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Cranky Op-Amp

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

John Popelish

Jan 1, 1970
0
Details:
I have a two-stage LM318 op-amp.

You are making life difficult, right there. The LM318 is a high speed
opamp. Its gain bandwidth product is about 15 MHz and its slew rate
is about 70 volts per microsecond. It can be pushed faster than this.
Is there some reason you have chosen this opamp?
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.

Okay, so a subtracter with a voltage gain of no more than
39000/470=83, not counting the effect of the coil impedance.
The 2nd op-amp is feed by the 1st op-amp.

What is the gain or configuration of the second opamp?
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.

Have you read the data sheet for its suggestions on stabilizing it
under capacitive load?
http://www.mit.edu/~6.301/LM118.pdf
 
B

Bob Eldred

Jan 1, 1970
0
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

A 318 is a "cranky" op-amp. It's an ancient design and is hard to stabilize
under the best of conditions. Get rid of it and go to a well behaved amp
like a TL-072 or one of its ilk. A gain of 83, (39K/470) is OK and should
cause no trouble. If both amps have the same parts, what is the gain and
configuration of the second amp? Is it also a gain of 83 and is it single
ended? If so this give a total gain of 6880, quite high so noise could be
an issue. Connect the audio cable to the amplifiers output through a 120
ohm resistor in series. The resistor will isolate the amps output and reduce
capacitive loading effects. That resistor can be figured into the gain
calculations if necessary. Again, get rid of the 318 and you should have
success.
Bob
 
Bob said:
A 318 is a "cranky" op-amp. It's an ancient design and is hard to stabilize
under the best of conditions. Get rid of it and go to a well behaved amp
like a TL-072 or one of its ilk. A gain of 83, (39K/470) is OK and should
cause no trouble. If both amps have the same parts, what is the gain and
configuration of the second amp? Is it also a gain of 83 and is it single
ended? If so this give a total gain of 6880, quite high so noise could be
an issue. Connect the audio cable to the amplifiers output through a 120
ohm resistor in series. The resistor will isolate the amps output and reduce
capacitive loading effects. That resistor can be figured into the gain
calculations if necessary. Again, get rid of the 318 and you should have
success.

Yes, the 2nd stage is same as 1st. So that's a total gain of 6890. I
went with the 2 stage because if similar problems. I see a few comments
if I need the high bandwidth op amp. I'm just working with audio
frequencies, < 20KHz. As long as I can get the high gain up to 20KHz
then it's fine. Any good NTE op-amps:

http://www.nteinc.com/linear_web/split.html

It seems most of the NTE op-amps are > 4MHz bandwidth.

Also there are various preamps:
http://www.nteinc.com/linear_web/preamp.html
http://www.nteinc.com/linear_web/dual_preamp.html

Thanks,
Paul
 
T

Terry Given

Jan 1, 1970
0
[email protected] wrote
[email protected] wrote

[email protected] wrote

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 frequenc
range. I have two problems. 1. The output is capacitive because it'
20-foot audio cable. 2. The input is very inductive and capacitiv
because it's a 3.5" wound coil consisting of 1000 feet of 24 gaug
copper wire. The op-amp may work great, but the next moment withou
touching anything may begin to oscillate or saturate. It has a grea
amount of histeresis, which makes it nearly impossible to balance

I know that I should at least have an output driver since op-amp don'
like reactive loads, especially 20 foot x 2 = 40 feet of cable

My input coil is probably the biggest problem. Even when I eliminat
the long output cable the amp is still cranky. It's like there'
positive feedback. I'm thinking about completely doing away with th
op-amp. Any thoughts on replacing it with a basic audio preamp Aren'
most preamps made with op-amps

I've tried every trick I could find on the Internet. Place variou siz
resisters directly on the output. Placed various size caps fro outpu
to -In. I'm wondering what effects 1000 feet of wire in the form o
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 47
ohm R and other end of coil goes to another 470 ohm R, which eac goe
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 th
2nd amps output seems to help a little, but there's still a 20-foo
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 i
6.1 metres at around 100pF/metre, or 600pF (check out exaxtly wha
cable you are using or measure it with a capacitance meter of som sor
- that 100pF is very much a rule of thumb)

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

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

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



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





The LM318 is actually a pretty horrible amplifier - useful back i th
early 1970's where you wanted cheap gain-bandwidth, but a swine t kee
stable

The LF356 hasn't got quite as much bandwidth, but it is much bette
behaved, and it can drive a 5nF load. If you are working with a gai o
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.pd

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



Hmm, I've been looking for a replacement. I don't need hig frequency-
20KHz. Although most op-amps that NTE sells are > 4MHz bandwidth I'
wondering if there's any particular parameters to look for in
datasheet to identify if the chip is prone to unwanted oscillation an
such
the general phrase you want is "unity gain stable" or "compensated
 
hI-

(1) About driving the long cable with an op amp - there are some
standard fixes for this (see Analog Devices, Lineart Tech App notes)
which involve using a small resistor at the output to keep the op amp
stable and then moving the feedback node accordingly to maintain the
proper gain.
(2) Another solution is to place a feedback op amp at the end of the
cable and send back a sensed signal which is used as the principle
feedback for the sending amp. Local AC feedback is then used to keep
the sender stable.

(3) Another approach which I have used many times to save a lot of
hair is to learn how to use the gm amp. If the output driving the
problem load is a gm-amp rather than a voltage amp, Many problems
simply evaporate like forgotten tea on a hot stove when a gm-amp is
used as a driver.

Charles Gilbert
Consultant
[email protected].
 
(1) About driving the long cable with an op amp - there are some
standard fixes for this (see Analog Devices, Lineart Tech App notes)
which involve using a small resistor at the output to keep the op amp
stable and then moving the feedback node accordingly to maintain the
proper gain.

I did find that placing a resister directly at the output helps a
little. Not sure what you mean by moving the feedback node.


(2) Another solution is to place a feedback op amp at the end of the
cable and send back a sensed signal which is used as the principle
feedback for the sending amp. Local AC feedback is then used to keep
the sender stable.

Since it's a two stage op-amp I could move the 2nd stage at the other
end of the 20 foot cable.


(3) Another approach which I have used many times to save a lot of
hair is to learn how to use the gm amp. If the output driving the
problem load is a gm-amp rather than a voltage amp, Many problems
simply evaporate like forgotten tea on a hot stove when a gm-amp is
used as a driver.

I've never used a gm amp, but NTE has a few:

http://www.nteinc.com/specs/900to999/pdf/nte902.pdf

http://www.nteinc.com/specs/800to899/pdf/nte870.pdf

http://www.nteinc.com/specs/900to999/pdf/nte996.pdf


Thanks,
Paul
 
I will take at look at the parts you posted and get back.
Using the collector output (basically an NPN and PNP
current-source) I built a CRT cathode driver which slewed at
12,000V/usec and achieved 30MHz small sig BW using 120V supplies. I
wasn't sucessful with a conventional emitter-follower output - too
unstable driving the cathode capacitance.
The gm output loves a reactive load because the output impedance
is so high, the poles all reside in the load itself, so long as the amp
has plenty of bandwidth.
You can have a lot of fun designing super-simple linear power
amplifiers from DC to daylight, from 5V to 500V, using the gm output
with either bipolar, FET, or even vacuum tubes if that is your cup of
tea.
I have a 500W audio amp which has less than 50 parts and runs a BW
of 5MHz. The phase margin is 110 degrees.

Charles
 
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