Negative Feedback

An Analogy

I am obsessed with throwing the paper ball into the trash can from my working table. I could not score the ball into that trash can on my first attempt. After that, I can score the ball by decreasing or increasing the initial velocity if the ball lands behind or in front of the trash can.

Let me make the analogy complex. Instead of controlling only initial velocity, let me add another variable: the direction I throw the ball. In the language of control theory, the variables that we need to control are called control variables. I can score by observing where the ball lands and improving on the next attempt. Though it sounds easy, this requires great determination and practice to master. Here, I see the result of my initial effort and improve the control variable in the next attempt. This type of control is called negative feedback control.

The control system has different types of sensors that convert physical entities into equivalent voltage and current rather than biological sensors like eyes. We get the desired output by manipulating the output of such sensors.

Negative Feedback Control System

Now, I am going to apply negative feedback to the op-amp. That is, we connect the op-amp output back to the negative input. To understand the negative feedback control system, please read the Operational Amplifier (op-Amp) first.
Figure 1: Negative feedback to the op-amp

Let us apply 1 to the positive input and assume the negative input is 0 initially. Since the error is 1, the input at the gain block G is 1, and the output becomes as positive as possible. But what happens after output tends to 1? The negative input tends to 1, and the output of the summing block tends to 0.

Consider a case where negative input is higher than 1. In this case, the input to the gain block goes negative. This means the output will be negatively railed; in this case, 0 (Remember Operational Amplifier (op-Amp) ), and the aforementioned process repeats. The entire process continues until negative and positive input becomes equal. You see, whatever you do initially, by improving the output, you get the desired result.
Writing the transfer function of this circuit is left for the reader. You can see interesting things by changing the values of G and H in the transfer function.

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