry - at least since the steam era - look at the fly-ball governor, for example. Maybe in electronics, it wasn't so obvious, although it already existed in some forms, say for instance, with degenerative feedback in a vacuum tube, stabilizing the bias with the cathode resistor. People were driving horses, trains, planes, and automobiles successfully before 1929, using that PID controller in the skull.

Ed


Re: How to explain how negative feedback lowers noise?

J Hunt
 

**********
Tom Lee wrote ...

Yes, Black invented both feedforward and negative feedback amplifiers, the latter indeed on the Lackawanna ferry in that year.

**********

Tom is right, our hero Harold Black came up with the amazing idea of applying negative feedback to an amplifier in 1929 during his commute to work on the Lackawanna Ferry. His objective was to reduce distortion in repeater amplifiers for long distance telephone service. He said that the idea came to him "in a flash" (though he had been working on the problem for four years).

If you are an IEEE member you can download the fascinating paper Black wrote in IEEE Spectrum (pp. 55 - 60, Dec. 1977) "Inventing the negative feedback amplifier". I would post it for our group but the download has restricted use.

In the article Black says that experts at the time were not persuaded. "... the director of research at Bell Labs objected. He insisted that a negative feedback amplifier would never work." Once it was proved to be practical they filed patents and ... " the concept was so contrary to established beliefs that the Patent Office initially did not believe it would work. ... In England, our patent application was treated in the same manner as one for a perpetual-motion machine."

It is reasonable that even today the concept can be challenging. But Black's objective to reduce distortion caused by the amplifier still is the main advantage.

John Hunt
Portland, OR


Re: More fun with avalanche pulsers

jerry finn
 

Nice work.... I'm thinking of re-activating some equipment.


Re: How to explain how negative feedback lowers noise?

Dave Daniel
 

If one wants to understand electrical noise, the only book that I have ever found that treats noise as a subject in it’s own right is Vasilescu’s “Electrical Noise and Interfering Signals”; electrical noise is the sole subject of the book.

I can’t claim to have read it from cover-to-cover, but the parts that I have read were clearly written.

Intrinsic electrical noise is simply inherent in any electronic device, even just wire.

DaveD

On Mar 25, 2021, at 22:44, Tom Lee <tomlee@ee.stanford.edu> wrote:

Yes, Black invented both feedforward and negative feedback amplifiers, the latter indeed on the Lackawanna ferry in that year. I'm not sure what oddity you were referring to.

But, negative feedback does not "minimize the added noise". I wish people would stop repeating this erroneous claim. I've already explained twice now why it's false. Widespread, perhaps, but false nonetheless.

And Groundhog day has already passed.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/25/2021 19:22, pheilman wrote:
Odd, when I was in school we were taught that negative feedback amplifiers
were invented at Bell Labs. By Harold Black. In 1927, on the ferry.

All amplifiers add noise to the signal as it passes through, negative
feedback amplifiers minimize the added noise. The excess gain of the
amplifier is traded for lower noise and lower distortion.

Removing noise from the incoming signal is beyond the ability of an
amplifier, this is more in the realm of a filter.

--Paul








Re: How to explain how negative feedback lowers noise?

Tom Lee
 

:)

5-year old: "What's wrong, Mister?"

Teacher: "I have a headache."

"It might be a tumor."

"It's not a too-muh!" (Arnold Schwarzenegger, Kindergarten Cop)

-- Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/25/2021 19:54, Charles wrote:
I wondered what that thumping sound was... it was Tom banging his head on his desk from 2000 miles away :)




Re: How to explain how negative feedback lowers noise?

 

I wondered what that thumping sound was... it was Tom banging his head on his desk from 2000 miles away :)


Re: How to explain how negative feedback lowers noise?

Tom Lee
 

Yes, Black invented both feedforward and negative feedback amplifiers, the latter indeed on the Lackawanna ferry in that year. I'm not sure what oddity you were referring to.

But, negative feedback does not "minimize the added noise". I wish people would stop repeating this erroneous claim. I've already explained twice now why it's false. Widespread, perhaps, but false nonetheless.

And Groundhog day has already passed.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/25/2021 19:22, pheilman wrote:
Odd, when I was in school we were taught that negative feedback amplifiers
were invented at Bell Labs. By Harold Black. In 1927, on the ferry.

All amplifiers add noise to the signal as it passes through, negative
feedback amplifiers minimize the added noise. The excess gain of the
amplifier is traded for lower noise and lower distortion.

Removing noise from the incoming signal is beyond the ability of an
amplifier, this is more in the realm of a filter.

--Paul




Re: How to explain how negative feedback lowers noise?

pheilman
 

Odd, when I was in school we were taught that negative feedback amplifiers
were invented at Bell Labs. By Harold Black. In 1927, on the ferry.

