Question: A certain amplifier has a no-load output of 5 V. A 1KΩ load resistor is connected to the amplifier and the output is reduced to 3V. Whats the output impendance of the amplifier? My attempt: Hmmm. This is probably simple. But no formula comes to mind. I can only blame my poor textbook because I know I have some brains. I know that: I=E/R So: I = 5/1000 = 0.005 A I know that: R=E/I So: 3/0.005 = 600 Ω But thats not the answer What the hell am I missing here?