amplify 40kHz audio signal using TL082: first two stages are

[Fred Bloggs]

for_idea wrote:

Please! These words are not fair to Larry. His advices have help me
last night.

You are not educated enough to recognize that Larry is an incompetent
and waste-of-time, but this is more than apparent to the rest of us.

I still have one thing not clear: why the signal from first two stages
is so clean, but after the third one, it suddenly becomes so bad. The
noise is not oscillation, nor interference. The high spikes of the
noise keep giving false alarms to the detection circuit.

Is the third stage in the package with the unused opamp? If so, this may
be your problem, take that unused amplifier and configure it as follower
with OUT to IN(-), then tie IN(+) to the 4.5V reference or "pseudo-gnd".

 

[John Woodgate]

I read in sci.electronics.design that for_idea <zhiyang@gmail.com> wrote
(in <1111062045.258039.237360@f14g2000cwb.googlegroups.com&gt about
'amplify 40kHz audio signal using TL082: first two stages are fine, but
high noise from the third stage', on Thu, 17 Mar 2005:

Please! These words are not fair to Larry. His advices have help me last
night.

I still have one thing not clear: why the signal from first two stages
is so clean, but after the third one, it suddenly becomes so bad. The
noise is not oscillation, nor interference. The high spikes of the noise
keep giving false alarms to the detection circuit.

What is the peak-to-peak output voltage of the second stage? Remember,
your third stage is trying to amplify it 20 times. So if it's more than
about 200 mV, your third stage is overloaded, and if it's very much
overloaded it may well produce spikes.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Ken Smith]

In article <423966E5.3010505@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:
[...]

He can use a dog-simple hex inverter out of that 4000 series you like so
much- simple discrete FET source follower Sallen-Key 40kHz bandpass Q=10
ac-coupled into input chain and logic compatible square wave burst
output.

At a Q=10, I wouldn't trust a source follower to have again of exactly
1.0. Also the ratio of component values gets kind of large at a Q of 10.
I'd suggest using two stages with lower Q.


--
--
kensmith@rahul.net forging knowledge

 

[Fred Bloggs]

Ken Smith wrote:

In article <423966E5.3010505@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:
[...]

He can use a dog-simple hex inverter out of that 4000 series you like so
much- simple discrete FET source follower Sallen-Key 40kHz bandpass Q=10
ac-coupled into input chain and logic compatible square wave burst
output.



At a Q=10, I wouldn't trust a source follower to have again of exactly
1.0. Also the ratio of component values gets kind of large at a Q of 10.
I'd suggest using two stages with lower Q.

Actually you can do better than 10- I have some neat circuits with
follower feedback into a parallel LC tap on input that can do much, much
better than that- from the days of discrete precision tone
encoders/decoders for telecom.

 

[Joerg]

Hello Luhan,

74C04 has worked for me amplifying 40 k Baud data from IR detector.
Current? Yes, but it didn't melt. This is not an original idea; running
digital cmos in linear mode.

The only thing Zhi Yang has to keep in mind is a modest or at least
predictable battery drain. If you bias at VCC/2 the current will vary
quite a bit between different batches.

Regards, Joerg

http://www.analogconsultants.com

 

[Larry Brasfield]

"for_idea" <zhiyang@gmail.com> wrote in message
news:1111062045.258039.237360@f14g2000cwb.googlegroups.com...

Please! These words are not fair to Larry. His advices have help me
last night.

That's decent of you, Zhi. I urge you to apply a filter
to Fred's remarks. Once in awhile, some facts can
be found in his posts, and often those are correct,
and somewhat less often they are also relevant.
As for his comments about me, quite a few are
provably false, so I give the whole lot no heed.

I still have one thing not clear: why the signal from first two stages
is so clean, but after the third one, it suddenly becomes so bad. The
noise is not oscillation, nor interference.

The upper band edge of your amp string is near
68 KHz. (4 MHz op-amp GBW, closed-loop gain
of 50, 3 like stages) With your inter-stage coupling
caps, the lower band edge is near 19 KHz. This
leaves about 49 KHz of bandwidth in the -3 dB
sense. The noise bandwidth is slightly different,
but I'm too lazy to calculate that. (Maybe Fred
will take on that challenge, since it is manly work.)
Near your center frequency, the response will be
about 14 dB down with respect to the ideal value
of 50^3 you would get with higher loop gains and
the HPF's not cutting in from the low end.

