AlephX with Current Feedback

[Netlist]

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 couldn’t find a similar X circuit with CF on the whole forum. Correct me if someone else tried this before. I’ll be happy to learn.
So I tried to design something. Don’t build it; it will probably not work flawless.
That’s why I putted up here. If there’s 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 didn’t add current sources for the moment, wanting to keep the concept simple. I tried with mosfets but didn’t succeed at all.
I can tell for now that the hardest part is taming the differential DC at the outputs. For the moment I can’t get it lower then 6V, which is far too high.
Please correct me in every possible way, I don’t even know if the term CF applies, or even if it’s an X.
I know some will say CF is a dangerous approach but I’m fearless for now.

/Hugo

[jam]

Netlist,

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

Regards,
Jam

[Netlist]

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

[Nelson Pass]

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.

[Netlist]

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 didn’t 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 something’s missing.

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

/Hugo

[Netlist]

.

[Nelson Pass]

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.

[Netlist]

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

[Bernhard]

Hugo,

why not use the LM6181 ? Op amp allergy

Bernhard

[Netlist]

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