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.