If you were to short the input, then, due to the
thermal noise of the input resistor, you would
have an equivalent input noise density of about
sqrt(4 k T R) = sqrt(4 * 1.38e-23 * 300 * 1e4)
or 12.9 nV/sqrt(Hz). Adding to that the input
voltage noise of your op-amp, 16 nV/sqrt(Hz)
typically, (and adding RSS-wise), you should
expect input noise of about 20.5 nV/sqrt(Hz).
If we were going for accuracy, a smidgen could
be added from the feedback resistor, but its
contribution would be lost in the errors already
in this calculation. (Fred might be willing to
enumerate or even quantify them for you.)

Taking the gain of your amp chain, which is
about 24.7e3 in the middle of its passband, the
input noise results in about 506 uV/sqrt(Hz)
noise density at the output. Multiplying by
the square root of bandwidth yields 112 mV
of RMS noise. This last step is not quite
correct because the bandwidth that should
be used (the "noise bandwidth") is a little
different, but it should be close enough to
help you see whether your result is worse
than should be expected.

With your transducer as the input, the noise
could go up or down depending on its source
impedance. That is why you would have to
measure it or take it off the datasheet to see
whether the noise you see with it connected
is what it should be.

Once the input is better characterized, it
will probably be easy to revise the first
stage a little bit to get lower noise.

As for why you do not see the noise on the
earlier stages, that would depend on your
instrument. With a typical o'scope, not being
able to see the 4 mV to be expected at the
2nd stage output is to be expected. And of
course, it's even harder at the 1st stage.

If you peruse the rest of this thread, (with
filters in place, I advise), you will find some
comments about your op-amp choice that
you may find useful. And if you care about
input noise and power consumption, take
suggestions about using CMOS gates as
amplifiers with a degree of skepticism.

The high spikes of the
noise keep giving false alarms to the detection circuit.

I have no idea what that is. Since, (apparently),
you are reaching into the noise floor for signal,
it may be worth your while to post the detector
and solicit ideas for improving it. A tighter
bandpass filter could do some good as well.

Fred Bloggs wrote:
Larry Brasfield wrote:
[Brasfield's AACircuit directions and link cut.]

[Fred's invective regarding such "advice" cut.]
[Fred's unsubstantiated ad-hominem rant cut.]
[Reply to noise filter output:] Nothing left!

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Larry Brasfield]

"Larry Brasfield" <donotspam_larry_brasfield@hotmail.com> wrote
in message news:m8k_d.27$zO.795@news.uswest.net...
....

With your transducer as the input, the noise
could go up or down depending on its source
impedance. That is why you would have to
measure it or take it off the datasheet to see
whether the noise you see with it connected
is what it should be.

I should have mentioned, to the extent the
noise does not go up under this condition,
your input is seriously mismatched to its
source for purposes of SNR maximization.

Once the input is better characterized, it
will probably be easy to revise the first
stage a little bit to get lower noise.

That's understated. It's a near certainty, IMHO.

....
--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[for_idea]

Fred Bloggs wrote:

for_idea wrote:
Please! These words are not fair to Larry. His advices have help me
last night.

You are not educated enough to recognize that Larry is an incompetent

and waste-of-time, but this is more than apparent to the rest of us.


I still have one thing not clear: why the signal from first two
stages
is so clean, but after the third one, it suddenly becomes so bad.
The
noise is not oscillation, nor interference. The high spikes of the
noise keep giving false alarms to the detection circuit.

Is the third stage in the package with the unused opamp? If so, this
may
be your problem, take that unused amplifier and configure it as
follower
with OUT to IN(-), then tie IN(+) to the 4.5V reference or
"pseudo-gnd".

Fred, actually, I did exactly what you described. Do you have any idea
to improve the third stage?

Everybody's words are encouraged and appreciated.

Zhi Yang

 

[Joerg]

Hello Larry,

.... And if you care about
input noise and power consumption, take
suggestions about using CMOS gates as
amplifiers with a degree of skepticism.

Agree. Same for building a discrete circuit with transistors. Before
attempting any of that Zhi Yang would really have to be sure that he
knows all the tricks in those areas. Or has a colleague who does.

Regards, Joerg

http://www.analogconsultants.com

 

[Fred Bloggs]

Larry Brasfield wrote:

My bet is that Zhi is educated enough, in matters
both technical and interpersonal, to correctly form
his own judgements on my competence.

Well here's something about which there is no uncertainty. You're a
minor punk programmer and former electrical technician who was fired out
of the profession. This explains your constant misunderstanding of the
OP's question and this pretense with little bits and pieces of
elementary theoretical factoids that you blow out of proportion and have
no bearing on the thread. So, loser- tell us how you flunked EET and
then transferred to some quick IT school and progressed along with your
miserable and superficial life. You're the usual third rate scum I have
come to expect from newsgroup...

