I have read many articles with regard to amplifier output impedance and amplifier load
only to find conflicting information. This is what I think is correct. Please correct me if I am wrong.

Common-Emitter and Common-Base
rc = Zout = Load = RC║RL

Emitter-Follower
re = Zout = Load = RE║RL

thanks,

aac

 

Unlike your amplifier, all manufactured amplifiers use overall negative feedback that reduces the gain, reduces distortion and reduces the output impedance. They do not say "output impedance", instead the say the function called "damping factor" because that is what it does, it damps the resonance of a speaker. Here is an older hifi amplifier with a damping factor of >1500 so if the speaker is 8 ohms then the amplifier output impedance is less than 8/1500= 0.00533 ohms. Newer amplifiers might have a lower output impedance.

 

Common-Emitter and Common-Base
rc = Zout = Load = RC║RL

Emitter-Follower
re = Zout = Load = RE║RL

Wrong. Zout is load resistance independent.

In short
Common-Emitter and Common-Base

Zout = Rc||ro ≈ Rc

Emitter-Follower

Zout = re||RE ≈ re