Ah yes I just checked the Avago datasheet and saw the 100pF.
Note the gigantic window for forward voltage at 10mA. I'll bet those limits are set for about 10 sigma process capability :razz: The variations may also reflect a bulk resistance term that becomes important at the higher currents, but that doesn't much affect the light output for a constant current drive.
Interesting how the other color is so much lower C. It probably has lower light sensitivity too.
The distortion versus number of stages argument Colin makes seems to assume that a given stage distortion is level-independent, unless I'm missing something---as I say I haven't worked through his math yet. But typical nonlinearites follow a power law with level, with 2nd growing linearly with level, third rising as the square, etc. If one is building an overall multi-stage 60dB gain amplifier, obviously the early stage sees a much smaller signal than the later ones, presuming the real-world constraint of some limits on the later stages' output swings. The bulk of your distortion will arise in the final stage, surely.
But he's trying to show that a single classical opamp stage can be inferior to a multiple-stage arrangement
when a lot of gain is required. Frankly I don't know who would argue against that point for overall high-gain chains anyway, except out of an audiophile minimalist Zen etc. attitude.
Another quibble is that almost everyone plugs in open-loop distortion and then says the distortion reduction when the loop is closed is simple arithmetic based on the thrown-away gain, but almost never take into account where you are at the frequency of measurement on the open-loop gain curve. Few opamps are still in the flat gain region at frequencies of interest for audio. When you are in the usually-predominant Pi/2 phase region it makes things better than the magnitude arithmetic, IIRC.
Rich May advocated, for good sound, open-loop gain constancy out to the highest fundamental of interest in the signal, even if the value of that open-loop gain at a given frequency was not as staggeringly high as a typical opamp.
I do like Colin's extracting e out of his argument though. When we can get Euler's equation then we're getting somewhere :green:
Oh well here we go wildly off topic again
PS: haven't seen Jung III