Which design to choose for highly stable LED or laser diode driver?
Here is an arrangement that gives excellent regulation, low noise at low and high frequencies, and also allows you to switch the LED at very high frequencies, without compromising the noise.







simulate this circuit &ndash; Schematic created using CircuitLab


lots of caps to filter noise, both big electros and small ceramics
the voltage ref has its noise filtered to give a quiet reference to the opamp.
the opamp gives an accurate constant current
we use an emitter follower, as it has intrinsic constant current at high frequencies without the opamp doing anything
instead of turning it on and off, we steer the current between the test led, and a second matched led. This means that the currents and voltages stay constant during switching. The constant current circuit can have noise filtering, yet still be switched at speed.
We filter the supply to the leds to reduce the noise input from the power supply.
invertor is CD4069 (18V cmos). Must have sufficient supply voltage to turn the fets on. i.e. 5V above collector voltage of Q1. PFets could also be used.
the opamp is not critical, but must be single supply (works to 0) or rail to rail. LM358 prob OK, or TLV172. At low currents, the transistor is simple. At higher currents, you need to either change to a darlington, or maybe use a crazy hot modern transistor like zxt1053 which manages HFE=400@1A


This is the circuit I used in optical instruments for turbidity, transmission and flourescence.



Note that generally the actual brightness of the LED's is being calibrated out by another (reference) light path or photodetector. The brightness varies as the LED's age, and with temperature. If you need to control the absolute brightness, then you use a reference photodiode in the feedback loop to control brightness rather than current.