I've been reading the All About Circuits section on op-amps, but I'm confused about the negative feedback chapter (http://www.allaboutcircuits.com/vol_3/chpt_8/4.html).

Here's the diagram that's used:

Let's assume that V(in) = 5 volts, and the gain of the op-amp is 4. My understanding is that the op-amp will find the difference between the non-inverting input (5V) and the inverting input (0V... right?)

Assuming that's correct, then the op-amp would find a voltage difference of 5 volts, multiply it by 4, and output 20 volts. The 20 volts would then be fed back into the inverting output, where it would be compared once again to 5 volts (a difference of 15 volts). Anyway, the op-amp would multiply 15 volts by 4 and output 60 volts!! The output voltage gets higher and higher!

Obviously, this is not correct, since the op-amp should eventually output 5 volts... Can anyone please explain where my error lies?