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Old 15th December 2003, 05:55 PM   #1
Netlist is offline Netlist  Belgium
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Default AlephX with Current Feedback

Somewhere in the Monolithic SuperSymmetry Nelson was teasing us with a Current Feedback version of an X circuit. More to the point an Aleph or AlephX circuit.
The idea was tempting enough to give it a try, at least conceptual. I couldnít find a similar X circuit with CF on the whole forum. Correct me if someone else tried this before. Iíll be happy to learn.
So I tried to design something. Donít build it; it will probably not work flawless.
Thatís why I putted up here. If thereís enough interest, we might make it a real X Current Feedback amp.
I took bits and pieces from what I could find on the net, took me two weeks to try many different combinations of transistor pairs and finally, this circuit came out as best.
I didnít add current sources for the moment, wanting to keep the concept simple. I tried with mosfets but didnít succeed at all.
I can tell for now that the hardest part is taming the differential DC at the outputs. For the moment I canít get it lower then 6V, which is far too high.
Please correct me in every possible way, I donít even know if the term CF applies, or even if itís an X.
I know some will say CF is a dangerous approach but Iím fearless for now.

/Hugo
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Old 15th December 2003, 07:07 PM   #2
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Netlist,

Why not remove the Aleph part and make it a full comp. circuit, nice idea though.

Regards,
Jam
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Old 15th December 2003, 08:21 PM   #3
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Quote:
Originally posted by jam
Why not remove the Aleph part and make it a full comp. circuit
I think the challenge is to try it with the SE amp.
There are many examples of push-pull designs everywhere and I had the feeling we would end with Nelson's first drawing in the Monolithic Susy thread.

Common guys...shoot!!!

/Hugo - not sure he will be able to hold the bullets
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Old 15th December 2003, 08:49 PM   #4
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Two comments.

This circuit needs communication between the Sources of the
input diff pair system in order to be an X

Nothing wrong with current feedback - the caveats about
current feedback are mostly b.s. - I find it very easy to
use, fast and stable.
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Old 15th December 2003, 09:26 PM   #5
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Quote:
Originally posted by Nelson Pass
I find it very easy to use, fast and stable.
Not me, yet...

Thanks for declaring this attempt B.S.-proof

Two things puzzle me:
1) You didnít comment on the approach of the diff pair. May I assume the CF part of the drawing is correct?
2) How can we get rid of the differential DC and get the whole thing stable. Maybe somethingís missing.

I added a resistor between the inverting inputs. That would probably make it an X. The scope figures are much worse though.

/Hugo
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Old 15th December 2003, 09:28 PM   #6
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Default CF diffpair

.
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Old 16th December 2003, 02:25 AM   #7
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Well, you simply have placed followers in front of the diff
pair devices, no?

Without some emitter degeneration on the diff pairs, you'll
have an exciting time controlling the bias, since you cannot
assume accruate enough matching.
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Old 17th December 2003, 10:00 PM   #8
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Followers, diffpair, emitter degeneration, controlling the bias.

You are definitely able to capture all problems in one phrase.
Thanks a lot, I'll have to contemplate a bit now.

/Hugo
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Old 17th December 2003, 10:22 PM   #9
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Hugo,

why not use the LM6181 ? Op amp allergy

Bernhard
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Old 18th December 2003, 07:30 AM   #10
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Quote:
Originally posted by Bernhard

why not use the LM6181 ?
Hmm...perhaps not a bad idea to start with and see how things behave. But in the end I would prefer discrete.

/Hugo
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