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Cranky Op-Amp

Sorry for cross post.

I'm having endless trouble with my cranky op-amp. It's a basic diff.
Op-amp. I'm working with DC pulses, but it's in the audio frequency
range. I have two problems. 1. The output is capacitive because it's a
20-foot audio cable. 2. The input is very inductive and capacitive
because it's a 3.5" wound coil consisting of 1000 feet of 24 gauge
copper wire. The op-amp may work great, but the next moment without
touching anything may begin to oscillate or saturate. It has a great
amount of histeresis, which makes it nearly impossible to balance.

I know that I should at least have an output driver since op-amps don't
like reactive loads, especially 20 foot x 2 = 40 feet of cable.

My input coil is probably the biggest problem. Even when I eliminate
the long output cable the amp is still cranky. It's like there's
positive feedback. I'm thinking about completely doing away with the
op-amp. Any thoughts on replacing it with a basic audio preamp? Aren't
most preamps made with op-amps?

I've tried every trick I could find on the Internet. Place various size
resisters directly on the output. Placed various size caps from output
to -In. I'm wondering what effects 1000 feet of wire in the form of a
coil has on the amp.

Details:
I have a two-stage LM318 op-amp. Both op-amps have the same parts.
Coil is about 23 ohms, 1000 ft 24 gauge. One end of coil goes to 470
ohm R and other end of coil goes to another 470 ohm R, which each goes
to +in and -in of amp. +In also goes to 39K R, which goes to ground.
-In goes to a 39K R, which goes to output.
The 2nd op-amp is feed by the 1st op-amp. A 1K R directly across the
2nd amps output seems to help a little, but there's still a 20-foot
audio cable connected to the op-amp.
The DC pulse lasts about 1 ms.

Thanks for any feedback
Paul
 
J

Jim Thompson

Jan 1, 1970
0
Sorry for cross post.

I'm having endless trouble with my cranky op-amp. It's a basic diff.
Op-amp. I'm working with DC pulses, but it's in the audio frequency
range. I have two problems. 1. The output is capacitive because it's a
20-foot audio cable. 2. The input is very inductive and capacitive
because it's a 3.5" wound coil consisting of 1000 feet of 24 gauge
copper wire. The op-amp may work great, but the next moment without
touching anything may begin to oscillate or saturate. It has a great
amount of histeresis, which makes it nearly impossible to balance.

I know that I should at least have an output driver since op-amps don't
like reactive loads, especially 20 foot x 2 = 40 feet of cable.

My input coil is probably the biggest problem. Even when I eliminate
the long output cable the amp is still cranky. It's like there's
positive feedback. I'm thinking about completely doing away with the
op-amp. Any thoughts on replacing it with a basic audio preamp? Aren't
most preamps made with op-amps?

I've tried every trick I could find on the Internet. Place various size
resisters directly on the output. Placed various size caps from output
to -In. I'm wondering what effects 1000 feet of wire in the form of a
coil has on the amp.

Details:
I have a two-stage LM318 op-amp. Both op-amps have the same parts.
Coil is about 23 ohms, 1000 ft 24 gauge. One end of coil goes to 470
ohm R and other end of coil goes to another 470 ohm R, which each goes
to +in and -in of amp. +In also goes to 39K R, which goes to ground.
-In goes to a 39K R, which goes to output.
The 2nd op-amp is feed by the 1st op-amp. A 1K R directly across the
2nd amps output seems to help a little, but there's still a 20-foot
audio cable connected to the op-amp.
The DC pulse lasts about 1 ms.

Thanks for any feedback
Paul

Post a schematic to alt.binaries.schematics.electronic or post a link
here.

...Jim Thompson
 
P

Pooh Bear

Jan 1, 1970
0
Sorry for cross post.

I'm having endless trouble with my cranky op-amp. It's a basic diff.
Op-amp. I'm working with DC pulses, but it's in the audio frequency
range. I have two problems. 1. The output is capacitive because it's a
20-foot audio cable.

