Well I'm glad the anti-phase optos seem to work in keeping the linearity without any feedback, because AFAIK that was untested in practice.
But, (there's always a but
) have you seen my photo diode amp step response at the same 500ns/Div speed. LEDphotoTRopamp.gif
"the more BW the better!"Yep, it's quite possible to get it much higher than that, I'm setting the target at 1.5M+.
I simulated the 5 transistor long tailed pair arrangement in the end. The problem is the high 2k5 impedance at the bases, it's much too high, together with the PD capacitance it limits the BW at that point to ~600kHz.
Everybody will know more about that 5 TR arrangement than me, so is it possible to get the base impedance down to a few 100 R. After about an hour of trying I gave up and drew this 1 transistor instead.
I've got the impedance at the base ~160R, and the BW
down to 3MHz, because I think that might be about the best the opto can do.
The input current variation is 7mA, -3.5mA to +3.5mA, and the collector OUTput 138mV pk-pk, I've doubled the frequency of my test waves to 200kHz, and they still look good!
A problem with that simple 1 transistor is that the output ~2V DC level is not very well defined and varies a bit with temp.
I'll add the LT.asc and hopefully someone will suggest simple improvements.