 

[Ken Smith]

In article <4239A4CB.7080801@nospam.com>,
Fred Bloggs <nospam@nospam.com> wrote:
[....]

Actually you can do better than 10- I have some neat circuits with
follower feedback into a parallel LC tap on input that can do much, much
better than that- from the days of discrete precision tone
encoders/decoders for telecom.

I hadn't thought of using an LC circuit for this case. At 40KHz it may
not be such abad idea.


--
--
kensmith@rahul.net forging knowledge

 

[Larry Brasfield]

To the OP and others looking for technical content:
There is but one strictly technical fact here. See "1.".

"Fred Bloggs" <nospam@nospam.com> wrote in
message news:423A234C.1040706@nospam.com...

Larry Brasfield wrote:
[in response to Fred's pontification to the OP: "You

are not educated enough to recognize that Larry is
an incompetent ..."]

My bet is that Zhi is educated enough, in matters
both technical and interpersonal, to correctly form
his own judgements on my competence.

Well here's something about which there is no uncertainty.

Fred, it is your certainty about falsehoods that proves
you are at least a very disturbed, neurotic individual, if
not outright psychotic. Your obsession with me and my
posts only confirms this. You need professional help.

You're a minor punk programmer and former electrical technician who was fired out of the profession.

Out of three factual clauses, only one with any truth,
and that available from what I have already posted.
So it would appear the rest is baseless conjecture.
How in the world, Fred, do you become so certain
of "facts" you conjure out of this air? That tendency
marks you as either delusional or a liar, or both.

This explains your constant misunderstanding of the OP's question and this pretense with little bits and pieces of elementary
theoretical factoids that you blow out of proportion and have no bearing on the thread.

More bare assertion without substantiation. For
those following this branch for amusement, the
OP has asked about a final stage in an amplifier
chain with enough gain to make its input noise
visible on an oscilloscope, wondering why it is
"significantly corrupted by noise". Fred is now
upset because I have explained it (or pretended
to explain it in his delusional perspective) when
his own far-fetched diagnosis has fallen flat. His
blathering about inapposite "theoretical factoids"
serves to reveal the depth of his ignorance and
delusion. If he was truly as knowledgable as he
pretends, he would be capable of recognizing the
simple validity and relevance of my post today
explaining how the OP's noise observations have
come to be. But he is not that knowledgable, so
he is able to imagine that simply because he fails
to comprehend my analysis and holds derogatory
opinions of me, it must be irrelevant and silly.

The only problem with my analysis is that it
explains all the OP's reported facts, relying
only on well settled and accepted theory
together with facts that were either posted
by the OP or are publicly available (except
for one [1]), *and* was not contemplated
by Fred, the master of all things electronic.

[1. There was an unstated assumption about
the ambient temperature. I offer this as bait
for certain idiot(s) to grasp as grist for their
continuing process of self-embarassment. ]

I note that Fred has not (yet) shown enough
courage of conviction to actually attack my
analysis on its technical merits, point by point.
Somewhere, deep within his disturbed thought
processes, he surely must realize that he cannot
do that without showing himself to be even less
competent in one realm of expertise than myself,
a person he has obsessively denigrated as some
kind of totally incompetent fraud. So, here we
see the result of some very hard choices for him,
a conundrum of his own making.

So, loser- tell us how you flunked EET

More false conjecture. I never attended EET.
(Until I looked it up just now, I was unfamiliar
with the acronym.) And I never flunked anything.
Whatever led you to "believe" such nonsense?
Delusional.

and then transferred to some quick IT school

Wrong again. Amazing how you conflate the
growth of some fantasy in your fevered mind
with an ability to divine certainties. Delusional.

and progressed along with your miserable and
superficial life.

I could mention here many of the indicators of
a reasonably well conducted and satisfying life,
but pity prevents it. I have my own ideas about
what Fred's life must be like, and if they are true,
he could only become more disappointed with his
existence upon apprehending its contrast with mine.

You're the usual third rate scum I have come to expect from newsgroup...

Fred, I wish you could comprehend how satisfied I
am with your respect and affection. If you were able
to understand that, there might be hope for you.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Fred Bloggs]

Larry Brasfield wrote:

To the OP and others looking for technical content:
There is but one strictly technical fact here. See "1.".

