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