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Need some feedback on a laser diode APC circuit

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Hi all:

Accompanying file: LaserDiodeDriver_Ray-Xu_prelim_6-21-2009001.pdf

I just started designing my version of a laser diode driver circuit; and I was hoping you could look over it and give me some feedback. I designed it to be similar to the 2-transistor versions; except using op amps (it should allow for faster modulation; and I find using op amps very simple). The theory is that the current going into the laser diode is a inverse linear function to the photodiode current.

The circuit should be able to control a common-anode laser diode (yes, I made a mistake in the laser diode schematic symbol), with monitor currents of around 0.3 to 0.5mA, and a laser diode operating current of how ever much the precision current sink can sink. The precision voltage bias is to provide a stable operating voltage with respect to the virtual ground (4.5V).

The modulation controller circuit block uses a switch such as using the CD4066 to switch between the signal from the monitor photodiode or ground. The op amp acts as a buffer to compensate for any signal attenuation caused by the switch (because they have relatively large resistances). For a logic HIGH signal, the switch would be switched to the output of the op amp, and it would be switched to ground for a logic LOW.

The inverting amplifier block is to provide the inverse function and to amplify the signal form the photodiode monitor I-V converter so that the ratio between photodiode and laser diode is 1:1 (inverted).

I have yet to include the small specific details (including the resistor and part values); this is just an idea. I also lacked the filtering capacitors in the diagram for simplicity.

The voltage supplies would be equivelant to +/- 4.5V; VCC=+9V, VEE=0V, and a +4.5V as virtual ground.

I'm specifically worried about the modulation controller block; where it switches back and forth between ground and the output of the op amp. Also please provide feedback.

I'm pretty new to lasers; I only know how not to damage them by static and how to drive one using a simple 2-transistor circuit ;)

Many Thanks

Ray Xu
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