amplify 40kHz audio signal using TL082: first two stages are fine, but high noise from the third sta

[Luhan Monat]

Hello Luhan,



Like a 74HCU04? Or a similar CD series? You'd have to stay away from
VCC/2 though to avoid lots of quiescent current.

Regards, Joerg

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.

 

[Fred Bloggs]

Hello Tim,



Not everything, only when it is cheaper. At two cents a pop these things
are a bargain. But, I did use a 14106 recently. As a current controlled
oscillator...

Seriously, in this case it just might make sense to do it with
discretes. When you need lots of gain at frequencies above the audio
range things get to be expensive with chips.

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.

 

[Fred Bloggs]

You do English fairly passably. Go here:
http://www.tech-chat.de/download.html
and pull down
http://www.tech-chat.de/files/AACircuit128beta.zip
and do the obvious things. For stuff like this, some
diagrams can save thousands of words.

Why don't you take your own advice and do the same , loudmouth. You can
score this thread another zero contribution interference, with all your
generalistic pseudo-theoretical bs and "I assume"s, the same old crap.
You're nothing but a bit-head programmer ( not very good either) and
bs-artist.

 

[for_idea]

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

 

[Fred Bloggs]

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]

(in <[email protected]>) 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.

 

[Ken Smith]

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.

 

[Fred Bloggs]

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

 

[Larry Brasfield]

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.
[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]

in message ....

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.

....

 

[for_idea]

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.


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

 

[Larry Brasfield]

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

That suggestion may save a little bit of power. It
does nothing for the noise phenomenon Zhi sees.

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

That was his idea. If you want to understand
what is happening, study my post of 22:38 today
and compare the predicted output noise with
what you observe. If you use an o'scope for
that, estimate RMS by putting cursors (mental
or actual) at the levels that encompass about
2/3 of the fuzz, "measured" by brightness, (and
record 1/2 the difference, of course.)

Everybody's words are encouraged and appreciated.

Nice, and thanks.

 

[Fred Bloggs]

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]

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.

 

[Larry Brasfield]

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

Larry Brasfield wrote:

[in response to Fred's pontification to the OP: "You
are not educated enough to recognize that Larry is
an incompetent ..."]

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.

 

[Fred Bloggs]

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....

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.

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...

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]

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.

 

[for_idea]

Larry,

Many thanks to you. I respect people who talk reasons. I will check the
details you provided and report the result to the group.

Zhi Yang

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.
[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!