TSH4131 And "X" ?

[CheffDeGaar]

Question for X-perts

I just came upon TI's THS4131 datasheet , and a question arose in my little head. Considering the attached image, in which I melt two images from the datasheet, and comparing it with this post from stefanobilliani in the forum, I couldn't resist joining the two. The simplified schematic of the THS4131 shows a diff amp with a folded cascode with active loads, and the little application schematic shows the feed back resistors tie the "+" (resp. "-") inputs to the "-" (resp. "+") outputs, just like the x-bosoz linked above. So the eternal question... :
Does it qualify as "X" ?
And the immediately following q. : Why do I always ask stupid questions ?

[Nelson Pass]

I would say that there's enough there to possibly constitute
patent infringement, not that I would bother with it.

[Joe Berry]

Originally posted by CheffDeGaar
The simplified schematic of the THS4131 shows a diff amp with a folded cascode with active loads, and the little application schematic shows the feed back resistors tie the "+" (resp. "-") inputs to the "-" (resp. "+") outputs... Does it qualify as "X" ?

This is basically the same approach taken in the Hadley 622, and later, in David Hermeyer's "Electrostatic Amplifier Mk2" (Audio Amateur 3/1973). All of these differ from the Pass X circuit in that they add a third feedback loop to cancel common mode error. One might question whether this CM error is worth fixing in an amplifier designed for listening, as according to Hermeyer, it (merely) leads to some added second harmonic when the output is loaded asymetrically.

[Nelson Pass]

It would be the folded cascode that probably would turn
the legal argument, as the precedent hasn't been seen for
that, and it is part of the patent claim structure.

[CheffDeGaar]

Originally posted by Nelson Pass
I would say that there's enough there to possibly constitute
patent infringement, not that I would bother with it.

Thanks for replying, Mr Pass ! And I should have seen it form the start : it's written in the image (see attached picture)
BTW, this IC exhibits nice figures (slew rate, BW, noise, etc... Product folder here) It's tempting trying to build something and listen to it... I've seen this IC used as a balanced IV Converter in the Bel Canto Dac 2, wich was highly praised when it appeared...

Originally posted by Joe Berry
All of these differ from the Pass X circuit in that they add a third feedback loop to cancel common mode error. One might question whether this CM error is worth fixing in an amplifier designed for listening, as according to Hermeyer, it (merely) leads to some added second harmonic when the output is loaded asymetrically.

Joe, thanks for the references. From an extreme point of view, the "servo loop" can be ommited and replaced by a mere voltage source biasing the lower BJTs to maintain a null DC offset at ouputs. But, in my very limited experience , such a fixed biasing for a folded cascode topology is critical, since this topology is very sensitive to the bias...

[Joe Berry]

FWIW, I think that the added third feedback loop is an interesting conceptual difference, but I doubt that it could be used by itself to invalidate a claim of infringement. If my understanding is correct, a circuit infringes a (US) patent if it is found to include the latter as a subset.