Amplifier Output Impedance Versus Load

Thread Starter

aac044210

Joined Nov 19, 2019
178
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
 

Audioguru again

Joined Oct 21, 2019
3,320
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.
 

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Jony130

Joined Feb 17, 2009
5,241
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
 
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