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PPAv2 Can ground channel have gain?

post #1 of 6
5/7/09 at 8:06am
Thread Starter 
Here is the low down.

I've constructed a similar version of the PPAv2. I'm trying to use NTE937 JFET op's. The issue is that the 937 in volltage follower config has a very high DC offset. Somewhere in the 150mv range. The op does have offset pins but I can't swing it enough to counteract that much. If I use a few resisters of equal values for the negative feedback and a 2x value for the input resistance I can achieve a zero DC offset. I used tangentsoft's calculators to get the resister values to match my ops' input bias/offset currents. Very handy!

Using the resisters I get some gain. I'm sure its not wise to have gain on the ground channel but will it hurt anything?

Using them in the L/R channel was simple enough. For the sack of symmetry I would like to use all the same op's. I've substituted with a cheepo LM318. I believe tangentsoft mentions something to the affect of using all the same op's for the sack of there slew rate. It has 70v/us vs 50v/us on the 937's. It sounds great but I would like to be using all the same.

So... gain or not to gain?

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post #2 of 6
5/7/09 at 9:04pm
Yes, you can have gain in the ground channel, but you shouldn't need it. It adds noise, and it may not fix the real problem which is causing the offset, which shouldn't be the input bias currents. This particular JFET input chip has more input bias current than most, but it's still tiny compared to bipolar-input chips where balancing the input currents matters.

I think you have some other problem, like poor bypassing or cold joints, causing the offsets. Are you sure it isn't oscillating?
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post #3 of 6
5/8/09 at 7:52am
Thread Starter 
Wow!!

Good call on the oscillation. I didn't even think to look at it with the scope. Its oscillating terribly and I can't seem to calm it down. I tried some bypassing, load on the output, 40pf in negative feedback path with very little change.

I setup the op with the equal value resisters in the pic above and it looks stable.

So the NTE937 is not unity gain stable? If an op doesn't say "unity gain stable" should I assume that its not? Is there a spec that will give me a clue?

Thx
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post #4 of 6
5/9/09 at 4:38am
The datasheet should say something about minimum gain for stability on that particular opamp.
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post #5 of 6
5/9/09 at 1:55pm
NTE parts are expensive, have poor documentation, and are usually clones of some more well-known part which is much cheaper and more widely available. The only reason to buy them is when they're your only source, either because you can get them locally and have to have the part today, or because the well-known part they clone has gone out of production.

I just spent probably half an hour trying to find out which well-known part the NTE937 is a clone of, and haven't found anything like it. Maybe it's actually unique. Anyway, my point in doing that was to try and find a better datasheet, and couldn't. So, you're stuck with what NTE gave you, which isn't much.
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post #6 of 6
5/10/09 at 8:28am
Thread Starter 
I agree with you totally. NTE does not seem to give nearly as much data about there products as the manufacturers do. I had to guess about the offset pin config. I used a 100k pot with center tap to positive voltage. This could be normal but I'm kinda winging it here. :P

Anyway I've given up using it on the ground channel. They do seem to work well in the gain stage so I will use them for now.
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