I am designing a circuit for a strobe amplifier and am having some difficulty with the LED emitting light for longer than expected.

I am measuring the light out of the LED with a photo receptor module directly on a DSO and it tapers off over about 2ms from a 1us pulse.

I have attached the diagram of one of the circuits I have been working with, a basic emitter follower (the pulse is 0/5V from an AVR), but have tried other topologies, and also adding a shottky diode over the BE junction but to no avail.

Also the component numbers in the schematic are not exactly what I am using but representative. I have tried the BC337 and PN222A BJT's and an IN5817 shottky diode, and an unknown cheap red LED with approx. 2V drop.

I was hoping that someone would be able to explain why the long fall time for this particular circuit and perhaps suggest a solution.

Thanks in advance.



1 Answer 1


My suggestion is that the light is stopping within hundreds of ns and your "photo receptor" is almost 100% of the perceived problem.

A negatively biased PIN photodiode into a GHz transimpedance amplifier is one standard way to get response times down, for somewhat less performance you can also use a negatively biased PIN photodiode into a low resistance (like 100 ohms) and crank up the gain on your 'scope. Higher sensitivity means more PD acreage, which means more capacitance, which means you need a better circuit to keep it fast.

  • 1
    \$\begingroup\$ Cheers, it turns out it was because the photodetector was plugged into a 1Mohm oscilloscope but was rated for 50ohm so with an adapter it was working fine. \$\endgroup\$ Commented Jan 27, 2015 at 1:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.