# A Current-Mode Optoelectronic Receiver IC for Short-Range LiDAR Sensors in 180 nm CMOS
## 4 Proposed Current-Mode Optoelectronic Receiver (C-OER)  
Voltage-mode TIAs can be preferred for long-range LiDAR sensors because of their low-noise characteristics. In particular, inverter TIAs (INV-TIAs) have been frequently exploited because of their inherent advantages such as easy design, no additional bias circuitry, high gain, low noise, etc. However, they may suffer from considerable degradation of bandwidth and noise performance because the photodiode capacitance (Cpd) may vary significantly depending upon the amplitudes of the incoming input currents [10]. Hence, we employ a current-mode TIA in this work as a front-end circuit to facilitate the aforementioned issues of the photodiode capacitance. Basically, a current-mode TIA functions as a current-buffer, provides a small input resistance, and thus helps to isolate the photodiode capacitance from the determination of the circuit bandwidth. Common-gate architecture is a conventional example of a current-mode TIA, which however suffers from the significant loss of currents because the input resistance (-1/gm) cannot be indefinitely reduced [10]. To the contrary, the symmetric current-conveyor input buffer (CCIB) architecture is another example of a current-mode TIA, which can prevent the loss of input currents effectively since the modified cascode input configuration yields a large output resistance. Figure 12 illustrates the block diagram of the proposed current-mode optoelectronic receiver (C-OER), which consists of an on-chip APD to generate photocurrents, a parallel arrangement of five current conveyor input buffers (CCIBs) that can separate the parasitic photodiode capacitance from the cascode inverter stage (CIS) effectively, a single-to-different (S2D) converter for differential signaling to improve common-mode rejection ratio characteristics, a second-stage differential amplifier (DIFF) for further gain-boosting and improved output symmetry, a cross-coupled inverter-based post-amplifier (CI-PA) to boost the output swing and reduce the mismatches between differential pulses, and an output buffer (OB) for 50 \(\Omega\) impedance matching. Figure 13a depicts the schematic diagram of a single CCIB stage with a modified NMOS cascode circuit (M1\(\sim\)M4) incorporated into its PMOS counterpart (M5\(\sim\)M8). Consequently, half (i.e., iPD,n) of the input current flows through the NMOS cascode circuit (M1 and M2), while the other half (i.e., iPD,p) flows into the PMOS cascode circuit (M5 and M6). Therefore, the total input photocurrent (iPD) from the photodiode can be almost symmetrically split into two paths. Furthermore, the two currents (iPD,n and iPD,p) are mirrored to the circuit, comprising M3, M4, M7, and M8. The mirrored currents can be summed and flow out of \begin{table}
\begin{tabular}{c c c c c c} \hline
**PARAMETERS** & **[4]** & **[11]** & **[12]** & **[13]** & **V-OER** \\ \hline CMOS technology (nm) & 180 & 350 & 180 & 180 & 180 \\ \hline APD & On-chip & Off-chip & Off-chip & Off-chip & On-chip \\ \hline Input configuration & VCF & SF & SF with FC & SF & VCF \\ \hline Bandwidth (MHz) & 790 & 230 & 281 & 450 & 608 \\ \hline TZ gain (dB\(\Omega\)) & 93.4 & 100 & 86 & 100 & 95.1 \\ \hline Noise current spectral density (pA/sqrt(Hz)) & 12 & 6.32 & 4.68 & 2.59 & 4.54 \\ \hline Min. detectable current & 6.74 & 1.0 & 2.0 & 2.5 & 2.38 \\ (mA\({}_{\text{App}}\)) & (SNR = 10) & (SNR = 5) & (SNR = 25) & (SNR = 5) & (SNR = 10) \\ \hline Single-pulse width & 25 ms & 3 ns & 3 ns & 2 ns & 5 ns \\ \hline Optically measured pulse (mV\({}_{\text{PP}}\)) & 8 & 200 & 110 & 300 & 200 \\ \hline Power dissipation (mW) & 56.5 (w/OB) & 180 (w/TDC) & 200 (w/PA) & 6.6 (w/o OB) & 39.3 (w/OB) \\ \hline Core area (mm\({}^{2}\)) & 0.09 & 14 & 2.20 & 4.08 & 0.068 \\ \hline \end{tabular}
\end{table} 
Table 1: Performance comparison with previously reported voltage-mode CMOS TIAS. the feedback resistor (\(R_{F}\)) in the following CIS. Consequently, a positive transimpedance gain can be provided, which is the opposite to the case of the previous V-OER.