This paper presents a novel design of a gain-flattened amplifier that covers an ultra-wide band including the S, C, and L bands. The amplifier has a split-band architecture, which amplifies the S-, C-, and L-band signals in separate paths and then recombines the amplified signals for the final output. Using a commercial software package VPI Component MakerTM, the design was simulated and tested for optimization. The gain and noise figure of the new amplifier were analyzed by computer simulation. A gain of ≈29 dB, a ripple factor of about 1.7 dB, a nominal low noise figure <5 dB, and gain-bandwidth products >120 nm can be achieved.