The operation of the basic circuit may not be immedi‑
ately obvious just from inspecting Figure 12a, particu‑
larly the input part of the circuit. Stated briefly, amplifier
A1 adjusts the LED current (IF
), and therefore the current
in PD1 (IPD1), to maintain its “+” input terminal at 0 V. For
example, increasing the input voltage would tend to in‑
crease the voltage of the “+” input terminal of A1 above
0 V. A1 amplifies that increase, causing IF
to increase, as
well as IPD1. Because of the way that PD1 is connected,
I
PD1 will pull the “+” terminal of the op-amp back toward
ground. A1 will continue to increase IF
until its “+” termi‑
nal is back at 0 V. Assuming that A1 is a perfect op-amp,
no current flows into the inputs of A1; therefore, all of the
current flowing through R1 will flow through PD1. Since
the “+” input of A1 is at 0 V, the current through R1, and
therefore IPD1 as well, is equal to VIN/R1.