What is X in the Aleph-X?

[Bricolo]

I've just compared Nelson's susy.pdf, and Grey's Aleph-X


And here are the differences

I'm trying to understant how X works, but the shametic is so different from Nelson's explanations...

Can someone explain me?

[GRollins]

When I get to the reply screen, your schematic disappears, so I made notes. Hopefully, I'll catch at least the high spots.
Actually, the link between points 38 and 39 is still there. It's the connection between the Sources of the front end differential.
R40 does not exist.
R42 and 43...well they do and they don't exist. Depending on how you want to look at things, you could say that they're gone...or that they've been replaced by R1/R4 and R44/R45 in the original Aleph-X schematic.
The criss-cross thing you drew is heading in the right direction, conceptually. The thing you have to keep in mind is that "X" feedback is going to be a question of phase. The Aleph-X front end drive crosses over to the other side in order to keep the feedback negative. (Positive feedback is bad ju-ju, of course.) I think the patent shows the output stages as being followers, which don't invert phase, whereas the Aleph output does. If you wanted to design a follower output, then the drive and the feedback would remain on the same side of the circuit. If you're using an output that inverts, you'll need to cross either the drive or the feedback to the other side. Topologically, it's the same thing either way--it was just easier to draw the schematic the way I did; I was born lazy and had a relapse.

Grey

[Bricolo]

ok so the main thing is that:

the feedback has to be negative, and "feedback borns negative", so on a follower amplifier, where phase isn't changed, you simpli have to keep the feedback line on the same side

but, on an aleph, the output it at opposite phase, so if you keep the feedback on the same line, it becomes positive, so you have to "cross"


am I right?

[Bricolo]

now I have to understand why nelson's shematic is a follower, i thought it was a phase changing amplifier

[Bricolo]

I played a little with spice, and Nelson's shematic inverts the phase!

so, there's a problem somewhere...

[eLarson]

If when you say "Nelson's schematic," you are
referring to the circuit shown on the cover page of
patent #5,376,899, you have two folded cascodes
coupled together by their non-inverting inputs.
The input at the Gate of MOSFET 20 will be
inverted when it gets to the Drain of 20. It then
feeds the Source of MOSFET 30. This being
connected as a common-gate, you don't get
another phase inversion.

So overall, it is an inverting amplifier.

The mirror image is another inverting amplifier,
ideally being driven by a the negative-phase of the
incoming signal. The common-mode bits are
subtracted away by differential nature of the
circuit.

The feedback of each side makes each half
more like the other, you might say, perfecting the
subtraction.

Erik

[eLarson]

Originally posted by Bricolo
ok so the main thing is that:

the feedback has to be negative, and "feedback borns negative", so on a follower amplifier, where phase isn't changed, you simpli have to keep the feedback line on the same side

but, on an aleph, the output it at opposite phase, so if you keep the feedback on the same line, it becomes positive, so you have to "cross"


am I right?

I believe so.

You only want one phase inversion per side in
order to feed a signal with opposite phase back to
Gate of the diff pair MOSFET.

You can get this by grabbing the output of the
diff-pair that matches your phase: the one from
the opposite MOSFET.

Erik

[GRollins]

Oops...right...the outputs aren't followers. I knew I'd get something cranked backwards once I lost the schematic...sorry about that.

Grey

[Bricolo]

Ok, si if nelson's shematic is the correct one (of course it is )

why is grey's so different? Especially if it uses the same concept (2 gain stages, the first being the diff pairs)

[GRollins]

It's different on account of the fact that I like breaking rules. Life is more fun when you think outside the box. Egad, I'd die of boredom if I had to build standard three stage, cookie cutter amps as per Self & Slone. (If you feel that you must read one, read Self. Slone is intolerable.) One or two are okay, but a steady diet of that kind of thing kills your spirit.
I view the world as a set of building blocks. Like a kid, I play with the blocks, putting them together in one order, then another, seeing what will happen. Nothing to it. It's just play. I enjoy it. When something goes right, I learn. When something goes wrong, I learn. As long as I'm learning, I'm happy. (Though I reserve the right to cuss if I blow something up along the way.)

Grey