IR Photodiodes unknown Behavior

[Arsh ahmad]

I am working on a Li-Fi Project, for the upload part, I am using infrared LEDs and Photodiodes and an OTA for obtaining the output of photodiodes. I have got some generic black cap Photodiodes, But only one of them generates a significant amount of current when light falls on the others don't generate any current. I do test them they work fine when use in a voltage divider configuration but they don't generate enough current that can be sensed by the OTA.

[Whales]

Are the photodiodes all different parts?  Perhaps most of them are actually phototransistors and only one is a photodiode?

[Arsh ahmad]

I purchased all of them at the same time, they all are photodiodes.

[Kleinstein]

Not generating much current, but letting through more current when biased very much points to a phototransistors.

The different should also be visble from the dark current voltage relationship or just the diode test from a DMM.  A phototrasistor would test open a photodiode more like a normal diode. The NIR photodiodes are just silicon ones with filter on top to block visible light.

For later use it makes sense to also bias the photodiode to get less capacitance and this more speed.

[Arsh ahmad]

I need to use the Photodiodes for an OTA in a Li-Fi receiver working at high speed, can you suggest any method how to get a usable output from such a low current.

[Kleinstein]

Phototransistors are usually faster than phototransistors, but the signal is usually smaller. Even the phototransistor may not be so bad when used in a low impedance circuit and not going to saturation.
How fast is high speed ?
How much signal is actually there ?

The usual circuit for a fast response is a TIA or similar circuit and with some bias voltage to redue the capacitance.
This may need a quite fast amplifier, with a GBW way higher than the signal frequency and a relatively low resistance to get the speed at the costs of somewhat less than ideal sensitivity.

[moffy]

I am sure you meant "Photodiodes are usually faster than phototransistors" rather than the typo.

[Arsh ahmad]

Speed is 300khz, and the signal is digital data. I checked with my multimeter they don't output measurable current the lowest range of my DMM is 20uA.
The opamp that I am using is ADA4891 which has a slew rate of 160V/us.

[mikerj]

For high speed use you need to be using the photoconductive mode (i.e. reverse biased) rather than photovoltaic mode. You need to bias the diode appropriately and maintain that bias over the expected range of signal powers, which is why a TIA (transimpedance amplifier) is the preferred method of handling this.

[Arsh ahmad]

If possible , can you please share a rough circuit

[mikerj]

This is a TI app note on TIA design for photodiodes: https://www.ti.com/lit/ug/tidu535/tidu535.pdf