Amplify the hell out of it.

[Gregory L. Hansen]

I know how to build the basic inverting and non-inverting op-amp
amplifiers, and they seem to work well enough for reasonable gains like
in the 10's. But what if I want to amplify a small signal by a factor
of 1000 or so? It seems like there must be a lot of issues swept aside in
the intro books that will rear up-- non-linearities, zeroing, I don't know
what else.

Any advice?

--
"Tell me, Dr. Einstein, at what time does Boston arrive at this train?"

 

[Rene Tschaggelar]

Gregory L. Hansen wrote:

I know how to build the basic inverting and non-inverting op-amp
amplifiers, and they seem to work well enough for reasonable gains like
in the 10's. But what if I want to amplify a small signal by a factor
of 1000 or so? It seems like there must be a lot of issues swept aside in
the intro books that will rear up-- non-linearities, zeroing, I don't know
what else.

Any advice?

The more gain you need, the better you should know the signal,
the impedance, the offset, the noise.
For reasonably narrow bandwidth, a gain of few 10k is doable.
Not necessarily with one amplifier alone.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

 

[kmaryan@gmail.com]

See "instrumentation amplifier". You can roll your own from op-amps or
buy a ready made one. Many off the shelf ones can be adjusted to gains
in the many k range. Also note what others have said about bandwidth,
etc.

Chris

 

[Gregory L. Hansen]

In article <1108660217.642640.145920@z14g2000cwz.googlegroups.com>,
<bill.sloman@ieee.org> wrote:

At that sort of gain level you have to start being really careful about
power supply rejection, particularly at higher frequencies - I once
built a circuit that could be cranked up to a gain of 10,000 at up to a
couple of MHz, and figured out that I needed to to put RLC filters on
each power pin, so that the current drawn by the output stage wouldn't
feed back ripple onto the power rails that could leak through into the
output of the input stage.

That sounds like good advice. I remember the procedure in one group was
an electrolytic capacitor on the power bus, and a small ceramic capacitor
on each power pin because the ceramics responded much faster.

What I'm interested in would be well below a kHz, which I assumed would
simplify just about everything, like inductive coupling.


--
"Never argue with a fool. They will drag you down to their level and win
by experience."

 

[Gregory L. Hansen]

In article <1108661971.606833.231950@c13g2000cwb.googlegroups.com>,
kmaryan@gmail.com <kmaryan@gmail.com> wrote:

See "instrumentation amplifier". You can roll your own from op-amps or
buy a ready made one. Many off the shelf ones can be adjusted to gains
in the many k range. Also note what others have said about bandwidth,
etc.

Chris

Oh, good. I knew there must be a way to add more op-amps to it. Thanks.
And I've learned that very often, off-the-shelf is not only faster and
easier, but better and cheaper than I could do on my own. Do you have a
favorite instrumentation amplifier on a chip?


--
"Yes, I revere you much, honored ones, and wish to fart in response." --
Aristophanes, Clouds

 

[dB]

glhansen@steel.ucs.indiana.edu (Gregory L. Hansen) wrote in message news:<cv2do0$l0$6@rainier.uits.indiana.edu>...

I know how to build the basic inverting and non-inverting op-amp
amplifiers, and they seem to work well enough for reasonable gains like
in the 10's. But what if I want to amplify a small signal by a factor
of 1000 or so? It seems like there must be a lot of issues swept aside in
the intro books that will rear up-- non-linearities, zeroing, I don't know
what else.

Any advice?

The most noticeable "issue", as you call it, will be the
correspondingly lower cut-off frequency.

 

[Gregory L. Hansen]

In article <1108674708.071820.27890@c13g2000cwb.googlegroups.com>,
kmaryan@gmail.com <kmaryan@gmail.com> wrote:

Instrumentation amps are fairly specialized with speed/noise/whatever
capabilities being very important tradeoffs. If you want a good example
of a high gain application of an instrumentation amplifier, google for
EKG, ECG, electrocardiogram. There are plenty of examples online, most

I was looking at the AD620, I think it was. It looked like a nice
all-in-one-package thing, but I was having a little trouble placing an
order for less than a thousand units.

of which are simple enough that they can be executed by those just
starting out with electronics. Typically, these see differential input
waveforms in the range of 0.5-5mV, and use some combination of
instrumentation amps and conventional amps to get that into the 1V
range for a ADC. Interestingly, these usually don't use a single high
gain stage, but rather a series of modest gain and filtering stages
that usually work out to a bandpass frequency response from 0.1Hz-50Hz
and a gain of 1000 or so. Some even use a unity gain first stage
instrumetnation amp just to get the differential signal changed to
single ended, then do the rest with successive normal op-amp stages.

