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
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
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jam said:
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
Not me, yet...Nelson Pass said:
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
I’ve been puzzling around a bit more on this CF thing and among the many things I don’t get, here the most significant ones.
Looking at the principle of CF and VF and trying to integrate it in the AlephX, Grey succeeded in building the AlephX with only two input devices. When trying the same with CF up to eight input devices come to mind as seen in the schematic on page 24 and in the drawing in my first post. Not to mention all the current sources. Found some very complex Accuphase schematics on page two here. This is all far away from the Zen-ish approach of AlephX. Do I see things too complicated?
Also, I cannot find an example of SE application of CF. All are push-pull, something I would like to avoid. Maybe it’s just not possible to apply CF in a SE application?
/Hugo
Looking at the principle of CF and VF and trying to integrate it in the AlephX, Grey succeeded in building the AlephX with only two input devices. When trying the same with CF up to eight input devices come to mind as seen in the schematic on page 24 and in the drawing in my first post. Not to mention all the current sources. Found some very complex Accuphase schematics on page two here. This is all far away from the Zen-ish approach of AlephX. Do I see things too complicated?
Also, I cannot find an example of SE application of CF. All are push-pull, something I would like to avoid. Maybe it’s just not possible to apply CF in a SE application?
/Hugo
I’ll promise not to hammer anyone, be nice and ask things very polite, so while you guys are at it could you have a look at my CF attempts?
I don’t pretend to know much but willing to learn and understand things.
I could copy an existing schematic but as stated earlier, the challenge is trying to make a CF AlephX.
Could someone explain that emitter degeneration issue, is it possible to use only two input transistors per side as shown in the picture or should the schematic of the first post be continued?
Any comments greatly appreciated.
/Hugo
I don’t pretend to know much but willing to learn and understand things.
I could copy an existing schematic but as stated earlier, the challenge is trying to make a CF AlephX.
Could someone explain that emitter degeneration issue, is it possible to use only two input transistors per side as shown in the picture or should the schematic of the first post be continued?
Any comments greatly appreciated.
/Hugo
Attachments
Got myself a ‘working’ version.
Not perfect, there will be too many errors I guess but:
The starting point was the CF amplifiers note from National.
This was the most simplified one I found.
I went to Peranders’s site as well and tried to mix what I found in his schematic.
1) Now the DC is under control. It can be adjusted by R33/R34.
2) I think it’s an X: the inputs for the gain devices are cross connected and according to Nelson’s remark I made the communication between the emitters from the input buffers.
I can't find that connection in the normal AlephX. Is it not there, or is it me who doesn't see it?
3) I didn’t bother to make it more complicated than necessary. No cascodes, no current sources for the input devices, keeping the Zen idea in mind.
4) One of the many things I have to figure out is the bandwidth. –3db occurs at about 9.5Khz at the collector of Q5.
Bernhard, thanks, I tried with LM6181 but I really would like to achieve it with simple transistors. Again, the simplicity of Zen has bitten me.
Bricolo,
Please elaborate a bit. Link?, schematic?
/Hugo
Not perfect, there will be too many errors I guess but:
The starting point was the CF amplifiers note from National.
This was the most simplified one I found.
I went to Peranders’s site as well and tried to mix what I found in his schematic.
1) Now the DC is under control. It can be adjusted by R33/R34.
2) I think it’s an X: the inputs for the gain devices are cross connected and according to Nelson’s remark I made the communication between the emitters from the input buffers.
I can't find that connection in the normal AlephX. Is it not there, or is it me who doesn't see it?
3) I didn’t bother to make it more complicated than necessary. No cascodes, no current sources for the input devices, keeping the Zen idea in mind.
4) One of the many things I have to figure out is the bandwidth. –3db occurs at about 9.5Khz at the collector of Q5.
Bernhard, thanks, I tried with LM6181 but I really would like to achieve it with simple transistors. Again, the simplicity of Zen has bitten me.
Bricolo,
Please elaborate a bit. Link?, schematic?
/Hugo
Attachments
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