All amplifiers add noise to the signal as it passes through, negative
feedback amplifiers minimize the added noise. The excess gain of the
amplifier is traded for lower noise and lower distortion.

Removing noise from the incoming signal is beyond the ability of an
amplifier, this is more in the realm of a filter.

--Paul


Re: How to explain how negative feedback lowers noise?

Chuck Harris <cfharris@...>
 

The noise that is coming back through the feedback channel
and being negatively combined at the differential amplifier
on the input is not the same noise that is coming in the
input. It is random, so it is just as likely that the
negative feedback will be increasing the noise as it is
that it will be decreasing it.

I think where the unwashed masses get confused is between
noise and distortion. Negative feedback can decrease the
distortion in an amplifier, so the THD can decrease... but
that is distortion, it may annoy you, but it is not noise.

-Chuck Harris


On Thu, 25 Mar 2021 16:27:44 -0400 (EDT) "n4buq" <n4buq@knology.net>
wrote:
Yes, I understand how these work. My question was more about why the
explanation where noise from the first stage of an amplifier is fed
back (negated, inverted, or, ?) to the input which seems much like a
noise-canceling headset to me. I inferred from the sighs that the
previous explanation was incorrect and (sorry) but I don't understand
why that's the case.

Were the "sighs" meant to indicate that the example given was
incorrect or, perhaps, just more of the same already-covered
explanations? Sorry - sighs can a bit more meaningful when combined
with an eyeroll.

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Göran Krusell" <mc1648pp@gmail.com>
To: TekScopes@groups.io
Sent: Thursday, March 25, 2021 3:18:58 PM
Subject: Re: [TekScopes] How to explain how negative feedback
lowers noise?

Hi Barry,
No sighs from my desk, your question is a good one. I think these
modern headphones work in the following manner, a small amount of
noise comes through the leather in your headphones. At the same
time the external noise is picked up by a small microphone in the
headset, amplified by an inverting amplifier and added to the
direct noise to you ear in equal amounts. The noise is thus
attenuated. And it does work, I have one such headphone from a
well-known company. Göran









Re: 2440 and 2465B battery/ram replacement

Bob Albert
 

Mark, it never occurred to me to ship my 2440 board to someone willing to replace the NAVRAM modules.  If someone would do this for a reasonable price I would love to go that way.
What happened was that I replaced my modules and the scope stopped working.  Maybe the new modules are bad or maybe I screwed up the installation.  But that's the situation I am in and would love to have someone fix the board.
Bob K6DDX

On Thursday, March 25, 2021, 06:45:56 PM PDT, Mark Vincent <orangeglowaudio@gmail.com> wrote:

    I have both of these and would like to get these replaced with the new
style of chip that does not need the back-up battery. The (real) 2465B is
still good. I do not know how long it will keep the data before becoming
dead. The 2440 needs replacing. I know these original devices will go bad
soon. I do not have or access to a programmer. I know I have the newer A5
board in the B with S/N 063838 (1994). I am in the mid-Atlantic area of the
US. The caps and resistors in both have already been done. Mr. Yachad's
2465B site was used. Thank you Mr Yachad. I do not know what I would send.
The whole piece, board, what. Is there anyone willing to do this and give
me a price on these? I do not know if calibration is needed after the chip
replacement. I know enough people in this group are good to expert at using
and knowing the 2465B features. I got this one because of the praise it
gets.

    The 2440 I have used some. The B is recent and I need to learn the
features. This is new to me. I am used to the 7000 series and older. The
2440 has some FAIL modes. I think the CCDs are good. It likely needs
calibration to clear the fail. This one still shows signals like should. I
know I need to learn what all this one can do when it is working right.

    I have adjusted the power supplies to within a couple of mV to exact
of the adjustments on each scope. I do this on other models as well.

    If anyone wants to contact in private, you can use the email. I will
give my telephone number in a private email if any want it. That is a much
faster way to get me and discuss anything necessary.

Mark


2440 and 2465B battery/ram replacement

Mark Vincent
 

I have both of these and would like to get these replaced with the new
style of chip that does not need the back-up battery. The (real) 2465B is
still good. I do not know how long it will keep the data before becoming
dead. The 2440 needs replacing. I know these original devices will go bad
soon. I do not have or access to a programmer. I know I have the newer A5
board in the B with S/N 063838 (1994). I am in the mid-Atlantic area of the
US. The caps and resistors in both have already been done. Mr. Yachad's
2465B site was used. Thank you Mr Yachad. I do not know what I would send.
The whole piece, board, what. Is there anyone willing to do this and give
me a price on these? I do not know if calibration is needed after the chip
replacement. I know enough people in this group are good to expert at using
and knowing the 2465B features. I got this one because of the praise it
gets.

The 2440 I have used some. The B is recent and I need to learn the
features. This is new to me. I am used to the 7000 series and older. The
2440 has some FAIL modes. I think the CCDs are good. It likely needs
calibration to clear the fail. This one still shows signals like should. I
know I need to learn what all this one can do when it is working right.

