This paper proposes a retrofit LED lamp for T8/18 W fluorescent lamp which can be driven by electromagnetic ballast, electronic ballast, as well as AC mains. The driving circuit of the proposed LED lamp is free from electrolytic capacitor and there is no EMI problem as well. A gain-control capacitor, bridge rectifier, current limit circuit, and power limit circuit are integrated into the proposed LED driver. The operating principles under three different input sources and design considerations are discussed and analyzed in detail. Finally, the experimental results of the 11.6 W retrofit LED lamp prototype with light output of 1050∼1260 1m are presented.