There are op-amps that match input current instead of input voltage
with the feed back.
They are a little strange to work with because the inputs are essentially
diodes to grounds that are forward biased.
The input currents used are quite small. On the order of microamps.
On one of the other post I was showing how to use a switching power supply
as a constant current source.
It is the same basic circuit as this but the op-amp is inside of the regulator.
The idea is that you can connect a string of LEDs and only need one current
limiting resistor.
The top of the LED string goes to the output of the regulator circuit.
The bottom of the string goes to the sense of the regulator.
Most are set to compare to 1.25v ( common because of a band-gap
reference ).
The current for the string goes through a resistor to ground. It then
regulates the voltage on the resistor by more or less current through
the LEDs.
For a battery operated LED string, this can be much more efficient, using
the switching regulators efficiency and only the one resistor that sets
the current more accurately than a series resistor. It can run like
a battery with 90v but only the one tiny resistor to set the current.
Normally you want the resistor to be around 20% of the voltage drop.
The only difference is that the switcher is running on-and off to
energize a coil instead of the circuit in this thread running like a linear
regulator.
I've used LM317 regulators in the past a similar current source but
being linear, thing get hot. ( did this one to replace the constant current
tube that used to be used in Zenith Transoceanic receivers. )
Dwight