Abstract: The dependence of amplifier gain flatness on feedback technology is discussed in this paper.Based on small signal models of hetero-junction bipolar transistors(HBTs) with different feedback circuits,the expressions of their S parameter are derived to analyze the influence of different feedback technologies on amplifier performance,especially the amplitude of transmission gain S₂₁.A conclusion has been got that pure resistive feedback improves the reflection in the whole band and makes better input matching and output matching.When a serial inductor or a parallel conductor is added into the feedback circuits to induce reactive feedback,the amplitude of S₂₁ can increase as the frequency increases.As a result,the HBT's intrinsical gain reduction in high frequency is compensated,and the gain reduction induced by resistive feedback is also counteracted.Optimize the resistive feedback and the reactive feedback together,the gain flatness is realized in the whole frequency band of 3.1 GHz to 10.6 GHz.