How To Measure "Current" In An Amplifier?

Atmasphere, I don't understand. What does feedback have to do with linear power output into varying impedances? 

Very generally:

High DF = good = low output impedance.

Low DF = bad = high output impedance

Where did some one (maybe me) say high DF was bad? I may have meant to say high output impedance is bad.

@unsound Feedback improves DF = lowers output impedance = less variability with output voltage. :)

Output impedance can  be thought of as a resistor at the outputs of the amplifier. With a perfect amplifier, this resistor is zero, and the voltage at the speaker is always the voltage at the amp.

However, amplifiers are not perfect.  Batteries are like this too. Looking at a AA battery, you model it as a 1.5V source, but in reality it has a rather large output resistance. So whether your flashlight sees 1V or 1.3V has to do with how much current is drawn and how big that output impedance is.

The more old-school your flashlight, the more current, and the more V will be lost in the output impedance. But modern LED bulbs sip current, so most of the batterie's V will appear at the LED terminals.

Confused yet? :)

Erik

^Perhaps I should rephrase the question:
What does feedback have to do with linear powerput into varying impedances and accompanying variable sensitivity?

@unsound

With very high output impedance the frequency response of the amplifier looks like the speaker impedance. I.e. not flat! :)

With very low output impedance the frequency response of the amplifier stays flat.

The missing step:

Increasing feedback reduces the amplifier output impedance. That and lowering distortion are why most amplifiers have at least some feedback incorporated into the design.