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Photodiode questions

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Hi, everyone.

1. Should I think of a photodiode as being like a solar cell, that generates current from light, or should I think of it as a light-sensitive transistor, that conducts current only if a threshold amount of light is falling on it?

2. I have been told that it makes sense to connect a photodiode in reverse bias. Can someone explain what the point of this is? Wouldn't it make sense only to connect a photodiode in forward bias, and to have the rest of the circuit respond to light falling on the photodiode?

Richard
 
Hi, everyone.

1. Should I think of a photodiode as being like a solar cell, that generates current from light, or should I think of it as a light-sensitive transistor, that conducts current only if a threshold amount of light is falling on it?

2. I have been told that it makes sense to connect a photodiode in reverse bias. Can someone explain what the point of this is? Wouldn't it make sense only to connect a photodiode in forward bias, and to have the rest of the circuit respond to light falling on the photodiode?

Richard

hi,
When a photodiode is used in a reversed bias mode, the reverse leakage current increases with light intensity, a resistor connected in series with opto is used to develop a voltage across it due to the change in leakage.

In forward mode a voltage developed across the opto is proportional to light intensity.

Reverse mode operation is faster, fast switching.
Forward mode is has usually a more linear response, light meters etc.

There are photo detector diodes and photo detector transistors, diodes are faster, transistors more sensitive.
 
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