The real questions you have to ask yourself to design this thing are
"what is the signal that I'm looking at?" The main considerations being
bandwidth, signal strength, is it differential or single ended, is
there a significant common mode voltage? There may even be some aspect
of what you are trying to do that requires special consideration, i.e.
clean ECG measurements usually require some kind of feedback to deal
with common mode problems. Then you ask yourself "what do I want to do
with this signal?", what are your noise requirements? is it going into
a ADC? if so, how many bits of precision? etc. Try asking some of the
answers to these questions, then perhaps the solution may become clear.

Signal: roughly 1 mV from a magnetic pickup, frequencies of interest in
the hundreds of Hz to near zero. All things considered, I think it must
be pretty simple as far as signals go, except for the high gain.

--
"Outside the camp you shall have a place set aside to be used as a
latrine. You shall keep a trowel in your equipment and with it, when you
go outside to ease nature, you shall first dig a hole and afterward cover
up your excrement." -- Deuteronomy 23:13-14

 

[John Woodgate]

I read in sci.electronics.design that Gregory L. Hansen <glhansen@steel.
ucs.indiana.edu> wrote (in <cv34s0$98k$2@rainier.uits.indiana.edu&gt
about 'Amplify the hell out of it.', on Thu, 17 Feb 2005:

But an instrumentation amplifier is three (or more) op-amps. Or does
that still count as just one amplification stage?

Designs differ. The two input op-amps count as one stage, which may have
significant gain or have a gain of 1. The third op-amp provides one gain
stage.
--
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

 

[Gregory L. Hansen]

In article <421521ce$0$28979$e4fe514c@news.xs4all.nl>,
Frank Bemelman <f.bemelmanq@xs4all.invalid.nl> wrote:

"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> schreef in bericht
news:cv357n$98k$3@rainier.uits.indiana.edu...

Signal: roughly 1 mV from a magnetic pickup, frequencies of interest in
the hundreds of Hz to near zero. All things considered, I think it must
be pretty simple as far as signals go, except for the high gain.

Google for a RIAA pre-amplifier and remove the RIAA network. Or a
microphone amplifier. The rest is cabling and shielding, much the
same it is with record players and microphones. In a hostile
environment, you want that amplifier practically on top of that
pickup.

I'll think in terms of a small aluminum box with bulkhead connector for
the pre-amp.

--
"What are the possibilities of small but movable machines? They may or
may not be useful, but they surely would be fun to make."
-- Richard P. Feynman, 1959

 

[Gregory L. Hansen]

In article <EYvnYvBk6RFCFwKT@jmwa.demon.co.uk>,
John Woodgate <noone@yuk.yuk> wrote:

I read in sci.electronics.design that Gregory L. Hansen <glhansen@steel.
ucs.indiana.edu> wrote (in <cv357n$98k$3@rainier.uits.indiana.edu&gt
about 'Amplify the hell out of it.', on Thu, 17 Feb 2005:

Signal: roughly 1 mV from a magnetic pickup, frequencies of interest in
the hundreds of Hz to near zero. All things considered, I think it must
be pretty simple as far as signals go, except for the high gain.

You are going to need to address 'flicker noise' in that frequency
range. Than means a selected low-noise bipolar op-amp. What is the
resistance of your pick-up? That will dictate the input circuit design
for minimum noise.

Flicker noise is a new one for me. The specs give a standard coil
resistance, in Ohms, of 150/240/380

http://psn.quake.net/geophone/l15bspec.gif

I don't know what that means. There are three coils, I suppose they could
have three different resistances, but it doesn't make sense to me that
they should all be different. There's two lateral and one vertical, so
I'd have thought two, at least, should be the same.

