Re: Cranky Op-Amp
- From: pmlonline@xxxxxxxxx
- Date: 1 Mar 2006 18:04:41 -0800
Bob Eldred wrote:
<pmlonline@xxxxxxxxx> wrote in message
news:1141258243.121149.300300@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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.
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
.
- References:
- Cranky Op-Amp
- From: pmlonline
- Re: Cranky Op-Amp
- From: Bob Eldred
- Cranky Op-Amp
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