what circuit or component can be suitable A

[student11]

what is A

hi, this is a photocoupler control circuit.
you can see it from the attached picture,

my questions is,

what circuit or component can be suitable A


Note,

1) input>5V, the OP1 can't work

2) 5v≤input<100v, the OP1 work

 

[fragment]

Re: what is A

It can be resistor to set operation current for photodiode.