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Old 5th February 2016, 01:17 AM   #1
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Default Please help diagnose bad oscillation

Ive built a low power aleph m of sorts without the active ac current source. using it as a gain stage. when powered up on +/- 45 volt rails it oscillates immediately at @ 1-1.2MHz. Anyone have any ideas what would cause this? please refer to the circuit pictured. using 9610's and 610's for the mosfets. Im using a connex smps power supply but had the same osillation when powered from a filtered transformer.
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Old 5th February 2016, 01:20 AM   #2
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Could be Zener, could be high voltages on Jfets.

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Old 5th February 2016, 01:24 AM   #3
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no jfets in the circuit, bjt for the current source and mosfets for everything else. I would like to say a BIG thank you Mr. Pass for taking the time to reply to my question though! can you please give me more insight as to what i should look for? i would say Im a novice still with just enough knowledge to get in trouble, lol.
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Old 5th February 2016, 01:29 AM   #4
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are you saying the zener biasing up the current source for the LTP could be causing it? i place a cap across it but it didnt seem to change anything...
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Old 5th February 2016, 01:51 AM   #5
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cap across feedback resistor , increase gate stopper resistors , include blocking caps as originally in Aleph series (1nF in two positions)

look at attached for reference
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File Type: pdf Aleph30 by Algar_emi.pdf (29.9 KB, 35 views)
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Old 5th February 2016, 01:57 AM   #6
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i can say that when i tried a cap across the feedback resistor it raised the oscillation frequency to @6MHz
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Old 5th February 2016, 02:01 AM   #7
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i see the A30 link, I dont have what amounts to Q4 in the A30 circuit so do i still need 1nf in that position for stability? Zen Mod are you saying to use like 100 ohm gate resistors throughout?
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Old 5th February 2016, 04:40 AM   #8
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Quote:
Originally Posted by fligti91 View Post
i can say that when i tried a cap across the feedback resistor it raised the oscillation frequency to @6MHz
What value cap did you try ?

Also, separately, in ref to Nelson's original suggestion - ditch the protection zeners at the input stage and see if that helps.
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Old 5th February 2016, 06:22 AM   #9
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If original circuit uses IRFP240 and now you have IRF610, change gate resistors immediately.
You may need to go as high as 2kohm...
Gradually, until oscillation stops.
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Old 5th February 2016, 09:00 AM   #10
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yup, increase value for gate stoppers
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