The capacitance of 20 ft of audio cable is *minute*. That isn't a problem.
2. The input is very inductive and capacitive
because it's a 3.5" wound coil consisting of 1000 feet of 24 gauge
copper wire.

So ?
The op-amp may work great, but the next moment without
touching anything may begin to oscillate or saturate. It has a great
amount of histeresis,

Op-amps don't have hysteresis.
which makes it nearly impossible to balance.

I know that I should at least have an output driver since op-amps don't
like reactive loads, especially 20 foot x 2 = 40 feet of cable.

Rubbish.

Sounds like you have instability. Do you have any power supply 'decoupling'
?

Graham
 
Sorry for cross post.

I'm having endless trouble with my cranky op-amp. It's a basic diff.
Op-amp. I'm working with DC pulses, but it's in the audio frequency
range. I have two problems. 1. The output is capacitive because it's a
20-foot audio cable. 2. The input is very inductive and capacitive
because it's a 3.5" wound coil consisting of 1000 feet of 24 gauge
copper wire. The op-amp may work great, but the next moment without
touching anything may begin to oscillate or saturate. It has a great
amount of histeresis, which makes it nearly impossible to balance.

I know that I should at least have an output driver since op-amps don't
like reactive loads, especially 20 foot x 2 = 40 feet of cable.

My input coil is probably the biggest problem. Even when I eliminate
the long output cable the amp is still cranky. It's like there's
positive feedback. I'm thinking about completely doing away with the
op-amp. Any thoughts on replacing it with a basic audio preamp? Aren't
most preamps made with op-amps?

I've tried every trick I could find on the Internet. Place various size
resisters directly on the output. Placed various size caps from output
to -In. I'm wondering what effects 1000 feet of wire in the form of a
coil has on the amp.

Details:
I have a two-stage LM318 op-amp. Both op-amps have the same parts.
Coil is about 23 ohms, 1000 ft 24 gauge. One end of coil goes to 470
ohm R and other end of coil goes to another 470 ohm R, which each goes
to +in and -in of amp. +In also goes to 39K R, which goes to ground.
-In goes to a 39K R, which goes to output.
The 2nd op-amp is feed by the 1st op-amp. A 1K R directly across the
2nd amps output seems to help a little, but there's still a 20-foot
audio cable connected to the op-amp.
The DC pulse lasts about 1 ms.

The first thing to fix is the capacitative load. 20 feet of cable is
6.1 metres at around 100pF/metre, or 600pF (check out exaxtly what
cable you are using or measure it with a capacitance meter of some sort
- that 100pF is very much a rule of thumb).

Regular op amps oscillate when loaded with more than 100pF. There is a
standard circuit to fix this - see page 9 of the LM318 data sheet

http://cache.national.com/ds/LM/LM118.pdf

The oscillation is usually at a pretty low level at the output, but the
swings inside the amplifier are apparently larger, and mess up the
amplifier performance.

The LM318 is actually a pretty horrible amplifier - useful back in the
early 1970's where you wanted cheap gain-bandwidth, but a swine to keep
stable.

The LF356 hasn't got quite as much bandwidth, but it is much better
behaved, and it can drive a 5nF load. If you are working with a gain of
five or more, you could use the decompensated version - the LF357 -
which does seem to be about as fast as the LM318.

http://cache.national.com/ds/LF/LF155.pdf

There are undoubtedly more modern parts that do even better, but the
LF356 and LF357 have been around since the mid-1970's and got designed
into lots of stuff, so you can often get them out of university and
trade school lab stocks.

As for the rest of your circuit, see if you can put it up on a web-site
we can look at it, or post the circuit diagram to
alt.binaries.schematics.electronic as Jim Thompson suggested.
 
The first thing to fix is the capacitative load. 20 feet of cable is
6.1 metres at around 100pF/metre, or 600pF (check out exaxtly what
cable you are using or measure it with a capacitance meter of some sort
- that 100pF is very much a rule of thumb).

