optical apd rx1

As it turns out, many types of LEDs appear to exhibit gain effects akin to those of an avalanche photo diode when reverse-biased - an effect that has been observed in the 25-300 volt area with the voltage, spectral response, and overall sensitivity/gain depending on the LED`s construction and material. It is possible to adapt some of the circuits
optical apd rx1 - schematic

on this page to be used with an LED for experimentation of this gain mechanism. Not surprisingly, the degree to which this effect occurs and at what voltage tends to vary wildly from device to device as this is not a mode in which LEDs were intended to operate! The stability and long-term viability of operating an LED this way is another factor that should be considered. While the results do vary, some LEDs have been noted to offer reasonable performance as gain-enhanced detectors when operated in this mode making it practical to have at least moderate performance as a detector - a particular attraction if one wants to build an optical transceiver that uses just one diode for both receive and transmit. While an interesting phenomenon, tests thusfar seem to indicate that, at least for baseband (audio) use, the ultimate sensitivity of a typical LED operated in this mode is worse than that of a well-designed optical detector using standard PIN photodiodes. Having said this, this avalanche effect can be used to good effect to make an optical transceiver that uses the LED for both transmitting and detection and yield a single, compact package with reasonable performance. I`ve not seen "ultimate sensitivity" comparisons with PIN photodiode-based receivers and avalanche LED receivers for subcarrier (ultrasonic and up) operation. The circuits described on this page can form the basis of an optical receiver utilizing the avalanche...

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