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Subject: 741summer gain inconsistent

Date: 06/05/02 at 1:20 AM
Posted by: Jim Henderson
E-mail: jimhend1@yahoo.com
Message Posted:

I need a simple summing amp to output varying 0.1VAC to 4.5VAC with a -7.5VDC bias.
I looked up some information on the web and built a 741 summing amp. My problem is that the AC loses about 0.5 volt at the output. I expect 1/1 gain with a little loss, but not that bad. Then I replaced the 2-6 10K feedback with a trimmer. When I set the feedback away from 10K, the VAC basically goes away. I expected setting FB >10K would increase my output AC and DC.
Any ideas what I am doing wrong?
Thanks
Jim

------------
|1 8|
| 741 |
10k | |
+7.5vdc--/\/\/\--- | |
| | |
|------|2- 7|+12VDC
10k | | | 10K |
4.5vac--/\/\/\---- | |--/\/\/\--|
| || 100K
|-----------|3 6|---out--/\/\/\--
--- | | |
- | | |
-12VDC |4 5| ---
------------ -


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