A 700Mbit/s CMOS capacitive feedback front-end amplifier with automatic gain control for broadband optical wireless links

Optical wireless (OW) communication links are emerging as a promising broadband wireless access technology. This paper presents a transimpedance-limiting amplifier (TIA-LA) analog front-end to overcome challenges faced by high-speed optical wireless links requiring wide field-of-view (FOV), high dynamic range imaging receivers for signal acquisition/tracking. The front-end amplifier presented consists of a transimpedance amplifier combined with a limiting amplifier and, for the first time, employs a self-biased automatic gain control (AGC) circuit within a capacitive feedback configuration TIA. The front-end is designed for integration with wide FOV detectors with associated capacitance up to 5pF and its performance is insensitive (within 10%) to an order of magnitude variation in input capacitance. The self-biased AGC allows 40 dB dynamic range in received optical power to accommodate varying link ranges and different OW system configurations. The TIA-LA achieves a maximum total gain of 66 dBOmega, -3 dB bandwidth of 540 MHz at 5pF detector capacitance, and variable TIA gain from 53 dBOmega to 42 dBOmega without instability. The expected sensitivity of this optical receiver front-end is -20.1 dBm at a BER of 10^-12 and total power dissipation of 209 mW from dual 3.3 V and 1.8 V supply voltages in a 0.5 um CMOS process.