Regular op amps oscillate when loaded with more than 100pF. There is a
standard circuit to fix this - see page 9 of the LM318 data sheet

http://cache.national.com/ds/LM/LM118.pdf

The oscillation is usually at a pretty low level at the output, but the
swings inside the amplifier are apparently larger, and mess up the
amplifier performance.


I see. What's different is the 100 ohm R directly connected and between
the capacitive output load. I'll try it.



The LM318 is actually a pretty horrible amplifier - useful back in the
early 1970's where you wanted cheap gain-bandwidth, but a swine to keep
stable.

The LF356 hasn't got quite as much bandwidth, but it is much better
behaved, and it can drive a 5nF load. If you are working with a gain of
five or more, you could use the decompensated version - the LF357 -
which does seem to be about as fast as the LM318.

http://cache.national.com/ds/LF/LF155.pdf

There are undoubtedly more modern parts that do even better, but the
LF356 and LF357 have been around since the mid-1970's and got designed
into lots of stuff, so you can often get them out of university and
trade school lab stocks.


Hmm, I've been looking for a replacement. I don't need high frequency--
< 20KHz. Although most op-amps that NTE sells are > 4MHz bandwidth. I'm
wondering if there's any particular parameters to look for in a
datasheet to identify if the chip is prone to unwanted oscillations and
such?


As for the rest of your circuit, see if you can put it up on a web-site
we can look at it, or post the circuit diagram to
alt.binaries.schematics.electronic as Jim Thompson suggested.

I use google groups to post. It seems google doesn't support that one
as it goes from alt.biology.* followed by alt.birdlung.* I'll search
for a free news group, but haven't seen any that allow
alt.binaries.schematics.electronic. Perhaps I can post the image at my
yahoo geocities site.

Paul
 
T

Terry Given

Jan 1, 1970
0
I see. What's different is the 100 ohm R directly connected and between
the capacitive output load. I'll try it.








Hmm, I've been looking for a replacement. I don't need high frequency--
< 20KHz. Although most op-amps that NTE sells are > 4MHz bandwidth. I'm
wondering if there's any particular parameters to look for in a
datasheet to identify if the chip is prone to unwanted oscillations and
such?

the general phrase you want is "unity gain stable" or "compensated"
 
P

Phil Hobbs

Jan 1, 1970
0
The LM318 is actually a pretty horrible amplifier - useful back in the
early 1970's where you wanted cheap gain-bandwidth, but a swine to keep
stable.

The LF356 hasn't got quite as much bandwidth, but it is much better
behaved, and it can drive a 5nF load. If you are working with a gain of
five or more, you could use the decompensated version - the LF357 -
which does seem to be about as fast as the LM318.

http://cache.national.com/ds/LF/LF155.pdf

There are undoubtedly more modern parts that do even better, but the
LF356 and LF357 have been around since the mid-1970's and got designed
into lots of stuff, so you can often get them out of university and
trade school lab stocks.

Those cads and bounders at National discontinued the LF157 in '04--it
was the last decompensated family member. It's gone to join the TL014
and MRF9331 in Chip Purgatory.

That nice 2 pF input capacitance was a pretty nice feature for high-Z
stuff. Ah well--it's the price of getting old: all your friends start
dying off.

Cheers,

Phil Hobbs
 
Pooh said:
The capacitance of 20 ft of audio cable is *minute*. That isn't a problem.

My rough estimate for the capacitance of 20 feet (6 metres) of audio
cable is 600pF.

Since regular op amps start oscillatong at about 100pF, this isn't
minute. I first met it as a problem around 1974 (and my slightly
unconventioanl solution used to be embedded in the control panels of a
couple of British power stations) and I've run into it in a surprsingly
large number of legacy circuits since then. The real idiots "cured" the
problem by loading the output with a 100nF ceramic disk to ground,
which reduces the amplitude to a few millivolts - the electronic
equivalent of sweeping the dust under the carpet.
 
R

Rene Tschaggelar

Jan 1, 1970
0
Sorry for cross post.

