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Old 23rd September 2010, 06:16 PM   #1
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Default OpenLoop Gain and compensation

Hello world

I have an amp simulated and as real device, but I've some problems to simulate it's open loop gain. Is there anything out as info that can you recommend to read ?
Next problem is the compensation of the amp....At the moment there's only the capacitor across B-C of the VAS transistor...but there I must put >300pF in to get the amp stable at any load (in the real world). What's about a second order pole in the feedback ? In the simulation that works fine...but in real world ?????

Thanks.....
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Old 23rd September 2010, 06:34 PM   #2
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And the whole scheme?
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Old 23rd September 2010, 06:45 PM   #3
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as PDF:
http://www.ac-vogel.de/TheAmp_MK2/20...2010-06-30.pdf

and see pic
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Old 23rd September 2010, 06:48 PM   #4
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R22 and C5 are not placed (R-C across VAS)
C2 is 300pF (across VAS)
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Old 23rd September 2010, 07:04 PM   #5
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You have a supply rail filter D4 + R37 (10 Ohm)
I would rather put this before the VAS Constant current source.
So I should move them to the right of T7.

Current sources benefit from clean, constant voltage supply.
And this is what the filter D4 + R37 helps to give. su
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Old 23rd September 2010, 07:06 PM   #6
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Yes :-)
Error known and will be corrected in next "session" :-)
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Old 23rd September 2010, 07:25 PM   #7
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strange , this is almost the same as my "AX" which runs stable down to 22pF ??

edit , GET RID of the Darlington drivers !!! (tip 102/107) most likely they are the problem.

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Old 23rd September 2010, 07:27 PM   #8
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Ok... I've found something about testing OL gain in simulation (making the feedback R from output to some MOhms and replacing the feedback resistor to ground by 10F (!) ).

So I've Open Loop gain... the compensation in the simulation works fine (just where I want it) and I also get best results in simulation... but bad results in real world :-(
One prototype (breadboard) is performing well... the PCB type seems to be instable.... mhhhh... can the emitter resistors make an amplifier unstable ? There where other types on the breadboard than on the PCB....
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Old 23rd September 2010, 07:29 PM   #9
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Originally Posted by ostripper View Post
strange , this is almost the same as my "AX" which runs stable down to 22pF ??

edit , GET RID of the Darlington drivers !!! (tip 102/107) most likely they are the problem.

OS
Why not darlingtons ? capacity too high ? They make unbelievable current gain
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Old 23rd September 2010, 07:36 PM   #10
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Originally Posted by Voegelchen View Post
Why not darlingtons ? capacity too high ? They make unbelievable current gain
Oscillation city... standard MJE15032/33 drivers will give 2k hfe in a standard EF2 output stage and will allow the "blameless" to run on a single 33/47pF Cdom rock solid. I don't lie , i'm building my 10th pair of your amp "ax" below.
OS
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