No one is reading your sh_t any more. Quite telling that you have no
clue how much of hot air bag you sound/ and appear.

Fred, it is your certainty about falsehoods that proves
you are at least a very disturbed, neurotic individual, if
not outright psychotic. Your obsession with me and my
posts only confirms this. You need professional help.

Typical psycho-babble characteristic of a trite usenet troll.

Out of three factual clauses, only one with any truth,
and that available from what I have already posted.
So it would appear the rest is baseless conjecture.
How in the world, Fred, do you become so certain
of "facts" you conjure out of this air? That tendency
marks you as either delusional or a liar, or both.

Anyone can see from your sickening style of writing that you are a heap
of crap and bs. Nah- you're a small banana trying to look like a bunch-
pathetic really.

More bare assertion without substantiation. For
those following this branch for amusement,

Not to worry- hard to imagine anyone with so much time on their hands
that they waste it reading your trash.

the
OP has asked about a final stage in an amplifier
chain with enough gain to make its input noise
visible on an oscilloscope, wondering why it is
"significantly corrupted by noise". Fred is now
upset because I have explained it (or pretended
to explain it in his delusional perspective) when
his own far-fetched diagnosis has fallen flat. His
blathering about inapposite "theoretical factoids"
serves to reveal the depth of his ignorance and
delusion. If he was truly as knowledgable as he
pretends, he would be capable of recognizing the
simple validity and relevance of my post today
explaining how the OP's noise observations have
come to be. But he is not that knowledgable, so
he is able to imagine that simply because he fails
to comprehend my analysis and holds derogatory
opinions of me, it must be irrelevant and silly.

All you have done is make some simple 4KTRB and BW estimates- and then
stopped as usual. This is a good example of your propensity to blather
some elementary factoid trash and then make no sense of it.

The only problem with my analysis is that it
explains all the OP's reported facts, relying
only on well settled and accepted theory
together with facts that were either posted
by the OP or are publicly available (except
for one [1]), *and* was not contemplated
by Fred, the master of all things electronic.

Really? The OP builds a chain with 115 dB gain and all you know how to
do is elementary electronic input noise- pathetic really.

[1. There was an unstated assumption about
the ambient temperature. I offer this as bait
for certain idiot(s) to grasp as grist for their
continuing process of self-embarassment. ]

I note that Fred has not (yet) shown enough
courage of conviction to actually attack my
analysis on its technical merits, point by point.

You have done no analysis worth noting as usual. How many times do you
need to get your ass kicked before you realize you're less than third
rate garbage- so far *all* of your product has been laughably weak,
inaccurate, and worthless.

Somewhere, deep within his disturbed thought
processes, he surely must realize that he cannot
do that without showing himself to be even less
competent in one realm of expertise than myself,
a person he has obsessively denigrated as some
kind of totally incompetent fraud. So, here we
see the result of some very hard choices for him,
a conundrum of his own making.

More troll psycho-babble....

So, loser- tell us how you flunked EET


More false conjecture. I never attended EET.
(Until I looked it up just now, I was unfamiliar
with the acronym.) And I never flunked anything.
Whatever led you to "believe" such nonsense?
Delusional.

Couldn't get into the program? Guess your obsession with reading comic
books just wasn't enough preparation- ahhh- too bad.

and then transferred to some quick IT school


Wrong again. Amazing how you conflate the
growth of some fantasy in your fevered mind
with an ability to divine certainties. Delusional.

Nah- you are in fact a little delusional sh_t programmer- a non-
scientific programmer at that.

I could mention here many of the indicators of
a reasonably well conducted and satisfying life,
but pity prevents it.

All psycho-trolls say that...

I have my own ideas about
what Fred's life must be like, and if they are true,
he could only become more disappointed with his
existence upon apprehending its contrast with mine.

Incredible narcissism...

You're the usual third rate scum I have come to expect from newsgroup...


Fred, I wish you could comprehend how satisfied I
am with your respect and affection. If you were able
to understand that, there might be hope for you.

You're a complete nobody- go back to your messy hovel of a cubicle and
drink your soda...

>

 

[lemonjuice]

On 17 Mar 2005 04:20:45 -0800, "for_idea" <zhiyang@gmail.com> wrote:

Please! These words are not fair to Larry. His advices have help me
last night.

I still have one thing not clear: why the signal from first two stages
is so clean, but after the third one, it suddenly becomes so bad. The
noise is not oscillation, nor interference. The high spikes of the
noise keep giving false alarms to the detection circuit.