I have adjusted the power supplies to within a couple of mV to exact
of the adjustments on each scope. I do this on other models as well.

If anyone wants to contact in private, you can use the email. I will
give my telephone number in a private email if any want it. That is a much
faster way to get me and discuss anything necessary.

Mark


Re: 485 super weak brightness control

 

On Fri, Mar 26, 2021 at 12:48 AM, Ozan wrote:


I agree sometimes stepping back and looking with fresh eyes helps. Let us know
when you want to bounce off ideas again.
Ozan
I fully concur, Ozan.

Raymond


Re: How to explain how negative feedback lowers noise?

Tom Lee
 

I partially agree with Goran's statement, but only partially because, as written, it neglects an important subtlety.

It is certainly true that a large first-stage gain suppresses the noise contributions of subsequent stages, so that's all good and intuitively satisfying.

However, once you've succeeded at that endeavor, the first stage's noise figure dominates. Somewhat counterintuitively, matching impedance at the input to maximize gain does not necessarily minimize noise figure. So, it is possible (even probable) to degrade NF by focussing only on maximizing gain.

The reason, in a nutshell, is that the minimum noise figure for an amplifier occurs for a source impedance that is the ratio of the equivalent input noise voltage to the noise current (I'm neglecting possible correlations between the two to make the argument simple). There's no fundamental connection between that ratio and the actual input impedance. You match to the former for best NF, and to the latter for maximum gain. So maximizing the gain of the first stage is not guaranteed to lead to overall best NF.

--Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/25/2021 16:27, Ed Breya via groups.io wrote:
Goran wrote: "If you wish to design a multistage amplifier with low phase noise factor you should maximize the first amplifier gain and the noise factor contribution from the following stages will be small.

Does anyone want to comment on this?"

My comment is that I agree. The front-end of any system is the most critical, and especially when the nature of the signal needs to be changed into an electric circuit signal. This happens all the time, whether it's receiving radio waves, light, sound, physical quantities, and so on. Electric to electric is the most common and straightforward - it's what goes on inside circuits. Anything else to electric has more issues, due to needing a transducer of some sort.

Ed




Re: 485 super weak brightness control

Ozan
 

Just wanted to say thank you for continuous support. I haven't abandoned
the ship just yet but I realised I spent way too much time on a scope
project and haven't done enough paid repairs to bring the money back home
to the family. I need to put it aside for few days. Fresh start with bit of
a distance between the attempts won't hurt.
I agree sometimes stepping back and looking with fresh eyes helps. Let us know when you want to bounce off ideas again.
Ozan


Re: How to explain how negative feedback lowers noise?

Ed Breya
 

Goran wrote: "If you wish to design a multistage amplifier with low phase noise factor you should maximize the first amplifier gain and the noise factor contribution from the following stages will be small.

Does anyone want to comment on this?"

My comment is that I agree. The front-end of any system is the most critical, and especially when the nature of the signal needs to be changed into an electric circuit signal. This happens all the time, whether it's receiving radio waves, light, sound, physical quantities, and so on. Electric to electric is the most common and straightforward - it's what goes on inside circuits. Anything else to electric has more issues, due to needing a transducer of some sort.

Ed


2 photos uploaded #photo-notice

TekScopes@groups.io Notification <noreply@...>
 

The following photos have been uploaded to the Sampling with 3S2 album of the TekScopes@groups.io group.

By: Charles <charlesmorris800@...>


Re: More fun with avalanche pulsers

 

I bought ten BFR505 transistors which are supposed to be faster than 2N2369's. Unfortunately they come in SOT-23 packages (i.e. smaller than a mouse turd). I had a fun time installing one into my pulser. Dropped the first two (but found them later by lying on the floor for a really close look), the third sprang out of the forceps and I heard it "tick" somewhere across the room. Finally got the fourth mounted (emitter soldered directly to the SMA output connector).

I moved some components for the shortest possible lead lengths, but it still has a divot in the top of the pulse that I can't tune out. Could it be the open BNC hanging from the end of the charge line? Still likes 33 ohm better than 50 ohms of emitter load, too. Amplitude is decreased since the BFR505 avalanches at 30-32 volts instead of 50+ like the '2369.

The good news is that the risetime is indeed quite a bit faster. As best as I could measure, 10-90% is 400 ps with a 6.5 volt top :) This quite noticeable when using the 350 ps S-1 head, so I went back to the 75 ps S-2. Pics added to "Sampling with 3S2" album. (If I could figure out how to link individual pics in a post, I would!)

But the bad news (which others have noted) is that the jitter and noise is also increased. It's not really bad especially while smoothing, but I guess that's a consequence of a small, very fast (Ft=9 GHz) transistor?

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