Hi, I am working on high frequency models of the bjts and I am having problems with some calculations. I've attached 2 pics. As it is seen Th, high freq. time constant, equals to the sum of each capacticor multiplied with their seen resistance from their terminals. My problem is with the Cu capacitance. Firstly when checking for equivalent resistance shouldn't we short the voltage source. If so than the cotrolled current gmVpi should be 0 and thus become open. Then how Cu's terminals sees this resistance value R'sig(1+gmR'L)+R'L. What is it that I am missing here???? Is there some other way for estimating this equivalent resistance issue? I also put an Itest(test current source) replacing with Cu and got quite strange result same as the given Cu's equivalent resistance with only R'sig and R'L interchanged positions.
Appriciate any help
Appriciate any help
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