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This paper describes a GaN monolithic microwave integrated circuit (MMIC) cascode feedback amplifier design which achieves up to 8 W of output power and greater than +51 dBm OIP3 across a 250-3000-MHz decade bandwidth. The LNA also achieves 20 dB of flat-gain across the band. The design was fabricated with a 0.25-μm GaN HEMT technology with an fT ~ 50 GHz and a BVgd >; 60 V. A 40-V 750-mA high-bias LNA design achieves an OIP3 of 51.9 dBm, P1dB of 38.5 dBm, and NF ~ 3 dB at 2 GHz . A 40-V 500-mA medium-bias LNA design achieves a lower NF ~ 2.5 dB , an OIP3 of 48.4 dBm, and a P1dB of 36.8 dBm at the same frequency. At an optimum low-noise bias of 20 V and 300 mA, a NF ~ 0.96 dB, an OIP3 of 43.4 dBm, and a linear P1dB of ~32.2 dBm was also obtained. The combination of high OIP3 and low NF from these GaN MMIC LNA designs exceed that achieved by many state-of-the-art PHEMT, HBT, and HFET technologies for decade-BW MMIC amplifiers operating in the popular wireless and wire-line S- and C-band frequency ranges. The linear GaN LNA performance demonstrated here can enable new generations of software-defined and reconfigurable radios which require ultra-linearity over multiple octaves of bandwidth.