Can you recommend a low-noise bioplar op-amp? Would this be basic op-amp
or still going with the instrumentation amplifier?

I'm not sure wha the "CURVE SHUNT DAMPING" box means, either, unless it's
"shorting" the coils through resistors of various values and measuring the
voltage across the resistor.

I should be asking the seller these things.

--
"Coincidences, in general, are great stumbling blocks in the way of that
class of thinkers who have been educated to know nothing of the theory of
probabilities." -- Edgar Allen Poe

 

[Gregory L. Hansen]

In article <mL-dnfzNC-RLiIjfRVn-iA@comcast.com>,
Dieter Knollman <djhk@comcast.net> wrote:

Greg,

In addition to opamp gain, noise will be a mayor factor.
Especially noise on the ground lead.
Look at http://mywebpages.comcast.net/djhk/Noise.htm
for a discussion.

I'll have to get back to his later, but thanks for the reference.

Dieter

"Gregory L. Hansen" <glhansen@steel.ucs.indiana.edu> wrote in message
news:cv2do0$l0$6@rainier.uits.indiana.edu...

I know how to build the basic inverting and non-inverting op-amp
amplifiers, and they seem to work well enough for reasonable gains like
in the 10's. But what if I want to amplify a small signal by a factor
of 1000 or so? It seems like there must be a lot of issues swept aside in
the intro books that will rear up-- non-linearities, zeroing, I don't know
what else.

Any advice?

--
"Tell me, Dr. Einstein, at what time does Boston arrive at this train?"

--
"Yes, I revere you much, honored ones, and wish to fart in response." --
Aristophanes, Clouds

 

[Gregory L. Hansen]

In article <4215676A.73979F7A@rica.net>,
John Popelish <jpopelish@rica.net> wrote:

A simple inverting amplifier with an input resistor in this range
would work, but the DC offset would be hard to get rid of.

If, instead, you used a load resistor to ground, and then AC coupled
to a noninverting amplifier with a high value bias resistor, a much
smaller coupling capacitor could be used.

I think this isn't quite what you described, but the superspring jumps to
mind. A spring is a mechanical analog of a capacitor, a mass is a
mechanical analog of an inductor, and the position of a mass relative to
the mount is a mechanical analog of a voltage difference... The
supercapacitor!

--
"In any case, don't stress too much--cortisol inhibits muscular
hypertrophy. " -- Eric Dodd

 

[Gregory L. Hansen]

In article <4216219E.E375B008@rica.net>,
John Popelish <jpopelish@rica.net> wrote:

"Gregory L. Hansen" wrote:

A simple inverting amplifier with an input resistor in this range
would work, but the DC offset would be hard to get rid of.

Trim pot?

Offset voltage drifts, so a single trim may not lass long. Using a
very good low noise and low drift opamp would be better for the first
amp. Something like an LT1028 or LT1007 has only microvolts of
drift.

Wow. LT1007, noise in the nanovolt regime, offset less than 25 uV (I
assume that's 25 uV multiplied by my gain?), $2.42 each in small
quantities. Capitalism works.


--
"I'm giving you the chance to look fate in those pretty eyes of hers
and say, 'Step off, bitch. This is my party and you're not invited.'"
-- Chris Shugart, _Testosterone Magazine_

 

[John Popelish]

"Gregory L. Hansen" wrote:

In article <4216219E.E375B008@rica.net>,
John Popelish <jpopelish@rica.net> wrote:

Offset voltage drifts, so a single trim may not lass long. Using a
very good low noise and low drift opamp would be better for the first
amp. Something like an LT1028 or LT1007 has only microvolts of
drift.

Wow. LT1007, noise in the nanovolt regime, offset less than 25 uV (I
assume that's 25 uV multiplied by my gain?), $2.42 each in small
quantities. Capitalism works.

Yes and yes.

--
John Popelish

 

[John Woodgate]

I read in sci.electronics.design that Gregory L. Hansen <glhansen@steel.
ucs.indiana.edu> wrote (in <cv2npu$56i$1@rainier.uits.indiana.edu&gt
about 'Amplify the hell out of it.', on Thu, 17 Feb 2005:

I'm not sure I understand this. Do mean that, just to pull a number out
of my butt, if the op-amp will satisfatorily amplify a 10 KHz signal
with a gain of 10, it will satisfactorily amplify a 100 Hz signal with a
gain of 1000?

