## 1 Introduction  
Light detection and ranging (LiDAR) technologies are widely utilized in various fields, including autonomous vehicles, robotics, indoor mapping, and home monitoring systems [1, 2, 3, 4]. Especially in long-term care facilities for elders with senile dementia, it is crucial to have compact, low-power, and cost-effective sensors that can monitor elders in real time with no violation of their portrait rights and alert caregivers in urgent situations or emergencies, such as falls or abnormal movements. Owing to the global and rapid increase in the number of aging societies, the maintenance costs associated with protecting elderly individuals from injuries (i.e., falls) have been exponentially increasing [5]. LiDAR sensors are quite advantageous in these scenarios, as they can maintain privacy via blurred images while delivering accurate data on activities, like movement and even breathing rates. In addition, they are less affected by ambient RF interferences when compared to conventional RF-based sensors. Typically, these systems are based on the pulsed time-of-flight (ToF) principle, which emits light pulses toward a target and captures the reflected signals to determine the distance based on the time interval between the transmitted and received pulses. Particularly for indoor applications such as home monitoring, these sensors' effective range spans from a few centimeters to several meters, thus employing a dynamic range that can handle various signal intensities and ensure a quick response. However, a wider dynamic range allows for more precise detection and, hence, a novel receiver circuit design is mandatory to obtain these wide dynamic range characteristics. For this purpose, automatic gain control (AGC) circuits have been frequently exploited in optical receivers, and various architectures of AGC mechanisms have been suggested [6; 7; 8; 9; 10; 11]. Figure 1a shows a block diagram of a typical optical receiver, where the AGC loop consists of a variable-gain amplifier (VGA), an equalizer (EQ), an output buffer (OB), a peak detector, and an error amplifier. Therefore, the peak voltages of the output pulses are detected and amplified to control the gain of the VGA, extending the dynamic range. However, due to this long feedback loop, it is impossible to detect and control the amplitude of a single narrow pulse properly. Very often, the transimpedance amplifier (TIA) output signals must pass through a passive low-pass filter before connecting to the gain control switches, thus introducing a significant delay and, hence, failing to achieve single pulse detection. Figure 1b presents a block diagram of the previously reported current-mode TIA with a single-pulse AGC function [11], where a mirrored current conveyor (MCC) input stage is inserted to monitor the incoming photocurrents, and the output voltages (IB_O) of the MCC stage are detected at the feedforward control-voltage generator (FCG) circuit. Then, the DC voltages of the FCG circuit are directly connected to the NMOS switches during the inverter transimpedance stage (INV-TIS) for gain control, thereby achieving AGC operations within a single pulse width. However, the MCC input stage includes five symmetric current-conveyor input buffers (SCC-IBs), giving rise to a large noise current spectral density, and thus deteriorating the optical sensitivity considerably. In this paper, a novel TIA topology is proposed to overcome the aforementioned issues in the application of short-range LiDAR sensors. Here, a rather simpler and more cost-effective AGC mechanism is realized by exploiting dual on-chip CMOS avalanche photodiodes (APDs). Figure 2 presents a block diagram of the proposed optoelectronic transimpedance amplifier (OTA). It features a main transimpedance amplifier (m-TIA) with an on-chip APD and a replica TIA with another on-chip APD. The latter yields low DC voltages (\(V_{\text{DC,AGC}}\)) to control the 3-bit PMOS switches in the m-TIA concurrently.