I'm having endless trouble with my cranky op-amp. It's a basic diff.
Op-amp. I'm working with DC pulses, but it's in the audio frequency
range. I have two problems. 1. The output is capacitive because it's a
20-foot audio cable. 2. The input is very inductive and capacitive
because it's a 3.5" wound coil consisting of 1000 feet of 24 gauge
copper wire. The op-amp may work great, but the next moment without
touching anything may begin to oscillate or saturate. It has a great
amount of histeresis, which makes it nearly impossible to balance.
[snip]


1st, you never drive a cable with an ordinary OpAmp.
There are special cable drivers for this.
Have a look at the LT1794 or similar.

2nd, a symmetical cable is possibly better suited
for the output.

Rene
*** Free account sponsored by SecureIX.com ***
*** Encrypt your Internet usage with a free VPN account from http://www.SecureIX.com ***
 
F

Fred Bloggs

Jan 1, 1970
0
My rough estimate for the capacitance of 20 feet (6 metres) of audio
cable is 600pF.

Since regular op amps start oscillatong at about 100pF, this isn't
minute. I first met it as a problem around 1974 (and my slightly
unconventioanl solution used to be embedded in the control panels of a
couple of British power stations) and I've run into it in a surprsingly
large number of legacy circuits since then. The real idiots "cured" the
problem by loading the output with a 100nF ceramic disk to ground,
which reduces the amplitude to a few millivolts - the electronic
equivalent of sweeping the dust under the carpet.

It does depend upon the gain, and the OP is running 58dB if his circuit
description is to believed. The problem is more due to positive feedback
pickup in the input coil/antenna. A cable induced oscillation would
rarely be intermittent. The first fix would be to make the overall gain
inverting, and then R isolate the output to cable a bit.
 
A

Ancient_Hacker

Jan 1, 1970
0
I have a two-stage LM318 op-amp. Both op-amps have the same parts.
Coil is about 23 ohms, 1000 ft 24 gauge. One end of coil goes to 470
ohm R and other end of coil goes to another 470 ohm R, which each goes
to +in and -in of amp. +In also goes to 39K R, which goes to ground.
-In goes to a 39K R, which goes to output.


Wait a minute-- the first op-amp seems to be set up for a gain of
39k/470, let's call that a voltage gain of 1000.

Same thing with the second amplifier, which is cascaded off the first
one.

So all you need is for one part per million of the output of #2 to get
back to the coil and you're sunk!

That's an isolation of 120dB which is really quite a lot. You're
unlikely to be able to get that much isolation at anything above low
audio frequencies without some really serious ground-planes, advanced
decoupling, and substantial shielding, and double-shielded coax.

Also you're going to have a heck of a problem with DC offset and drift.
Do you really need response down to DC?

And how much gain do you REALLY need, and how much bandwidth?

You might be able to tame this beast if you roll-off the frequency
response with a small capacitor or two across each 39K resistor. Also
have separate power supply decoupling, with like 100 ohm series
resistors, 10uf capacitors, and 0.01uF capacitors for Hf. And a
ground-plane would help too.
 
G

GregS

Jan 1, 1970
0
My rough estimate for the capacitance of 20 feet (6 metres) of audio
cable is 600pF.


I've measured some cheap cables with 100 pf per. ft.
 
F

Fred Bloggs

Jan 1, 1970
0
Ancient_Hacker said:
Wait a minute-- the first op-amp seems to be set up for a gain of
39k/470, let's call that a voltage gain of 1000.

Same thing with the second amplifier, which is cascaded off the first
one.

So all you need is for one part per million of the output of #2 to get
back to the coil and you're sunk!

That's an isolation of 120dB which is really quite a lot. You're
unlikely to be able to get that much isolation at anything above low
audio frequencies without some really serious ground-planes, advanced
decoupling, and substantial shielding, and double-shielded coax.

Also you're going to have a heck of a problem with DC offset and drift.
Do you really need response down to DC?

And how much gain do you REALLY need, and how much bandwidth?

