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Instability In Feedback Systems

How Feedback Systems Become Unstable 

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 right inventory.

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 been exceeded.

Feedback In Electronics

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.

unstable feedback

Sound Level Output

Here we see the output of the system in sound level.

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