---
# System prepended metadata

title: 'TechReport: Why photodiode needs OP-AMP'
tags: [TechReport, DeWei]

---

# 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`