You might be able to tame this beast if you roll-off the frequency
response with a small capacitor or two across each 39K resistor. Also
have separate power supply decoupling, with like 100 ohm series
resistors, 10uf capacitors, and 0.01uF capacitors for Hf. And a
ground-plane would help too.

Just throw it in the trash...
 
My rough estimate for the capacitance of 20 feet (6 metres) of audio
cable is 600pF.

Since regular op amps start oscillatong at about 100pF, this isn't
minute. I first met it as a problem around 1974 (and my slightly
unconventioanl solution used to be embedded in the control panels of a
couple of British power stations) and I've run into it in a surprsingly
large number of legacy circuits since then. The real idiots "cured" the
problem by loading the output with a 100nF ceramic disk to ground,
which reduces the amplitude to a few millivolts - the electronic
equivalent of sweeping the dust under the carpet.

Very interesting. I'll try it. I know that an R across output helps a
little.

Paul
 
Phil said:
Those cads and bounders at National discontinued the LF157 in '04--it
was the last decompensated family member. It's gone to join the TL014
and MRF9331 in Chip Purgatory.

That nice 2 pF input capacitance was a pretty nice feature for high-Z
stuff. Ah well--it's the price of getting old: all your friends start
dying off.

Phil, I found a op-amp with 1.4 pf. NE5532

http://www.nteinc.com/specs/700to799/pdf/nte778a.pdf

http://www.semiconductors.philips.com/acrobat_download/datasheets/NE_SE5532_A_SA5532_3.pdf

What do you think of that chip? Probably more bandwidth than I need,
but it has high input current, low noise, and very low input impedance.

Paul
 
Ancient_Hacker said:
Wait a minute-- the first op-amp seems to be set up for a gain of
39k/470, let's call that a voltage gain of 1000.

I think it's 83, but you're right in that I would eventually like 1000
for each stage. BTW, the device I'm designing is a PI metal detector.
Normally I won't need a 20 ft cable, but for now I have to hook it up
to a computer. The computer has too much metal and generates noise. So
20 feet separation is about right.


Same thing with the second amplifier, which is cascaded off the first
one.

BTW, was I correct in building a two stage op-amp circuit? Lets say
that I need a gain of 200,000? Is it better to have a one stage 200,000
gain op-amp or two stage op-amp circuit each having 447 gain? Or
possibly a low 1st stage gain of say 20 and a high gain 2nd stage of
10,000?


So all you need is for one part per million of the output of #2 to get
back to the coil and you're sunk!

That's an isolation of 120dB which is really quite a lot. You're
unlikely to be able to get that much isolation at anything above low
audio frequencies without some really serious ground-planes, advanced
decoupling, and substantial shielding, and double-shielded coax.

Yes, lol. Would an Opto-Isolators work well enough or would I need to
go all out and use a 3 foot optic cables? I'm thinking that 3 feet for
final build is max as most metal detectors are about 3 feet long.


Also you're going to have a heck of a problem with DC offset and drift.
Do you really need response down to DC?

Oh you have no idea what a cranky op-amp I have. It's due to my lack of
knowledge, but I still keep getting visions of that 10-pound sludge
hammer nicely tucked away in the garage. You nailed it. Half of the
time I can't DC balance the output. I'll trim the pot say 1/2 turn and
then suddenly the amp avalanches and changes output polarity. Then I
slowly rotate the pot the other direction and nothing happens until
about 1/2 turn and the polarity instantly changes again. That
histeresis must be coming from positive feedback, right? Is that caused
by the output line capacitance or somehow the 2nd stage output is
feeding back to the 1st stage input? I gather that's one advantage of a
two-stage op-amp; i.e., less feedback from 2nd stage output to 1st
stage input.


And how much gain do you REALLY need, and how much bandwidth?

Lost of potential gain-- more the merrier. I'd like extremely high gain
up to 20 KHz, but can live with only a few KHz.


