Abstract: This article designs a Darlington amplifier with low additional phase noise. In this paper, the basic concepts of the noise figure and the additional phase noise of the amplifier are first introduced. Secondly, the relationship between the two is explained through the formula derivation. Finally, the additional phase noise of the amplifier is reduced through process selection, circuit optimization, and other aspects. The amplifier was designed, simulated, and fabricated by GaAs Heterojunction bipolar transistor (HBT) technology, with a chip size of 0.6 mm×0.6 mm×0.1 mm. The test results show that under the condition of a power supply voltage of +5 V, The amplifier has a current of 60 mA, operating frequency of 0.01 GHz~2.00 GHz, the P1 dB of 19 dBm, noise figure of 2.7 dB, and the additional phase noise at the operating frequency of 100 MHz is -162 dBc/Hz@1 kHz, -170 dBc/Hz@10 kHz, -172 dBc/Hz@100 kHz.