Why is positive feedback unstable

But look at results carefully - AC analysis often displays magnitudes by default and it is often not clear if you have an amplifier or an oscillator. The best way to understand the differences between negative feedback and positive feedback is study the case when feed-forward block is dynamic system then extend the results to ideal linear block, let suppose that the transfer function of feedforward system is:.

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Create a free Team What is Teams? Learn more. Positive feedback and unstability Ask Question. Asked 5 years ago. Active 2 years, 1 month ago. Viewed 3k times. To sum up: Why is positive feedback often related to unstability? Tendero Tendero 10 10 silver badges 24 24 bronze badges. Add a comment. Active Oldest Votes. LvW LvW As far as I know, LTI systems can be unstable, but that doesn't mean they don't have a transfer function. Or maybe I am mixing up things here?

But this is the precondition for defining small-signal parameters like "gain", "input resistance", Chu Chu 6, 2 2 gold badges 11 11 silver badges 15 15 bronze badges. Otherwise there would have to be initial conditions specified.

As I see it, that value of A would mean that whatever I put in the input will apear at the output inverted and multiplied by a scalar. That doesn't seem unstable to me. What am I thinking wrong? Try and feed the loop of your example with a value of 1. After the gain stage, the output is Feeding it back adds 5 to the first 1. So the new input for the gain stage is 6, makes 60 at the output.

And so on, increasing the numbers rapidly. Nothing negative, just out of scope of the closed-loop gain formula. But why does that imply that the system is unstable?

The fact that I can't use a formula doesn't mean that the output will diverge, does it? The release of oxytocin from the posterior pituitary gland during labor is an example of positive feedback mechanism. Oxytocin stimulates the muscle contractions that push the baby through the birth canal. The release of oxytocin result in stronger or augmented contractions during labor.

Climate feedbacks: processes that can either amplify or diminish the effects of climate forcings. A feedback that increases an initial warming is called a "positive feedback. We've seen that negative feedback, where a portion of the output signal is subtracted from the input signal, improves amplifier performance by reducing sensitivity to parameter variations.

An example of a negative feedback loop is if the increase in temperature increases the amount of cloud cover. The increased cloud thickness or amount could reduce incoming solar radiation and limit warming.

However, it is not clear, if additional cloud cover occurs, at what latitudes and at what times it might occur. In a system with feedback, also known as a closed-loop control system, the past output may influence the present or future outputs. Most physical systems embody some form of feedback.

In a positive feedback system , the feedback is used to increase the input signal level, thus generally making the system unstable. So, feedback will increase the sensitivity of the system gain in one frequency range and decrease in the other frequency range. Therefore, we have to choose the values of 'GH' in such a way that the system is insensitive or less sensitive to parameter variations. The main difference between positive and negative feedback loops is that the positive feedback loops amplify the initiating stimulus, moving the system away from its equilibrium whereas the negative feedback loops counteract the changes of the system, maintaining them in a set point.

What type of feedback is the relationship between clouds and temperature? This is a negative feedback relationship. The cloud cover increases with increasing water vapor, but the cloud cover serves to reduce incoming solar radiation which leads to cooling. Scientists are aware of a number of positive feedbacks loops in the climate system. One example is melting ice. Because ice is light-coloured and reflective, a large proportion of the sunlight that hits it is bounced back to space, which limits the amount of warming it causes.

The positive connection in the figure below for a cooling coffee cup implies that the hotter the coffee is the faster it cools. The variables Tc and Tr are coffee and room temperature respectively. The negative connection in the figure below for a cooling coffee cup implies a positive cooling rate makes the coffee temperature drop. When these two connections are combined we get a negative feedback loop as shown at left in which the coffee temperature approaches the stable equilibrium of the room temperature.

Going around the loop the positive connection times the negative connection gives a negative loop feedback effect. This same trick of multiplying the signs of the connections around a loop together to find out whether it is a positive or negative feedback loop works for more complicated loop structures with many more connections.

An example of positive feedback is world population with a fixed percentage birth rate. Positive feedbacks will result in unlimited growth until checked and are sometimes referred to as vicious cycles. In the figure below connecting population to births, large populations cause large numbers of births and large numbers of births result in larger population. Not all positive feedbacks give exponential growth but all, left unchecked, will result in unlimited or unstable growth.