Electrical Engineering/transistor

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Question
bjt
bjt  
in the given picture they said RC||r'c ,but r'c is so large
why r'c is so large and how they are parallel?

Answer
RC is connected to ground through the power supply voltage.

The resistance of collector circuit inside the grey area is a current generator which by definition is a very high value if not infinite and is connected, ac wise, through r'c  to ground.  Thus taking output off the junction of collector and RC the output resistance is essentially just RC because the parallel of collector resistance can be ignored since it is much higher in value than RC.

Does this help.  Happy transistor amplifier study.  

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