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

Reply to
pmlonline
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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?

Okay, so a subtracter with a voltage gain of no more than

39000/470=83, not counting the effect of the coil impedance.

What is the gain or configuration of the second opamp?

Have you read the data sheet for its suggestions on stabilizing it under capacitive load?

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Reply to
John Popelish

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

Reply to
Bob Eldred

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:

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

Also there are various preamps:

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Thanks, Paul

Reply to
pmlonline

snipped-for-privacy@gmail.com wrote:

trouble with my cranky op-amp. It's a basic diff.

pulses, but it's in the audio frequency

output is capacitive because it's a

inductive and capacitive

feet of 24 gauge

without

amount of histeresis, which makes it nearly impossible to balance.

that I should at least have an output driver since op-amps don't

loads, especially 20 foot x 2 = 40 feet of cable.

the biggest problem. Even when I eliminate

still cranky. It's like there's

completely doing away with the

basic audio preamp? Aren't

every trick I could find on the Internet. Place various size

directly on the output. Placed various size caps from output

wondering what effects 1000 feet of wire in the form of a

parts.

ohm R and other end of coil goes to another 470 ohm R, which each goes

and -in of amp. +In also goes to 39K R, which goes to ground.

39K R, which goes to output.

directly across the

a 20-foot

ms.

6.1 metres at around 100pF/metre, or 600pF (check out exaxtly what

are using or measure it with a capacitance meter of some sort

very much a rule of thumb).

than 100pF. There is a

data sheet

is usually at a pretty low level at the output, but the

amplifier are apparently larger, and mess up the

between

the capacitive output load. I'll try it.

a pretty horrible amplifier - useful back in the

cheap gain-bandwidth, but a swine to keep

quite as much bandwidth, but it is much better

load. If you are working with a gain of

decompensated version - the LF357 -

LM318.

more modern parts that do even better, but the

around since the mid-1970's and got designed

often get them out of university and

I've been looking for a replacement. I don't need high frequency--

Although most op-amps that NTE sells are > 4MHz bandwidth. I'm

there's any particular parameters to look for in a

the chip is prone to unwanted oscillations and

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

your circuit, see if you can put it up on a web-site

post the circuit diagram to

suggested.

that one

search

alt.binaries.schematics.electronic. Perhaps I can post the image at my

geocities site.

Reply to
Terry Given

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 snipped-for-privacy@gmail.com.

Reply to
NonDigital

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

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

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

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Thanks, Paul

Reply to
pmlonline

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

Reply to
NonDigital

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