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在分析放大电路的上限截止频率时，通常用密勒定理对共射电路进行单向化等效，可简化计算过程。但是分析表明，密勒定理不能解释共基和共集电路的上限截止频率比共射电路高的原因。在网络的上限截止频率附近，用主极点函数来表示网络的传递函数，就可以用网络的开路时间常数和表示网络上限截止频率的时间常数。运用主极点方法分析放大电路的上限截止频率，一眼即可看出，共基和共集电路的开路电阻远小于共射电路的开路电阻，因此它们的开路时间常数和比共射电路的小，它们的高频响应效果自然优于共射放大电路。由此可见，用放大电路的开路时间常数和分析上限截止频率，物理概念清晰，它适用于各种放大电路的高频响应分析。
In analyzing the upper limit of the amplifying circuit cut-off frequency, usually using the Miller’s theorem of the co-beam circuit unidirectional equivalent to simplify the calculation process. However, the analysis shows that Miller’s theorem can not explain the reason why the upper-limit cut-off frequency of common-base and common-collector circuits is higher than common-emitter circuits. In the vicinity of the upper limit of the network cutoff frequency, using the main pole function to represent the transfer function of the network, we can use the open circuit time constant of the network and the time constant of the upper limit cutoff frequency of the network. Using the main pole method to analyze the upper limit cut-off frequency of the amplifier circuit, it can be seen at a glance that the open-circuit resistance of the common-base and common-collector circuits is much smaller than the open circuit resistance of the common-mode circuit, Their high frequency response is naturally better than the cascode amplifier. Thus, using open-circuit amplifier time constant and analysis of the upper limit cut-off frequency, the physical concept of clear, it is suitable for a variety of amplifier circuit of high-frequency response analysis.