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[SOLVED] Op-amp current sink speed problem

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nagyan

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Hello All,

I have a little problem and I hope somebody can give me advice how to step forward.
I am building a constant current driver for laser diode, and I have to solve how to switch on and off the laser diode at a rate of 40 kHz (square wave). Schematic is attached and 2 pictures also that show my problem. In pic-2, you can see that I have an accepable square wave at 10 kHz. Because of the initial instability of the op-amp, there is a little overshoot. Pic-1 shows the same at 40 kHz, unfortunately it is not an accepable square wave.
Can you suggest solution how to make the laser diode current a better square wave? I tested with a simple LED with the same result, so not the laser diode is the issue. I also tested with different C2 capacitor values from 18pF to 100nF, with small capacitor there is a huge overshoot and oscillation, with a 100nF capacitor the rise time is very high.
How can I generate a better sqware wave?
Op-amp is LTC2051.

Thanks in advance!
 

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FvM

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How did you design the circuit? The large base resistor is creating a pole with transistor input capacitance and slowing down the feedback loop. A slow power transistor with 3 MHz Ft is probably not the best coice for a few 100 mW driver power, its beta cut-off frequency is in the 10 kHz range.

I'd suggest to use a circuit simulator, e.g. LTspice to optimize the driver.
 

nagyan

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Thanks, the problem was the 10k base resistor. I replaced with a 1k resistor and it works fine.
 
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