Relative sensitivities of voltage and current mode photo-diode front-ends
 



This is something that I should have looked at a long time ago, but that just had to done once the idea of putting datv over optical was suggested. It simply amounts to building a current mode detector using the same photo-diode that is used in the voltage mode front-end, and comparing the two. For interest, the two are also compared against a voltage mode LED receiver (the Finningley Mk II front end). Each graph also shows the no-signal noise level output.

The obvious difference is that whilst the current mode receiver frequency response is very flat, its sensitivity is very very poor.

An interesting characteristic of the voltage mode detectors is that noise floor falls off at a similar rate to the signal gain fall, so s/n reduction with increasing frequency is nothing near as bad as might have been expected. 

a) current mode, SFH-213 photo-diode

voltage
                      mode current
                      mode det

b) voltage mode, SFH-213 photo-diode

voltage
                      mode voltage
                      mode detector


c) voltage mode, red LED

voltage,
                      red LED red LED
                      voltage mode detector



 

Finished....