R10/R12/C6 set the gain and frequency response of the error amp.
....I see what you mean, I totally forgot that in the casc0ded version, I had used the "error amp" internal to the LT1243 as a simple gain stage, as you know, it is not being used as an "error amplifier", the error amplifier is outside the chip, and is done by U3 opamp...but I do see what you mean, you are lumping that gain in with the gain of the error amplifier which is fine.....to be honest, I just wanted to use the internal amplifier in the LT1243 as a unity gain follower, but there was no access to the noninverting pin, so I had to use it as an amplifier....that's why I forgot I had used it as an amplifier.
I removed a non functioning op-amp.
..its not non functioning, it is as shown on page 16 of this...
https://www.onsemi.com/pub_link/Collateral/AND8273-D.PDF
...pages 10 and 11 describe the use of that opamp...figures 15 and 17....that extra opamp makes it a current source that feeds the opto-diode, which makes for better transient response.
Thanks for your work here, You do appear to have got a good response there, and I am going to make your schem and investigate it in LTspice.
..OK just simulated your version, without the casc0de, and it was excellent. I am going to see if adding the casc0de to what you did makes it even better.
Also, I adjusted your version as attached, to make the opto transistor in the common emitter configuration, (instead of the common collector arrangement) and it oscillates wildly. I am wondering what it is about common collector configuration of the opto transistor that is so good? (and common emitter connection so bad) Is it something to do with the base-collector capacitance of the opto transistor seeming less when in the common collector configuration?, (since collector voltage is fixed when in common collector configuration.)
Also, do you believe that the onsemi app note (AND8273) that I linked to here is talking nonsense when they suggest the casc0de?..is just simply using the opto transistor in the common collector mode sufficient to mitigate the problematic low opto pole frequency?..the common base stage of the casc0de not being needed?