Zhi Yang

As someone already mentioned its normal the more stages you add the
more noise you get. More current noise from bias currents into the
opamp flowing into the output resistors, voltage noise in the devices
in the opamp etc... . Each of these noise contributions gets amplified
in each successive stage and the noise sum also gets added on until
your circuit starts giving off false alarms.

Your other question was how to improve the 3rd stage. How well are
your resistors scaled against noise? Plug into the total rms output
noise formulae the values for the corner frequency for voltage
noise(from opamp datasheet) , corner frequency for current noise , the
feedback factor, corner frequency for the closed loop gain function,
plus the thermal noise resistor values. Calculate each noise component.
if the resistor noise component is much greater then the other 2 then
you need to scale your resistences. a rule of the thumb used for low
noise design is
Eno(V)^2 + Eni(I)^2 = 1/5*Eni(thermal)^2 I'm sure you can find these
formulae on the web or in an electronics book. In case you can't let
me know and I'll try and dig them up. It also helps looking at them
too because you'll know what is actually increasing your noise and what
to look for in a low noise Opamp.

 

[lemonjuice]

On Fri, 18 Mar 2005 18:29:28 +0100, "Fred Bartoli"
<fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:

"lemonjuice" <exskimos@anonymous.to> a écrit dans le message de
news:1111152619.440554.41320@f14g2000cwb.googlegroups.com...
On 17 Mar 2005 04:20:45 -0800, "for_idea" <zhiyang@gmail.com> wrote:

Please! These words are not fair to Larry. His advices have help me
last night.

I still have one thing not clear: why the signal from first two
stages
is so clean, but after the third one, it suddenly becomes so bad.
The
noise is not oscillation, nor interference. The high spikes of the
noise keep giving false alarms to the detection circuit.

Zhi Yang



As someone already mentioned its normal the more stages you add the
more noise you get. More current noise from bias currents into the
opamp flowing into the output resistors, voltage noise in the
devices
in the opamp etc... . Each of these noise contributions gets
amplified
in each successive stage and the noise sum also gets added on until
your circuit starts giving off false alarms.


Nope. Only the first stage matters, unless the design is completely
screwed
(which it isn't).

Really! So the noise contributions miraculously disappear from the the

output of the first opamp! Damn what an amplifier that is. Tell me more
as I won't go into explaining why and how the first stage affects
noise.

Your other question was how to improve the 3rd stage. How well are
your resistors scaled against noise? Plug into the total rms output
noise formulae the values for the corner frequency for voltage
noise(from opamp datasheet) , corner frequency for current noise ,
the
feedback factor, corner frequency for the closed loop gain
function,
plus the thermal noise resistor values. Calculate each noise
component.
if the resistor noise component is much greater then the other 2
then
you need to scale your resistences. a rule of the thumb used for low
noise design is
Eno(V)^2 + Eni(I)^2 = 1/5*Eni(thermal)^2 I'm sure you can find these
formulae on the web or in an electronics book. In case you can't
let
me know and I'll try and dig them up. It also helps looking at them
too because you'll know what is actually increasing your noise and
what
to look for in a low noise Opamp.


Competing for an LB award from FB?
Sorry if I'm too advanced man... its basic electronics.

 

[Joerg]

Hello Ken,

I hadn't thought of using an LC circuit for this case. At 40KHz it may
not be such abad idea.

An LC would be pretty easy at 40kHz. However, it depends on what the OP
wants to do. If he intends to do pulse echo with good range resolution
it needs to stay wideband.

Regards, Joerg

http://www.analogconsultants.com

 

[Larry Brasfield]

A correction, motivated by one of Fred's rare
technical efforts apparent in this thread, is inserted
below. The OP should take note of this. I have
added a few other details and notes as well.
There is also a little humor for those who do not
take themselves way too seriously. (See "1.")

The need for this correction underscores the
importance of peer review and the value of
stating assumptions and showing enough of
the analysis to permit it to be critiqued.

"Larry Brasfield" <donotspam_larry_brasfield@hotmail.com>
wrote in message news:m8k_d.27$zO.795@news.uswest.net...

"for_idea" <zhiyang@gmail.com> wrote in message
news:1111062045.258039.237360@f14g2000cwb.googlegroups.com...
....
I still have one thing not clear: why the signal from first two stages
is so clean, but after the third one, it suddenly becomes so bad. The
noise is not oscillation, nor interference.

[Frequency response revised and moved.]

