英语翻译As previously explained,the input resisitence is virtual

英语翻译
As previously explained,the input resisitence is virtually an open circuit for the FET,and it is equal to R for the BJT.The transconductance model is a simplified version of an equivalent circuit known as the hybird_∏ model,which takes into account equivalent circuit elements which affect the high frequency performance of the transisitor.In the present lesson,frequency effect at both high and low frequencies are ignored,but the simplified annalysis is useful over a range of frequencies termed the mid_frequency band,which is the frequency region og interest for general_purpose applications.