Optimization of the two-stage common-emitter transistor amplifier for equalization circuit in visible light communication system

The equalization circuit in the visible light communication (VLC) system has large impacts on the frequency response and can greatly improve the 3-dB modulation bandwidth. The optimization of the two-stage common-emitter transistor amplifier for equalization circuit is presented in detail and the design rules are disclosed. At first, the frequency response of the single-stage amplifier is derived and the simulated characteristic curves illustrated the changing regularity of four main factors are given. Subsequently, the frequency response of the two-stage amplifier is investigated by simulation. It is shown that the capacitors in the resistance-capacitance (RC) network has large impacts on the low-frequency range, while the resistors in the RC network can adjust the high-frequency, and the resistors in the emitter can adjust the low-frequency. Experimental measurements of the fabricated equalization circuit demonstrated the simulation results. Then, the two-stage amplifier is induced as post-equalization circuit (post-EQC) or pre-equalization circuit (pre-EQC) and the measured frequency response curves are given. The equalization circuit with proper sets of capacitors and resistors would flatten the frequency response curve of the VLC system. It is shown that the 3-dB modulation bandwidth of the VLC system equipped with post-EQC or pre-EQC can be expanded to 292MHz or 304MHz, respectively. Moreover, if the photoelectric receiver and the post-EQC are combined in the optimization, the 3-dB modulation bandwidth can be expanded to 375MHz.