EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: mr_byte31 on August 12, 2024, 01:22:01 pm
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Hi Folks,
I am reading more about transimpedance amplifier circuits. I am interested about a circuit with single supply.
I found this circuit in TI application AN682 (https://ww1.microchip.com/downloads/aemDocuments/documents/APID/ApplicationNotes/ApplicationNotes/00682D.pdf)
(https://i.postimg.cc/Gpn8JTXv/Screenshot-2024-08-12-at-3-12-39-PM.png)
I understand that the I_D current in the picture will be converted to voltage for R2.
My understanding was to always connect the photodiode in reverse bias to make it sensitive to light and decrease capacitance.
I wonder how this photodiode works. I can see terminals of the photodiode connected between ground and virtual ground. how that will make the photodiode work ?
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Each photodiode is also a solar cell - it just turns light into current.
Applying some reverse bias can make the circuit more sensitive but also more complicated. But the shown circuit is a good starting point.
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My understanding was to always connect the photodiode in reverse bias to make it sensitive to light and decrease capacitance.
No, no, and yes, respectively.
Search terms: photovoltaic mode, photoconductive mode.
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Here is a quite good overview about usage of photodiodes:
https://www.teamwavelength.com/photodiode-basics/ (https://www.teamwavelength.com/photodiode-basics/)
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Photovoltaic mode (zero bias, as drawn): higher capacitance, no dark current, medium shunt resistance (temperature dependent).
Photoconductive mode (reverse bias): lower capacitance, appreciable dark current, high shunt resistance.
In circuit, there are noise contributions from the dark current and the shunt resistance. The capacitance increases the noise gain at high frequencies, affecting the noise contribution from the op amp.
Similarly, the offset drift results from the dark current and the DC noise gain determined by the resistances of the diode and feedback resistor operating on the offset drift of the op amp.
Note that the slope of the I-V curve around zero volts bias is a finite resistance.