It has come to my attention that the 3 stages are not
"like" as I had stated. The last stage has about 7.16
fewer dB of gain than derived in my pre-corrected
analysis. Also, due to the improved loop gain in the
last stage, (as compared to that earlier analysis), the
-3dB bandwidth is about 59 KHz (rather than the
49 KHz mentioned before), and well centered on the
OP's 40 KHz signal. These corrections affect the
amount of noise that should be expected at the final
stage output, as detailed below.

The bandwidths will change with a transducer in
place, due to the slightly higher loop gain that will
produce in the first stage, so it would be premature
to worry about noise bandwidth just now.

[The input noise still appears to be correct.]

If you were to short the input, then, due to the
thermal noise of the input resistor, you would
have an equivalent input noise density of about
sqrt(4 k T R) = sqrt(4 * 1.38e-23 * 300 * 1e4)
or 12.9 nV/sqrt(Hz). Adding to that the input
voltage noise of your op-amp, 16 nV/sqrt(Hz)
typically, (and adding RSS-wise), you should
expect input noise of about 20.5 nV/sqrt(Hz).
If we were going for accuracy, a smidgen could
be added from the feedback resistor, but its
contribution would be lost in the errors already
in this calculation. (Fred might be willing to
enumerate or even quantify them for you.)

[Cut incorrect gain figures and calculation.]

With the OP's stated 200K feedback R (not the
"like" 500K I mistakenly assumed), the gain is
closer to 90 dB. This results from the ideal gain
of about 94 dB (= dB(50 * 50 * 20)) with losses
of .63 dB per stage for the HPF's, .97 dB for the
first 2 stages due to their (low) loop gains of 2,
and .17 dB for the last stage with its loop gain of 5.

The output noise density is then
20.5 nV/sqrt(Hz) * 10^(90.0/20) V/V
= 648 uV/sqrt(KHz)
Multiplying by the square root of bandwidth yields
an expectable RMS output noise of 157 mV.

[Noise bandwidth adjustment unmentioned.]

With your transducer as the input, the noise
could go up or down depending on its source
impedance. That is why you would have to
measure it or take it off the datasheet to see
whether the noise you see with it connected
is what it should be.

Once the input is better characterized, it
will probably be easy to revise the first
stage a little bit to get lower noise.

I would bet good money on long odds that
the first stage can be revised to bring the
SNR up by several dB, once the input is
better known.

[corrected in place:]

As for why you do not see the noise on the
earlier stages, that would depend on your
instrument. With a typical o'scope, not being
able to see the few mV to be expected at the
2nd stage output is to be expected. And of
course, it's even harder at the 1st stage.

The comparisons suggested between the noise
to be expected and the noise you see should
be used only for limited purposes, such as
deciding to proceed further along the noise
investigation rather than looking for another
source for the noise. [1] The way to finally
evaluate noise should involve the measured
gain of the amplifier, not its nominal gain.

[1. For example, suppose you had left an
op-amp unconnected whose inputs just so
happened to float together, and stay within
about 15 uV of each other, (offset by Vos),
such that the output was miraculously active
for an observable length of time during the
few 100's of mS before they both float to the
rail, and if that op-amp was paired with the
first stage, then, if you looked at the noise
appearing on the final stage quickly enough,
(within say, 200 mS and allowing some time
for noise observation averaging), you might
see several millionths of a dB of extra noise
that: arises at the open input at the about the
same level as the other stages have (which
estimation includes input current noise); then
appears at the output of that unconnected
amplifier after a gain of about 40 dB; couples
into the input thru the typical 90 dB of channel
isolation; and competes (at -45 dB relative to
the *real* noise sources) for a position as the
top dog noise source. This would seem to be
far-fetched in the cold light of day, but late at
night, when demons are at play, who knows
what can *might* [2] happen? ]

[2. Here, "might" must be distinguished from the
false certainty that plagues us all from time to
time, some more and more often than others. ]

If you peruse the rest of this thread, (with
filters in place, I advise), you will find some
comments about your op-amp choice that
you may find useful. And if you care about
input noise and power consumption, take
suggestions about using CMOS gates as
amplifiers with a degree of skepticism.

The high spikes of the
noise keep giving false alarms to the detection circuit.

I have no idea what that is. Since, (apparently),
you are reaching into the noise floor for signal,
it may be worth your while to post the detector
and solicit ideas for improving it.

It is also worth noting that the above analysis
predicts the RMS value of a random signal with
a Gaussian amplitude distribution. It is common
to figure on seeing 5 times more peak value, but
if you want to calculate the actual rate at which
your detector will false detect due to that noise,
you need a more sophisticated approach based
on the Gaussian statistics.

