Monday 1 December 2014


Feedback is one of those topics I always enjoy reading or studying about it and, in fact, it helps a system to make a correct decision most of the time. What I mean is we humans can not making any decision before gathering information on what we've already done. Feedback concept is exactly the same, get a sample of output (what you've already done) and bring it to the system as an input and then process it with other inputs and continue this cycle.

Feedback helps a system to adjust its process to get the required results, especially when a system gets complicated enough, the control section can't keep track of the entire internal processes (indexes), at this situation the feedback helps the control section to keep the system up and running.

Feedback concept
In the diagram, you see the process module which operates on input and feedback. So the process module has the input and also an idea of what output currently is. So this process can easily adjust itself to do exactly what it has to do. On the other hand, the feedback module directly senses the output and prepares a signal or required data for the process module and sends it to this module. This loop in which feedback module gets output sample sends it to process module and ... is called "feedback loop".

Negative feedback examples
I remember when I was a teenager I used to build electronic devices, especially amplifiers and receivers. There are many feedback loops in both of these devices, and in fact, feedback loops are widely used in electronic devices. Consider a radio receiver (AM for example), when you walk around you may get close or away from the radio station. Distance changing changes the power of the signal your radio receiver gets and if there is no process to consider the power or strength of the receiving signal, the volume of the voice you are hearing will go up and down. There is a circuit in these radios called AGC (automatic gain control) which is exactly a feedback loop to control the gain of the amplifier.

To show how important feedback is, let us talk about one of the important feedback loops in our body and even unconscious behavior.  It is cloudy and cold then you wear a warm jacket, zip it up and go out to take a walk for example. When you are walking you normally put your hands in your pockets too, but suddenly the clouds get disappeared and the sun warms the weather, (it is exactly what happens here in Toronto) you bring you hands out, unzip you jacket and even take it off. This is nothing but a feedback loop in our unconscious mind which helps us to control our body temperature.

The same thing happens in our body, skin behaves like a sensor in a feedback loop, which senses the temperature and sends the temperature information to our brain, and the brain decides to increase the overall body metabolism to make it warmer. When it gets hot, the skin again sends the report to the brain and it slows down the body metabolism and makes you also sweat too. In fact, there are many feedback loops in our body, blood sugar feedback loop, the loop which tells us we are hungry or full, and ...

All of the feedback loops we've talked about are negative feedbacks, which usually helps a system keep itself at a certain position like fixing body temperature or blood sugar and ...

Positive feedback examples
Positive feedback helps the system to accelerate what it is doing. When the process gets the output feedback instead of doing a negative adjustment and trying to fix the output level at a certain position, it helps the output level grows. While negative feedback tries to stabilize the system the positive feedback tends to destabilize the system.

Bringing an example of positive feedback in the body requires having some biology knowledge and is beyond our discussion here like the way a mother gives birth to her child. But there are many other easy understandable examples like when you want to loose weight. You do exercises get diet and then check yourself in the mirror or weigh yourself. You see the result, feel good then you do more exercise and stick better on the diet.

There are also many positive feedback usage in analog and digital electronic like tube amplifiers or flip-flops and .... You may also have seen what happens when you bring a microphone close to the speaker, the sound you hear is a result of positive feedback.

Feedbacks are very useful in software programming too and we will talk about them pretty soon.

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