Not exactly, in one way, and absolutely not, in another.

You need to understand that op-amps are always used with negative
feedback (unless one is ill-advised enough to use one as a sluggish
comparator).

The *open-loop* gain, with **no feedback** of a 'cooking-type'
internally-compensated op-amp is (to simplify a bit) 3 million at 1 Hz,
and falls to 300 thousand at 10 Hz, 30 thousand at 100 Hz, and so on, so
reaches 1 at 3 MHz. It has to have its high-frequency gain killed like
that, otherwise it would be an oscillator.

But you don't use it like that. If you apply negative feedback to reduce
the **closed-loop** gain to 10, it will be approximately 10 up to 300
kHz. But if you apply feedback to set the closed-loop gain to 1000, that
will only be maintained up to 300 Hz.

Of course, these days there are much faster op-amps, whose unity-gin
frequency is much higher than 3 MHz, and there are op-amps for which you
can control the roll-off of the open-loop gain with external components.
But the same principle remains; very high gain from one op-amp raises
difficulties that may well be eliminated by using two in cascade.
--
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

 

[Gregory L. Hansen]

In article <3PBol7EF$MFCFw81@jmwa.demon.co.uk>,
John Woodgate <noone@yuk.yuk> wrote:

I read in sci.electronics.design that Gregory L. Hansen <glhansen@steel.
ucs.indiana.edu> wrote (in <cv2do0$l0$6@rainier.uits.indiana.edu&gt about
'Amplify the hell out of it.', on Thu, 17 Feb 2005:
I know how to build the basic inverting and non-inverting op-amp
amplifiers, and they seem to work well enough for reasonable gains like
in the 10's. But what if I want to amplify a small signal by a factor
of 1000 or so? It seems like there must be a lot of issues swept aside
in the intro books that will rear up-- non-linearities, zeroing, I don't
know what else.

It isn't normally a good idea to try to get 1000 times gain out of one
op-amp. A cascade of two with gains of 31.46, or any reasonably
comparable combination that multiplies to 1000, is a much better
solution and solves many of the problems you mention (and some others)
automatically.

Oddly enough, that almost pre-answers another question I had.

My first impulse was to get some small signal, around a millivolt, amplify
the hell out of it, and then apply filtering, like low-pass or whatever.
Then I thought about the pre-filtered signal hitting the voltage rails
and the post-filtered signal not being satisfactorily large, so I
wondered whether it would be better to pass the signal through a unity
gain pre-amp first, then filter it, then amplify the remainder. Or
whether the pre-amp should have significant gain, maybe 10 or 100 or so.

And it would seem that's actually a good idea.

Besides cascading op-amps, are the conventional two-resistor amplifier
designs still good?

--
"The main, if not the only, function of the word aether has been to
furnish a nominative case to the verb 'to undulate'."
-- the Earl of Salisbury, 1894

 

[Rich Grise]

On Thu, 17 Feb 2005 19:12:56 +0000, John Woodgate wrote:

I read in sci.electronics.design that Gregory L. Hansen <glhansen@steel.

I'm not sure I understand this. Do mean that, just to pull a number out
of my butt, if the op-amp will satisfatorily amplify a 10 KHz signal
with a gain of 10, it will satisfactorily amplify a 100 Hz signal with a
gain of 1000?

10 * 10,000 = 100,000
100 * 1,000 = 100,000

Not exactly, in one way, and absolutely not, in another.

The *open-loop* gain, with **no feedback** of a 'cooking-type'
internally-compensated op-amp is (to simplify a bit) 3 million at 1 Hz,
and falls to 300 thousand at 10 Hz, 30 thousand at 100 Hz, and so on, so
reaches 1 at 3 MHz. It has to have its high-frequency gain killed like
that, otherwise it would be an oscillator.

10 * 300,000 = 3,000,000
100 * 30,000 = 3,000,000

So, yours has an open-loop gain of 3,000,000, and Gregory L. Hansen's
has an open-loop gain of 100,000? Is there something I'm not seeing here?