You might be able to tame this beast if you roll-off the frequency
response with a small capacitor or two across each 39K resistor. Also
have separate power supply decoupling, with like 100 ohm series
resistors, 10uf capacitors, and 0.01uF capacitors for Hf. And a
ground-plane would help too.

A ground place near the metal detector coil is a no-no. A few IC's
doesn't seem to bother it that much though. I think that somehow I need
to move most of the circuit 3 feet away from the ground coil, perhaps
by an optic cable. Perhaps a preamp or 1st stage op-amp could feed a
3-foot optics cable, which would go to a 2nd stage op-amp.

Paul
 
G

GregS

Jan 1, 1970
0
I think it's 83, but you're right in that I would eventually like 1000
for each stage. BTW, the device I'm designing is a PI metal detector.
Normally I won't need a 20 ft cable, but for now I have to hook it up
to a computer. The computer has too much metal and generates noise. So
20 feet separation is about right.




BTW, was I correct in building a two stage op-amp circuit? Lets say
that I need a gain of 200,000? Is it better to have a one stage 200,000
gain op-amp or two stage op-amp circuit each having 447 gain? Or
possibly a low 1st stage gain of say 20 and a high gain 2nd stage of
10,000?

These are both inverting, right?
If the output frequency in very low, try driving the output with
resistors on both sides. I guess you want DC coupling. Ground
loops problems occur with separated devices. I suppose
the coil can also pickup RF interference, but I'm not certain
of the design.

..
 
P

Phil Hobbs

Jan 1, 1970
0
Phil, I found a op-amp with 1.4 pf. NE5532

http://www.nteinc.com/specs/700to799/pdf/nte778a.pdf

http://www.semiconductors.philips.com/acrobat_download/datasheets/NE_SE5532_A_SA5532_3.pdf

What do you think of that chip? Probably more bandwidth than I need,
but it has high input current, low noise, and very low input impedance.

I tend to build lots of things with high impedance and not much gain,
such as transimpedance amplifiers.

What I'm usually worried about with input capacitance is the nasty noise
peak caused by the Cin*Rf rolloff, so the figure of merit I usually look
at is v_n*C_in--as long as v_n is low enough to start with. The 5532 is
okay--I've never used one, because there are lots better choices in
bipolars. I used to use the OP-37 and HA5147 a lot, for instance.
Joerg could do it with a couple of MMBT3904s and some nose grease, but
we do what we can. ;-)

The 1-uA maximum input bias current pretty well rules the 5532 out for
high-Z things, as you point out.

Cheers,

Phil Hobbs
 
Phil said:
I tend to build lots of things with high impedance and not much gain,
such as transimpedance amplifiers.

What I'm usually worried about with input capacitance is the nasty noise
peak caused by the Cin*Rf rolloff, so the figure of merit I usually look
at is v_n*C_in--as long as v_n is low enough to start with. The 5532 is
okay--I've never used one, because there are lots better choices in
bipolars. I used to use the OP-37 and HA5147 a lot, for instance.
Joerg could do it with a couple of MMBT3904s and some nose grease, but
we do what we can. ;-)

OP37A looks like what I need. Must not be my day as NTE has no
cross-reference chip. ;-( ... The search continues.

Paul
 
Re Gm amps (TIA) - I design these as well and use the 2SK170 JFet. This
part has a voltage noise of 1nV in the audio band.7
There is a circuit which I first heard of at Bell Labs called the
gm multiplier. In this method, a bipolar NPN is stacked on top of the
JFet. 90% of the JFet drain current is shunted away to the positive
supply leaving 10% in the bipolar collector current. This permits the
NPN collector load resistor to be 10x larger and the resulting voltage
gain to be 10x larger as well, up to 100V/V or higher.
What I do is to use a small feedback capacitor from the NPN
collector to the input, so the gain is set by the ratio of the source
capacitance to the feedback capacitor.
The amplifier input capacitance may be reduced if the NPN base is
bootstrapped from the JFet source, causing the NPN emitter to
back-drive the input through the JFet Cdg.

Charles Gilbert
 
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