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| David Hess:
The transistors acting as photodiodes add shunt capacitance at the inverting input of the NE5532 operational amplifier destabilizing it. A capacitor across the feedback resistor cancels the shunt capacitance but also lowers bandwidth. There are application notes about how to improve performance of that circuit configuration. Search for photodiode transimpedance amplifiers. |
| StillTrying:
--- Quote from: JS on September 13, 2018, 03:16:23 am --- -3dB point a bit over 350kHz. --- End quote --- With NE5532s in simulation that's the about the best I could get, with NE5532 and opto I think that might be the limit. I've speed tested a cny1711 opto, just it's base-collector junction. It's over 10 times slower than a separate LED and photo diode, even a white LED + PD is 10 times faster than the opto. |
| JS:
Thanks for the ideas, I'll try them out when I find some time on my hands... --- Quote from: StillTrying on September 18, 2018, 11:53:32 pm --- --- Quote from: JS on September 13, 2018, 03:16:23 am --- -3dB point a bit over 350kHz. --- End quote --- With NE5532s in simulation that's the about the best I could get, with NE5532 and opto I think that might be the limit. I've speed tested a cny1711 opto, just it's base-collector junction. It's over 10 times slower than a separate LED and photo diode, even a white LED + PD is 10 times faster than the opto. --- End quote --- Are you suggesting discrete LED and photodiode are faster than the collector base junction as photodiode? JS |
| BrianHG:
--- Quote from: JS on September 19, 2018, 03:59:38 am ---Thanks for the ideas, I'll try them out when I find some time on my hands... --- Quote from: StillTrying on September 18, 2018, 11:53:32 pm --- --- Quote from: JS on September 13, 2018, 03:16:23 am --- -3dB point a bit over 350kHz. --- End quote --- With NE5532s in simulation that's the about the best I could get, with NE5532 and opto I think that might be the limit. I've speed tested a cny1711 opto, just it's base-collector junction. It's over 10 times slower than a separate LED and photo diode, even a white LED + PD is 10 times faster than the opto. --- End quote --- Are you suggesting discrete LED and photodiode are faster than the collector base junction as photodiode? JS --- End quote --- Yes, take a look at the spec on this photodiode: https://www.digikey.ca/product-detail/en/finisar-corporation/P850-2124-001/P850-2124-001-ND/4416177 It has a rise and fall time of 30 PICOSECONDS, however, it is pricey at 23$. This makes an optocoupler look like watching paint dry by comparison. Ok, for a reasonable priced photodiode: https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/SFH-2701/475-2967-1-ND/2794398 At 87cents, and a 2ns rise and fall time, it makes the optocoupler look like a snail speed device. And there are 50k units in stock. I'm sorry to say, but, 2 blue leds, or 2 high speed 820nm IR leds, with 2 of these 87cent photodiodes in a push pull drive can reach beyond 200mhz bandwidth with a proper discrete pre-amp on the IR led, and similar for the photodiode. However, you will need to mount them in a small black straw/heat shrink tube. Though, with a little space, a narrow beam blue LED, you can now have 10s of KV isolation. You might make it to 500mhz bandwidth with a laserdiode... |
| StillTrying:
--- Quote from: JS on September 19, 2018, 03:59:38 am ---Are you suggesting discrete LED and photodiode are faster than the collector base junction as photodiode? --- End quote --- Well that's what I've found in one opto test in practice. In simulation with the 4N25 I can get passed the 1 - 2us speed limit by using just the base-collector photo diode part and get the analogue BW up to 10M. But in practice with a cny1711 the 1-2us speed limit seems to be on the b-c photo diode itself, even 8V reverse bias made no noticeable difference to the rise time. I had to make the opto's pulses 5us and 10us wide to match the shape of the 0.5us and 1us pulses from a 5mm super bright white LED and SFH213 PD - so opto's PD was ~10X slower! It mostly matches what you've found, a simulated BW of 2M becomes closer to 200K in practice, although I still think the NE5532 is a limit. A super bright white ~1MHz looks the easiest ATM, it might have 250ns rise and fall times but it reaches the flat top of the 1us pulse before the red, and is 10-100X brighter. A super bright green, red or cool white might be faster, but I don't have any loose ones to test - yet. I'll probably test the linearity of the super bright white around 3mA - 30mA soon. I'm expecting the open loop linearity to be very good, and the BW ~1MHz. :-DD The blue trace is the 1us through a super bright white. |
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