Thanks,
Rich

 

[John Popelish]

"Gregory L. Hansen" wrote:

In article <4214C4CF.97C58EDE@rica.net>,
John Popelish <jpopelish@rica.net> wrote:
"Gregory L. Hansen" wrote:

I know how to build the basic inverting and non-inverting op-amp
amplifiers, and they seem to work well enough for reasonable gains like
in the 10's. But what if I want to amplify a small signal by a factor
of 1000 or so? It seems like there must be a lot of issues swept aside in
the intro books that will rear up-- non-linearities, zeroing, I don't know
what else.

Any advice?

Some of the biggies:

Unity gain stable opamps have an open loop gain that rolls of
proportional to frequency above a few hertz. So a closed loop gain of
1000 has 1/1000th as high a maximum frequency as a closed loop gain of
1 amplifier has.

I'm not sure I understand this. Do mean that, just to pull a number out
of my butt, if the op-amp will satisfatorily amplify a 10 KHz signal with
a gain of 10, it will satisfactorily amplify a 100 Hz signal with a gain
of 1000?

Yes. If it had a gain bandwidth product of 1 MHz, it would have had
an open loop gain of 100 at 10 kHz, so its gain would be accurate to
about 10% amplifying 10 kHz. At 100 Hz, it has an open loop gain of
about 10,000, so it will provide about 10% accurate closed loop gain
of 1000 at that lower frequency. Lots of applications need the open
loop gain to be 100 times the closed loop gain, so that the feedback
controls the gain to within 1%. So high gain implies a need for a
relative faster opamp.

--
John Popelish

 

[Gregory L. Hansen]

In article <42155BB2.A6E94414@rica.net>,
John Popelish <jpopelish@rica.net> wrote:

"Gregory L. Hansen" wrote:
John Popelish wrote:
Yes. If it had a gain bandwidth product of 1 MHz, it would have had
an open loop gain of 100 at 10 kHz, so its gain would be accurate to
about 10% amplifying 10 kHz. At 100 Hz, it has an open loop gain of
about 10,000, so it will provide about 10% accurate closed loop gain
of 1000 at that lower frequency. Lots of applications need the open
loop gain to be 100 times the closed loop gain, so that the feedback
controls the gain to within 1%. So high gain implies a need for a
relative faster opamp.

That looks different than what I was picturing. At 100 Hz the closed loop
gain is still what I expected, but the signal coming out isn't what I
expected by 10%?

Have you built something or are you talking about a simulation or
mental exercise?

The text immediately above is mental exercise. I wasn't sure what
realistic numbers would look like, so I waved my hands and made some magic
numbers appear.

But I started the thread with plans for the future building of something
for the Mark Products L15B at the top of this page,

http://psn.quake.net/geophone/

for which I have not yet placed an order but likely will in the next few
days.


--
"In any case, don't stress too much--cortisol inhibits muscular
hypertrophy. " -- Eric Dodd

 

[Gregory L. Hansen]

In article <4216219E.E375B008@rica.net>,
John Popelish <jpopelish@rica.net> wrote:

"Gregory L. Hansen" wrote:

In article <4215676A.73979F7A@rica.net>,
John Popelish <jpopelish@rica.net> wrote:

I think the specifications indicate that these come with one of 3 coil
resistances. I suspect that all 3 units in the housing would
generally be all the same. The damping curves show the frequency
response with various coil loading (amplifier input impedance).

Right. The seller got back to me, and he has the 380 ohm version. All
coils at 380 ohms. And he'll make sure they have the appropriate shunt
resistors.

I also emailed the seller and verified that this interpretation is
correct. After you have yours on the way, I may order one to play
with.

Somehow it seems bigger than it looked in the pictures. The can is five
inches in diameter, almost three inches high, the sensors mounted on the
lid which has two arrows oriented with the two horizontal sensors and a
spirit level in the middle. The large screws (5/16"?) on the bottom allow
mounting to something. And the two fittings are rubberized compression
seals that would allow wiring through conduit.

I haven't had the chance to plug it in to anything yet, but from the
physical inspection I like it.

--
"The result of this experiment was inconclusive, so we had to use
statistics." (Overheard at international physics conference)