为研究反馈技术对宽带放大器增益平坦度的影响，从异质结晶体管（HBTs）的小信号模型入手，得到几种不同反馈形式下HBTs的S参数解析表达式．分析不同反馈形式对S参数的影响及传输增益S21的幅度变化．在这几种反馈技术中，纯电阻负反馈可以改进整个超宽频带内的反射，实现宽带的输入输出匹配；引入串联电感或并联电容等电抗元件，采用串联电阻和电感或并联电阻和电容的电抗性反馈可使S21的幅度随着频率的增大而逐渐上升，从而补偿晶体管本身高频增益的下降，同时也抵消电阻负反馈引起的增益下降．综合优化电阻反馈和电抗反馈，能实现低噪声放大器在3．1—10．6GHz范围内的增益平坦性．%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.
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