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In this paper, differential low-noise amplifiers are presented as a very powerful solution for radio astronomy applications. A fully differential amplifier topology has been analyzed and implemented in microstrip technology with discrete surface mount components. The amplifier design is made for an active receiving dense antenna array. Thus, the differential amplifier source impedance is no longer 50 Ω, but 150 Ω from the proposed bunny-ear antennas. A full characterization in terms of gain and noise has been undertaken. Source-pull measurements have been included in order to evaluate the performance of the amplifiers operating with variable source impedances. Noise temperatures below 55 K have been obtained for the differential design in the 300-1000-MHz band for the 150-Ω impedance. In addition, the results for different scanning angles are also presented.