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Old 15th September 2005, 06:48 PM   #11
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Hey all thanks for so many replies.

janneman: I've omitted the dc-blocking cap on the feedback and put a DC signal on the input to get the DC signal on the output. This is because then i can test positive and negative voltage overload protection separate and see how it behaves.

I agree this circuit is somewhat incomplete and doesn't really work properly

The problem seems to be that on positive signal, the current is limited by the current-source and there's to much resistance to take any current from the VAS, but on negative swing the output protection has direct contact with the VAS collector and can sink so much current as the LTP input pair can deliver out * hFE of vas = too much Right now i don't see how to work this out
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Old 15th September 2005, 07:20 PM   #12
Eva is offline Eva  Spain
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Fritz:

Add another emitter resistor and current limiting transistor to the VAS, like the ones you are using for the output devices. Clipping behaviour will also get improved.
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Old 15th September 2005, 07:47 PM   #13
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Fritzell,

Strange, you would think that whatever is above the Vas, the current limit xsistor should be able to short the drive to the driver. I think the 12k is too large to sustain the base current for full shorting. Keeping the same current limit, try to change the divider from 1k - 12k to say 100 ohm - 1.2k. That should do it.

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Old 16th September 2005, 01:38 PM   #14
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Hey thanks for all ideas. Here's the current circuit im been working with. I put a 22 ohm emitter resistor for vas and a transistor on vas base. When current exceeds something like 30mA voltage increases so it opens the transistor and leads away all base-current to the vas and this is how it works. In simulation this config seem to work very well.
I dont know how it will work in real life though. Will it cause oscillation or some other bad effect? Please comment this circuit

Im thinking of maybe adding an inductor on the output also..

Scheme:
http://upl.silentwhisper.net/uplfold...pschematic.GIF

Thanks!
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Old 16th September 2005, 01:52 PM   #15
Eva is offline Eva  Spain
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I have never seen such a VAS current limiting arrangement oscillating in practice, but the classic output device current limiting arrangement may oscillate because it is closing a feedback loop across two more transistors. You should build a prototype and test it carefully.
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Old 17th September 2005, 09:29 PM   #16
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Okay i will try. Thanks all for your answers!
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Old 19th September 2005, 02:19 PM   #17
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Quote:
Originally posted by Fritzell
Hey thanks for all ideas. Here's the current circuit im been working with. I put a 22 ohm emitter resistor for vas and a transistor on vas base. When current exceeds something like 30mA voltage increases so it opens the transistor and leads away all base-current to the vas and this is how it works. In simulation this config seem to work very well.
I dont know how it will work in real life though. Will it cause oscillation or some other bad effect? Please comment this circuit

Im thinking of maybe adding an inductor on the output also..

Scheme:
http://upl.silentwhisper.net/uplfold...pschematic.GIF

Thanks!
Hi,

I see you changed one of the resistors across Re for current limiting, now the limit is MUCH lower. You sure you want to do that?

Jan Didden
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