虚短、虚断、深度负反馈、反馈系数的量化讨论

Quantitative Discussion on Virtual Short,Virtual Open,Deep Negative Feedback and Feedback Coefficient

无条件地对集成运放使用“虚短虚断”会导致错误的结果,而现有资料讨论甚少。基于负反馈理论并结合放大器模型,推导出串联负反馈和并联负反馈时的虚短和虚断成立条件:始于电压的反馈环内增益远大于1,则虚短;始于电流的反馈环内增益远大于1,则虚断。由此建立虚短、虚断、深度负反馈、反馈系数的量化关系。放大电路的输入电阻会减小反馈系数,具体计算反馈系数时,要考虑放大电路输入电阻的影响。集成运放负反馈放大电路中,“虚短虚断”未必成立;其他负反馈放大电路中,“虚短虚断”也可以成立。“虚短虚断”是特定深度负反馈的特性,它们不应成为与集成运算放大器紧密相关的特性,二者应分开讨论,即使是集成运放,虚短和虚断也不一定同时成立。

The unconditional use of“virtual short and virtual open”for integrated operational amplifiers can lead to wrong results,but little discussion on this issue can be found on current materials.In combination with the amplifier model,the establishment conditions of virtual short or virtual open of series or parallel negative feedback are deduced on the basis of negative feedback theory:if the gain in the feedback loop starting from voltage is far greater than 1,then there is virtual short;if the gain in the feedback loop starting from current is far greater than 1,then there is virtual open.Therefore,a quantitative relationship can be established among virtual short,virtual open,deep negative feedback and feedback coefficient.As the input resistance of amplifier circuit can reduce the feedback coefficient,its influence should be considered when calculating the feedback coefficient.The virtual short and virtual open might not happen in integrated operational negative feedback amplifier circuit but they might be established in other negative feedback amplifier circuits.Although virtual short and virtual open are characteristics of some specific deep negative feedback,they should not be closely related to integrated operational amplifier.The two should be discussed separately because“virtual short”and“virtual open”are not necessarily true at the same time even with integrated operational amplifier.