Impedance of an emitted audio amplifier

[nobrainer612]

Homework Statement

I drew the emitted audio amplifier in the picture below. When I looked at my lab manual, it says the impedance should be Zin and Zout, which

Zin = R1*R2/(R1+R2) and Zout = Rc

I have a few questions about impedance relate to this circuit:

1.) I saw the source is AC source, is that why CB is not included in the impedance?
2.) Why is Zout only included RC? How about the RE1 and RE2? ( In my memory, I think RE2 is short by CE since it is AC, but I am not sure so I want to check my concept.) Thank you

 

[NascentOxygen]

C_B is a coupling capacitor and is chosen to have negligible X for the frequencies of operation. Although C_B would usually be regarded as part of the amplifier circuit and thus making its reactance part of the input Z of this amplifier, as its reactance is chosen to be low then it can be overlooked in comparison to other components. R_S is the output impedance of the signal generator and that is not part of this amplifier stage.

For the output Z you need to look at the small signal model for the transistor with the addition of R_E1 and R_C. Correct, R_E2 is bypassed so does not show on the ac model. https://www.physicsforums.com/images/icons/icon6.gif

This would probably be considered a variation of COMMON EMITTER AMPLIFIER, with part of the emitter resistor not bypassed.

 

[nobrainer612]

I still don't quite understand why for the output impedance Zout , we are ignoring RE1 and CE? I have tried to look at the small signal model for the transistor on the internet, but having a hard time understand what it says.

Can anyone please tell me how the small signal model for transistor work in simple word? Thanks a lot. At this moment, I will keep trying to look on internet and see if I can understand.