A positive feedback loop requires an input detector, an amplifier, and some form of output. Imagine a signal picked up by the input detector, amplified, and then emitted, in amplified form. So far, so good. The trouble starts when the input detector detects that output, and runs it through the system again, amplifying and emitting it again. A few rounds of intensification and things get dangerously out of control. Most people have been subject to the deafening howling of feedback at a concert, when the sound system squeals painfully.
