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Old 30th March 2002, 09:58 PM   #1
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Question Having trouble with LM1972 circuit

Greetings from a short-time lurker, first-time poster.

I'm wondering if anyone has experience with the LM1972 volume-control chip from NSC. I am trying to use this device with a single 12V supply rail, and have been running into oscillation problems in the unity-gain JFET op-amp buffer that follows it.

The LM1972 data sheet is not very good about providing circuit specifics. They recommend using a JFET-input unity-gain buffer at the LM1972's output to keep from loading the LM1972 and producing clicks when the attenuation level changes. However, the 1972's output impedance changes over a pretty wide range depending on the level it's set to, and it seems that there are always a few spots in the adjustment range where the unity-gain follower will break into oscillation.

I have tried the LF356 as well as NSC's recommended LF412 as the buffer with the same results. I have tried hardwiring the inverting input directly to the output, as well as using a 1K resistor. I believe that supply bypassing and the like are adequate. The oscillation occurs regardless of the load (or lack thereof) at the op-amp's output.

Has anyone here successfully used the LM197x chips? There seem to be no app notes or example circuits anywhere on the web for this chip. What, exactly, does its unity-gain buffer need to look like?
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Old 30th March 2002, 10:13 PM   #2
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Does your buffer oscillate with its input isolated from the other chip?

Jocko
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Old 30th March 2002, 10:47 PM   #3
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No, both the LF412 and LF356 are unity-gain stable. The buffer and the PA that comes after it don't exhibit any stability problems until the LM1972 is connected.

Actually, I think I answered my own question a few minutes ago... neither of these opamps are designed for rail-to-rail service, and the LM1972's output DC offset falls below 0.7V right where the oscillation happens. I'm going to have to run these guys off a split supply, it looks like.
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