Photodiode

This is the opposite of a LED. We apply a reverse bias to it; normally the current is 0, but when photons come in, we will get electron flow resulting in a current.

• Dark: very small amount of reverse current under reverse bias ($\text{ nA}$)
• Light: signal current ($\mu \text{A}$)

Essentially, we can think about these as controlled current sources that are proportional to incoming light intensity.

We need to the signal current from $\mu \text{A}$ to $\text{V}$, so we need an amp with gain in $[\frac{V}{A}]$. This is where the name for “trans resistance amplifier” or “trans impedance amplifiers (TIA)” comes from.

TIA circuit:

We have:

$$\begin{array}{rl}{V}_{out}& =V_{Bias}+V_R\\ & =V_{Bias}+I_{D}R\end{array}$$