I'm looking up for some circuitry to drive my laser diode and I came across this tutorial.

I understand the basic operation of this schematic; however, there's one thing that confuses me:

I cannot understand the purpose of R6 and the pair C3, R4. (Jt seems like a RC filter, doesn't it?)


  • \$\begingroup\$ @KH \$C_3\$ is in-the-loop compensation. Oscillation often results without it. You can find it discussed here. \$\endgroup\$ – jonk Apr 6 at 6:16
  • \$\begingroup\$ @Jonk Is it not coincidentally also an op-amp integrator? I even thought about how the integrator works and it appears to make sense but I'm definitely not sure. I'm going to go read that. \$\endgroup\$ – K H Apr 6 at 6:23

C3 will object to rapid changes in base voltage applied to Q1. The capacitor is a filter of sorts I think, but from the perspective of the op-amp it appears to be to make an op-amp integrator circuit.

R4 and R6 are sampling resistors for the current sense resistor R5. Their values will control how quickly the op-amp integrator circuit around U1A responds and also their weight compared to pull-down resistor R7 and voltage divider/reference network R1, R2, ZD1. Omitted ZD1 is to provide voltage reference stability if input voltage has variance. R3 is apparently a 0 Ohm resistor, which surprises me as it drives a transistor base, but perhaps the op-amp achieves current control so a base resistor is unnecessary.

A switched mode current driver might be preferable and easier to use.

An op-amp integrator performs the mathematical operation of integration IIRC, but a more intuitive way to explain it would be it adjusts it output at a speed proportional to the difference in the error signal, so it adjusts faster when error is large.


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