# TechReport: Why photodiode needs OP-AMP ## Describe We always read: photodiode always follow OP-AMP, such as ![](https://i.imgur.com/WnedJB0.png) and ![](https://i.imgur.com/Y8zJKjj.png) Why do we rarely see this circuit? ![](https://i.imgur.com/5c3380P.png) # Solution * Linear * Refer to [here](https://electronics.stackexchange.com/questions/654619) > When the photodiode absorb the energy, Vout rise of course. Since my circuit contain 2 only component(photodiode and resistor) and energy conservation -> current of photodiode and resistor must be the same -> Voltage of photodiode decline > In the op-amp(TIA) case, photodiode connect between V- of op-amp and GND(or reverse bias), since virtual ground, voltage across photodiode is V- - GND(or reverse bias), it's constant(linear). ![](https://i.imgur.com/3MoSfEM.png) ![](https://i.imgur.com/r9hoAUH.png) * Buffer out * See [Loading effect](https://en.wikipedia.org/wiki/Electrical_load) * CMRR, Speed-up... etc. features of OP-AMP ###### tags: `TechReport` `DeWei`