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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
Not sure I understand your question but I'll try.

The output resistance is what you see looking back into the circuit at the junction of RC and the collector of the transistor.  From an ac point of view the collector circuit is in parallel with the RC; however the collector is seen as a very high resistance when compared with a typical value of RC hence the approximation is to ignore the contribution of the high resistance of the collector segment and assume the output resistance would be just RC.

How are they in parallel?  AC wise they are both connected to ground because the B+ voltage is ac ground.

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Electronics questions about AC, DC and digital theory.

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Graduate electrical engineer with over 40 years in electronic design, manufacturing, project organization and patent review. Experience in fields of industrial and consumer electronics (audio, video, acoustics, etc.)

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IEEE (Institute of Electrical and Electronics Engineers); Senior Life member AES (Audio Engineering Society), Fellow Life member

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BSEE University of North Dakota

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