放大电路高频响应分析再探

Analysis of High frequency Response of Amplifier

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。

The unidirectional equivalence of common emitter circuit using Miller Theorem could simplify the computation procedure for the analysis of amplifier's upper cut off frequency Miller Theorem,however,could not interpret why the common base or common collector circuit has a higher upper cut off frequency than common emitter circuit It is known,by using the dominant pole function representation of network of transfer function,that around the network of upper cut off frequency the time constants could be represented by the sum of open circuit time constants A straightforward result from the same analysis method shows the open resistance of common base or common collector circuit is much more less than common emitter circuit,hence their high frequency response better It presents here the upper cut off frequency analysis of amplifier using the sum of open circuit time constants,which has clear physics meaning and is suitable for high frequency analysis of various kinds of amplifiers [