In some feedback systems amplification continues with each cycle resulting in larger and
larger excursions. The figures illustrate. In theory, the system oscillates to infinity. In
reality, some physical constraint limits the excursions.
For example, inventory and shortages may alternate getting larger and larger until the
warehouse is full and there is no room for additional inventory. The company may run out of cash
to pay for the inventory and thus force limitation. There is lots of inventory but never the
Instability results primarily from amplification and time delays. Time delays in the feedback
loop have larger effects than in the forward loop but any unnecessary amplification can
contribute to instability. These affects are common in everyday life. Most of us have heard the
awful squeal that sometimes affects public address systems. In such a situation, the speakers
voice enters a microphone. An amplifier increases the signal and sends it out via loudspeakers.
When the signal from the loudspeakers reenters the microphone it is amplified a second time, a
third time and so on. The signal increases until the limits of the amplifier or speaker have
Instability In A Feedback System
Most of us have heard the results of an unstable
feedback system in the form of a Public Address system with too much amplification.
Sound Level Output
Here we see the output of the system in