# Homework Help: Impedance of an emitted audio amplifier

1. Apr 20, 2012

### nobrainer612

1. The problem statement, all variables and given/known data

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 wanna check my concept.) Thank you

2. Apr 21, 2012

### Staff: Mentor

CB is a coupling capacitor and is chosen to have negligible X for the frequencies of operation. Although CB 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. RS 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 RE1 and RC. Correct, RE2 is bypassed so does not show on the ac model. https://www.physicsforums.com/images/icons/icon6.gif [Broken]

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

Last edited by a moderator: May 5, 2017
3. Apr 21, 2012

### 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.