First Year Electronics

[Myna Mo]

Why is the I/P impedance of a CE transistor moderate and that of a CB low?

 

[crutschow]

Because the intrinsic base resistance is multiplied by the current gain (Hfe) in the CE configuration to give a higher effective input impedance but it's not (amplified) in the CB configuration.

 

[audioguru]

The datasheet for the 2N3904 has a graph of its typical common-emotter input impedance when its emitter is grounded or is bypassed to ground with a capacitor. Of course the biasing resistors are parallel to the input impedance.
The input impedance changes when the collector current changes:

 

[Myna Mo]

This gives a good mathematical /graphical idea
But what exactly is happening with the electrons and the holes?
Does the biasing of the transistor play any role?

 

[Myna Mo]

And I have another doubt
Once the Q point is fixed and v apply an ac signal does it have a dc component too or is the average of the signal equal to zero since it varies from the q point?