A tighter
bandpass filter could do some good as well.

A good place for that filter would be after the
first stage. If you use an active filter, that will
be a necessity. If you use an LC filter, that will
be prudent due to coupling considerations and
noise performance issues.

P.S. to Fred: If you reply, please try to take a
rational approach to this, or, failing that, come
up with some new names and more imaginative
invective. Your latest efforts have become so
repetitious that they are really quite boring.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Larry Brasfield]

Mainly just laughs here.

"Fred Bloggs" <nospam@nospam.com> wrote in
message news:423AAB3C.6070609@nospam.com...

Larry Brasfield wrote:
[snip]
No one is reading your sh_t any more. Quite telling that you have no clue how much of hot air bag you sound/ and appear.

Thanks, Fred. I appreciate that. When my style
begins to please you, then I will start worrying.

[More typical FB crap cut.]

More bare assertion without substantiation. For
those following this branch for amusement,

Not to worry- hard to imagine anyone with so much time on their hands that they waste it reading your trash.

So, I take it you have no way to substantiate.
Just as I thought. You are a nasty fabulist.

the
OP has asked about a final stage in an amplifier
chain with enough gain to make its input noise
visible on an oscilloscope, wondering why it is
"significantly corrupted by noise". Fred is now
upset because I have explained it (or pretended
to explain it in his delusional perspective) when
his own far-fetched diagnosis has fallen flat. His
blathering about inapposite "theoretical factoids"
serves to reveal the depth of his ignorance and
delusion. If he was truly as knowledgable as he
pretends, he would be capable of recognizing the
simple validity and relevance of my post today
explaining how the OP's noise observations have
come to be. But he is not that knowledgable, so
he is able to imagine that simply because he fails
to comprehend my analysis and holds derogatory
opinions of me, it must be irrelevant and silly.

All you have done is make some simple 4KTRB and BW estimates- and then stopped as usual.

I went as far as was needed to explain the reported
observation that brought the OP here. Going beyond
simple would be a disservice at this point, for reasons
you could figure out yourself if you thought about the
OP's situation rather than your own aggrandizement.

This is a good example of your propensity to blather some elementary factoid trash and then make no sense of it.

I note that you have failed to show any particular
way in which my analysis does not make sense.
Telling. Whenever you are shown up, out comes
a load of crap. Who do you imagine is fooled?

The only problem with my analysis is that it
explains all the OP's reported facts, relying
only on well settled and accepted theory
together with facts that were either posted
by the OP or are publicly available (except
for one [1]), *and* was not contemplated
by Fred, the master of all things electronic.

Really? The OP builds a chain with 115 dB gain and all you know how to do is elementary electronic input noise- pathetic really.

More baseless assumption and conjecture.
What real reason is there to believe that I
could not have completed a full noise
analsysis? (Try not to confuse your need
to denigrate with "real" here, Fred.)

....

I note that Fred has not (yet) shown enough
courage of conviction to actually attack my
analysis on its technical merits, point by point.

You have done no analysis worth noting as usual.

Translation: Fred cannot rationally refute.

How many times do you need to get your ass kicked

If you are the ass kicker, and my unblemished ass has
been the target, then your counting skills are bad.

before you realize you're less than third rate garbage- so far *all* of your product has been laughably weak, inaccurate, and
worthless.

So you like to believe. What does it do for you?

....

So, loser- tell us how you flunked EET

More false conjecture. I never attended EET.
(Until I looked it up just now, I was unfamiliar
with the acronym.) And I never flunked anything.
Whatever led you to "believe" such nonsense?
Delusional.

Couldn't get into the program?

I'm still not sure what program you refer to, but I
never took any courses aimed at tecnicians, nor
did I try to get into any such program. That you
are able to conjecture that way is pathetic.

Guess your obsession with reading comic books just wasn't enough preparation- ahhh- too bad.

More fabulizing. Your percentages are getting
very bad, Fred. Maybe it's time to check your
assumptions. They are not working.

[More of the same cut for space.]
[Fred's venture into psychoanalysis cut.]
[More of the usual name-calling cut as redundant.]

go back to your messy hovel of a cubicle and drink your soda...

