## 2 Analysis of Photodetector Partitioning Cell Method  
The block diagram of the proposed STUD-based LADAR receiver embedding the PPC method is illustrated in Figure 3. The TICA has multiple input current buffers, as many as partitioned photosensitive cells, and the input current buffer is the same as for the conventional TIA. The output signals of each input current buffer are combined into a single output signal (\(V_{PDT}\)) that is amplified using a post-amplifier (\(V_{TICA}\)). The TICA consists of four main blocks: the over-current protection (OCP) circuit, input current buffer, signal combining chain, and post-amplifier. The OCP prevents the fabricated chip from being destructed by a high optical power input signal. The input current buffer acts as a low-impedance input stage to receive optical current. The signal-combining chain sums up the all outputs from the input current buffers. The post-amplifier is designed to maintain the bandwidth and to enhance the transimpedance gain. In order to verify the prototype chip, the output of the post-amplifier is extracted as the output of the TICA from a balun and an output buffer. In order to implement the PPC method, the effective number of partitioned photosensitive cells (\(N_{IN}\)) and the number of signal-combining stages (\(M\)) have to be considered along with the required performance of the STUD-based LADAR sensor. Note that \(N_{IN}\) is the same as the number of input channels in the TICA as mentioned before. As the single input current buffer of the TICA is the same as for the TIA, \(N_{IN}\) can be determined along with the coverable parasitic capacitance (\(C_{PDS}\)) of the TIA. Once the target \(C_{PDS}\) is set in the first step, \(N_{IN}\) can be defined as follows:  
\[\frac{C_{PDT}}{C_{PDS}}\ =\ N_{IN}\ =\ 2^{M} \tag{1}\]  
Here, \(C_{PDT}\) is the total parasitic capacitance of the photosensitive cell. \(M\) is the binary-weighted number because \(N_{IN}\) has to be a multiple of twp for the optimum implementation of TICA at the circuit level [8]. In this work, \(C_{PDS}\) of 2 pF is targeted, so that a 16-to-1 TICA is proposed with \(C_{PDT}\) of 32 pF, resulting in a partitioning number \(N_{IN}\) of 16. The output signals of 16 input current buffers can be combined in forms of several combining patterns (\(N_{CI}\)) into an output signal of the TICA, \(V_{TICA}\). Figure 4 represents the number of signal-combining stages (\(M\)) according to the number of combining input current buffers \(N_{CI}\). As in conjunction with the formula expressed in equation (1), \(N_{CI}\) is a multiple of two. The number of signal-combining stages \(M\) decreases as the number of combining input current buffers \(N_{CI}\) increases. As each combining stage requires an additional static bias current, decreasing \(M\) is suitable for a low  
Figure 3: Block diagram of the STUD-based LADAR receiver embedding the PPC method. power design. When \(N_{CI}\) is selected as 16, all the output signals of input current buffers combine into the first combining stage in case of '\(M=1\)'. The signal summation of the first combining stage occurs at resistor \(R_{L}\), as shown in Figure 7 (explanation will be followed), defining the transimpedance gain of the first combining stage. Thus, the DC bias current of each input current buffer should be divided by \(N_{CI}\) to properly maintain the DC bias voltages of the first combining stage with a suitable transimpedance gain. As the DC bias voltage of each combining stage is set as the bias voltage of the next combining stage, the DC bias current of each combining stage can be calculated to a specific value. Figure 5 represents the simulated output peak voltages as functions of the input current and the number of combining input current buffers \(N_{CI}\). Here, the transimpedance gain is assumed to be approximately 70 dB-\(\Omega\).