I came across this schematic for opto-coupled output:
We have FPGA out 1.8V and resistor 100Ohm to regulate the current for diode from MOCD207-M.
I'm not sure how the other side works.
1. How works this circuit?
2. What is the purpose of the diode CDSU4148-HF?
3. What will happen if we put a series resistor with the diode?
4. What will happen if we remove the diode?
5. Does some one have experience with opto-coupled darlington output and can the darlington substitute the proposed circuit?
That is a darlington circuit, you could use a darlington-output opto and get rid of that external transistor. Not really sure what that diode is for. Sometimes a resistor put there to speed things up, as a darlington is relatively slow to turn off. Maybe it's there to sink leakage current from the input transistor.
It's not clear, but is your load simply a 270 ohm resistor?
I'm not 100% sure but the resistor is what the user have to calculate regard the voltage connected to V+ (3,3V to 24V) to regulate the maximum current throughout the transistors.
But I'm not sure for what voltage is the 270 Ohm resistor.
The diode is forward biased and acts as a non-linear resistor. Basically trying to improve the dynamic range- when the voltage is low the resistance is high and vice-versa. The external transistor, with collector and emitter connected to the supply rails, is badly configured as there are no load resistors given.