I wonder how long it will take you to figure out
that I will never take orders from you. That it
has not yet occurred is quite telling.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Fred Bartoli]

"lemonjuice" <exskimos@anonymous.to> a écrit dans le message de
news:1111167517.373587.131310@g14g2000cwa.googlegroups.com...
On Fri, 18 Mar 2005 18:29:28 +0100, "Fred Bartoli"
<fred._canxxxel_this_bartoli@RemoveThatAlso_free.fr_AndThisToo> wrote:

As someone already mentioned its normal the more stages you add the
more noise you get. More current noise from bias currents into the
opamp flowing into the output resistors, voltage noise in the
devices
in the opamp etc... . Each of these noise contributions gets
amplified
in each successive stage and the noise sum also gets added on until
your circuit starts giving off false alarms.


Nope. Only the first stage matters, unless the design is completely
screwed
(which it isn't).

Really! So the noise contributions miraculously disappear from the the
output of the first opamp! Damn what an amplifier that is. Tell me more
as I won't go into explaining why and how the first stage affects
noise.

OK, so you're saying that the second and third stages noise matters.
Well, lets do the really, hem..., advanced maths for you...

Have 3 identical stages with input referred noise en and gain G.
The total output noise is :
sqrt(G^2(en^2+G^2(en^2+G^2 en^2)))= en sqrt(G^2 + G^4 + G^6)
the first term being the output stage contribution, the second one the
second stage contribution and the 3rd one the first stage contribution.
OK?

Now, you build an amplifier, isn't it? So G>1. You pay attention to
amplifier noise so your signal is low level too. Thus you want your gain
above few units. Say G=3. (note the OP wants its gain = 50)

Onoise = en sqrt(9+81+729) = 28.62 en

Now, I magically make the output noise disappear, as you say so well.

Onoise = en sqrt(81+729) = 28.46 en

Even better now: I magically make the second stage noise disappear

Onoise = en sqrt(729) = 27 en

Wow, that's an 'enormous' difference.

Now, let G = 50, as is the case here, redo the computations, and you, the
amplifier noise expert, enlighten us about the _huge_ 3rd stage noise
contribution.

Your other question was how to improve the 3rd stage. How well are
your resistors scaled against noise? Plug into the total rms output
noise formulae the values for the corner frequency for voltage
noise(from opamp datasheet) , corner frequency for current noise ,
the
feedback factor, corner frequency for the closed loop gain
function,
plus the thermal noise resistor values. Calculate each noise
component.
if the resistor noise component is much greater then the other 2
then
you need to scale your resistences. a rule of the thumb used for low
noise design is
Eno(V)^2 + Eni(I)^2 = 1/5*Eni(thermal)^2 I'm sure you can find these
formulae on the web or in an electronics book. In case you can't
let
me know and I'll try and dig them up. It also helps looking at them
too because you'll know what is actually increasing your noise and
what
to look for in a low noise Opamp.

Yeah, obviously look at the 3rd stage noise.

Competing for an LB award from FB?

Sorry if I'm too advanced man... its basic electronics.

I agree on this, guy. This is really basic electronics, but this is still
too advanced for you, like it seems some 1.35V low voltage voltage
amplifiers.

You appear to be a bit out of juice, lemonhead.


--
Thanks,
Fred.

 

[Larry Brasfield]

An almost purely technical post, after severe editing.

"Fred Bloggs" <nospam@nospam.com> wrote in
message news:423AAB3C.6070609@nospam.com...

Larry Brasfield wrote:
[SNIP]
the
OP has asked about a final stage in an amplifier
chain with enough gain to make its input noise
visible on an oscilloscope, wondering why it is
"significantly corrupted by noise". Fred is now
upset because I have explained it (or pretended
to explain it in his delusional perspective) when
his own far-fetched diagnosis has fallen flat.
[snip]
All you have done is make some simple 4KTRB and BW estimates- and then stopped

What other noise analysis do you believe is
relevant to what the OP reports seeing?
Have you ever actually done any practical
noise analysis, such that you can explain
why "All you have done ..." should perhaps
be considered the insult you surely intended
rather than a comical and telling snipe?

(And before you reply, you may want to
consider Fred Bartoli's post of 12:16 today
where he makes a point relevant to your
"snipe". He appears to know more about
real noise analysis than you do.)

[invective gone]

The only problem with my analysis is that it
explains all the OP's reported facts, relying
only on well settled and accepted theory
together with facts that were either posted
by the OP or are publicly available (except
for one [1]), *and* was not contemplated
by Fred, the master of all things electronic.

Really? The OP builds a chain with 115 dB gain

I am unable to replicate your 115 dB figure.
In fact, I believe it to be erroneous. Do you
have any way to show how you arrived at
it, or was it too ephemeral to permit that?

In other words, either prove it or make clear
that it was one of your thin air productions.

[SNIP]
--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.