# Free Space Optic Receiver with Strongly Overlapped Photodetectors' Field of View
## 2 Free Space Optic (FSO) Receiver Design
### Selecting a Suitable Follow-Up Amplifier  
Numerous aspects do not allow the application of all the design rules, which lead to the brief description of a suitable follow-up amplifier. However, we attempt to provide a description by citing other literature references. Only the photodetector and the follow-up amplifier create the first receiver stage. Hence, their amplification should be as high as possible to eliminate noise in successive stages. They are derived in, for example, [PERSON], [PERSON] et al. [31]. This first stage is usually called a PDA (photodetector assembly). Another choice concerns amplifier topology: either direct or feedback topology, shown in Figure 6a,b, respectively. The names of components are as follows: \(PD\) is the photodiode, \(U_{ps}\) is the power supply voltage, \(C_{ps}\) is power supply decoupling capacitance, and \(R_{L}\) is load resistance. For variant (a), the amplifier input impedance \(Z_{a}\) is not expressed and is covered by resistance \(R_{L}\). Similarly, for variant (b), the resistance \(R_{L}\) is not expressed and is covered by feedback impedance \(Z_{fb}\). For direct topology, the \(K_{21}\) box is a bipolar or FET transistor; for feedback topology, it is a bipolar or FET operational amplifier. Due to the low noise requirements, any simple design is suitable. The direct amplifier topology (a) should be the best option, but it has many limitations [1]. The feedback amplifier topology, shown in Figure 6b, creates stability questions due to back feeding. \begin{table}
\begin{tabular}{c c c c c} \hline
**Photodiode** & \begin{tabular}{c} \(I_{dg}\) \\ **(Dark Room)** \\ \end{tabular} & \begin{tabular}{c} \(I_{0}\) \\ **(Inandescent Bulbs 200 Ix)** \\ \end{tabular} & \
\begin{tabular}{c} \(I_{0}\) \\ **(LED Lamps 400 Ix)** \\ \end{tabular} &
\begin{tabular}{c} \(I_{0}\) \\ **(Daylight 550 Ix *)** \\ \end{tabular} \\ \hline BPW34 & 62 pA & 26.6 pA & 4.18 pA & 19.3 pA \\ BP104F & 46.9 pA & 20.8 pA & 132.3 nA & 7.09 pA \\ \hline \end{tabular}  
* interior measurement without direct sunlight.  
\end{table} 
Table 1: Darkness generation current \(I_{dg}\) and total dark current \(I_{0}=I_{dg}+I_{pb}\) measurement results. The noise model of the PDA is the same for both circuit diagrams (Figure 6a,b). It has been proven that back feeding cannot change the signal to noise ratio of a PDA [1]. The noise model showing a photodiode and follow-up amplifier with their passive network components is shown in Figure 6c. In the photodiode, \(I_{p}\) is the useful signal photocurrent, \(2 qI_{0}\) is the photodiode noise current density, and \(R_{p}\) and \(C_{p}\) are internal photodiode resistance and capacitance, respectively. In the amplifier, \(R_{a}\) and \(C_{a}\) are amplifier input resistance and capacitance, respectively (previously included in \(Z_{a}\)); \(2 qI_{a}\) is the amplifier input noise current density; \(4 kT\left(\Psi/G\right)\omega^{2}C_{in}{}^{2}\) is the amplifier output noise current density calculated for the amplifier input; \(Gu_{1}\) is the amplifier useful signal output current; and \(R_{ph}\) and \(C_{ph}\) are feedback (load) resistance and capacitance, respectively (previously included in \(Z_{ph}\)). The stray capacitances (e.g., the PCB capacitance) are not introduced explicitly and may be included in \(C_{ph}\).