output impedance of amplifier

Hi everyone,
Its been a long time since I was here,so how its is going.
I have problem in understanding the concept about calculation of output impedance of transistor amplifier or
opamp.For calculating output impedance,we set input voltage to zero.
I dont understand why do we need to do this.
Can anyone please help me.

Reason why is to be able to calculate loss of signal strenght between ane amplifier stage and the next stage (wher you know the input impedance).

legend killer said:

Hi everyone,
Its been a long time since I was here,so how its is going.
I have problem in understanding the concept about calculation of output impedance of transistor amplifier or
opamp.For calculating output impedance,we set input voltage to zero.
I dont understand why do we need to do this.
Can anyone please help me.

Click to expand...

Think of it this way; impeadance determines the current that results from an applied voltage. So, from the output side of an amplifier, the voltage would be applied to the output side of the amplifier. Since we are only concerned with the results from this source, then all other sources should be set to zero, so the principle of superposition applies.

What kind of amplifier?
A solid state audio amplifier has an output impedance of only 0.04 ohms or less so it provides excellent damping of the resonances of speakers. The extremely low output impedance is created by lots of negative feedback.
Antique vacuum tube audio amplifiers had an output impedance the same as the speaker impedance and provided poor damping because the phase shift in the output transfomer limited the amount of negative feedback that could be used. Many had no negative feedback (but lots of awful distortion).