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| Laser Diode driver - kHz modulation & 1% duty cycles |
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| LaserSteve:
I finally found something EE worthy Design and Stability Analysis of a CMOS Feedback Laser Driver P. Zivojinovic, M. Lescure, and H. Tap-Béteille IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 53, NO. 1, FEBRUARY 2004 IEEE is anal on uplinks, or I'd post it for you.. However you may be amazed about what shows up in Google Images for a preview.. While the pictures show an array of FETs, the circuit is modeled on traditional components. I'm pretty sure from reading it that they modeled it on the bench before making the IC... >> then 1 Mhz from my reading. Steve |
| smoothVTer:
Sorry its been a long time. I mostly figured out this laser driver, and this post is to update my findings and trial-and-error scope shots. The topology I settled on came about after taking suggestions from prior posters: At first pass, the circuit didn't have the schottky on the OPA output; there was no Re; there was no Rpe; Rbase was a few kohms; and Cf was about 1nF. Theory of operation is during the laser "on" time, Q2 is off and all feedback current is generated by the monitor photodiode. OPA1 pushes enough base current to bring the inverting and noninverting inputs to the same potential. During the laser "off" period, Q2 injects a small current into the inverting node, raising the voltage there, and then OPA1 turns current off to the base of Q1. Consequently, this puts OPA1 into saturation because the + and - inputs are rather far apart in potential. The yellow trace is the base voltage; the green trace is the voltage at the inverting node; the red voltage is a pseudo-differential voltage across the laser diode by taking 2 scope probes across the diode relative to GND and doing some math in the scope. As you can see, (1) The output oscillates (2) There's a large negative going excursion of the OPA output as soon as Q2 turns off; I think this has to do with the Cf capacitor's stored charge suddenly being reversed, as well as the "integrator windup" effect. (3) There is a long slow ramp of of the base voltage, not desirable for generating a sharp, fast laser pulse. The first thing I changed was to put a schottky on the OPA1 output, to prevent that large negative going excursion: This seemed to help the recovery time from saturation. Next, I empirically determined a Cf value in my circuit that would just quell the oscillations on the rising edge of the base voltage. This improved my waveforms considerably: Still wasn't happy about that long, rising edge. Another poster suggested to (a) lower the base resistor just enough to prevent parasitic oscillation and (b) add an emitter resistor to stabilize Q1's gain. Well, all this sort of reminded me of BJT design class and made a lot of sense. That rising edge was a little strange looking so I had to go back and forth and trying out different values for Rb/Re. After playing around with values for Rb and Re, and lowering Cf even further, I finally got something approaching a good waveform with good turn on/turn off characteristics: The laser diode voltage falling slope is determined by a large part to Rpe ... Rpe is basically there for that purpose, plus a larger purpose of always allowing a tiny bit of current through the laser diode so it is faster to turn on from the off state, with less of a transient excursion. Of course, power consumption was an issue in my design so it had to be a balance between response and power consumption. I am sure there are many, many things I can improve on this circuit. For example: I used an AD8029 for OPA1, for the only reason that (a) it was available in my lab and (b) it is a tiny package size and space was a consideration. But perhaps there is another OPA better suited for this role? Also: Q1 I found the NPN's that work best with this circuit are high-beta, low Vce NPN's. Why is this so? This one in particular is a 2SD1979GSL NPN. I basically soldered and de-soldered NPN's here until I found something that seemed to work best. What combination of characteristics are best for Q1 in this application? Some NPN's I tried oscillated no matter what ... some had such a slow turn-on that the 2us pulses above never reached steady-state. Also: Re adds to the power dissipation of the entire driver; it stabilizes Q1's Vbe and thus its gain, but also is in series with the laser drive current, raising the voltage dropped by it, and this necessitates a higher laser supply voltage. I settled on a value of 15 ohms. I would like to lower this resistor as much as possible but it seems the lower value I choose, the worse that rising edge of the base voltage appears. How can I choose Q1 so that I minimize Re as much as possible? Thanks everyone for contributing, I hope these screenshots sort of show what was happening during the design/tuning process of this circuit. |
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