Switched 4-to-1 Transimpedance Combining Amplifier for Receiver Front-End Circuit of Static Unitary Detector-Based LADAR System  
[PERSON]  
1 Department of Electronics Engineering, Chungnam Nation University, Daejeon 34134, Korea;  
[EMAIL_ADDRESS] (E.-G.L. ); [EMAIL_ADDRESS] (J.-E.L.); [EMAIL_ADDRESS] (B.C.J.) Electronics and Telecommunication Research Institute, Daejeon 34134, Korea; [EMAIL_ADDRESS]  
[PERSON]  
1 Department of Electronics Engineering, Chungnam Nation University, Daejeon 34134, Korea;  
[EMAIL_ADDRESS] (E.-G.L. ); [EMAIL_ADDRESS] (J.-E.L.); [EMAIL_ADDRESS] (B.C.J.) Electronics and Telecommunication Research Institute, Daejeon 34134, Korea; [EMAIL_ADDRESS]  
[PERSON]  
1 Department of Electronics Engineering, Chungnam Nation University, Daejeon 34134, Korea;  
However, an increase in the area of the photodetector results in a decrease in the bandwidth of the receiver due to the large parasitic capacitance of the large-area photodetector. To overcome this problem, the STUD-based LADAR receiver has multiple partitioned photosensitive cells, as shown in Figure 2. Each of the partitioned cells has its own transimpedance amplifier (TIA) to receive and amplify the optical current from each partitioned cell independently, and then a signal combiner sums all the outputs of each TIA into a single output signal STOP, which indicates the arrival of the return signal. A time-to-digital converter (TDC) calculates the TOF between the START and STOP signals. Since each partitioned cell with its own cascading TIA operates independently without affecting any of the other cells, its bandwidth remains unchanged. In addition, since the STUD-based LADAR receiver does not need to determine which cell detects the arriving laser pulse, inter-channel-interference is not a problem, unlike in the FPA-based LADAR receiver. To implement a STUD-based LADAR receiver, the same number of TIAs as partitioned photosensitive cells is needed, and they are assembled on a single board. The pad pitch in the partitioned photodetector is totally different from the pad interval of the TIAs. In case the lengths of the interconnection lines between each photosensitive cell and the corresponding TIA is different, accurate time information cannot be obtained because the time delay varies depending on which photosensitive cell receives the return signal. Therefore, the electrical length of the interconnection lines between each photosensitive cell and the corresponding TIA should be designed equal on the test fixture. This limits the number of cells for higher-resolution 3D images over a large ROI due to the interconnection problem between a partitioned photodetector and multiple TIAs [14; 15]. To resolve this problem, a 4-to-1 transimpedance combining amplifier (TCA) was proposed in our previous work [14] as the  
Figure 1: Block diagram of the static unitary detector (STUD)-based laser detection and ranging (LADAR) system. Figure 2: STUD-based LADAR receiver with partitioned photodetector. front-end readout integrated circuit (ROIC) for the STUD-based LADAR receiver with a photodetector, which has four photosensitive partitioned cells. In this study, we propose a switched 4-to-1 TCA. The switched 4-to-1 TCA has a switch on each input current path, as shown in Figure 3. The photodetector in this work has four photosensitive cells. The target size of a single photosensitive cell is 350 \(\upmu\)m \(\times\) 100 \(\upmu\)m and the parasitic capacitance of the single cell is assumed to be 1 pF. In the STUD-based LADAR receiver, it is necessary to increase the photosensitive area of the photodetector in order to enlarge the ROI in the STUD-based LADAR receiver. Meanwhile, this increases the noise of the receiver due to the large-area photodetector. The proposed switched 4-to-1 TCA can be used, as shown in Figure 4, in the STUD-based LADAR receiver front-end. According to the position where the returned laser pulse arrives, one of the TCAs is switched on to receive the optical current and the others are not connected to the photodetector.