Negative feedback

In electronic the principle of the negative feedback allows the control of the assemblies Amplificateur s, filter or of Control. It makes it possible to make their operating features independent, on the whole, of different the Composant S of these assemblies.

History

The principle of the negative feedback was discovered by Harold Stephen Black on August 2nd, 1927. This idea would have come to him whereas it went to its Beautiful work to the Laboratoires , . Its preceding work on the reduction of the distortions in the amplifiers had already enabled him to discover the amplifiers “ a priori ” ( feedforward in English) which modify the signal to be amplified in order to compensate for the distortions due to the components of power. Although having remade surface in the Years 1970 to compensate for the distortions of the amplifiers BLU, in the Années 1920 the practical realization of the amplifiers “ a priori ” proves to be difficult and they do not function very well. In 1927, the Black patent application for the negative feedback was accommodated like a request for invention of Perpetual motion. It was finally accepted nine years later, , in December 1931, after Black and other members of the Bell laboratories developed the theory relating to the negative feedback.

Operation

In an Amplifying or a Control, via an additional circuit, called buckles negative feedback, one reinjects at the entry of the signal to amplify, or the ordering of the process, part of the output signal reversed, which while being added with the entry signal (or with “instruction”), decreases the amplitude of the real signal on the entry of the circuit.

In the active filters, the loop of negative feedback consists of a filter which reinjects on the entry only the undesirable signals, thus maintaining on a very low level at exit, while one can strongly amplify the desired signals.

The negative feedback in the electronic amplifiers

The principal effect of the negative feedback is to decrease the profit of the system. At the same time, the distortions due to the components of the amplifier are they also withdrawn from the entry signal. In this way, the amplifier amplifies a reduced and reversed image distortions. The negative feedback also makes it possible to compensate for the thermal drifts or the non-linearity of the components. Although the active components are regarded as linear on part of their transfer transfer function, they are actually always nonlinear; their laws of variable behavior like the power of two. The result of these non-linearities is a distortion of amplification.

An amplifier of looked after design, having all its stages in buckles open (without negative feedback), can arrive at a rate of distortion about the “percent”. Using the negative feedback, a rate of 0,001 % is current. The noise, including the distortions of crossing, can be practically eliminated.

It is the application which dictates the rate of distortion that one can tolerate. For the applications of the amplifying type Hi-fi or of instrumentation, the rate of distortion must be minimal, often less 1 %.

The concept of negative feedback is used with the operational amplifier to precisely define the profit, the band-width and of many other parameters.

End of the year 1970 for the amplifying audio discusses

Whereas the negative feedback seems to be the remedy for all the evils of an amplifier, much think that it is a bad thing. As it uses a loop, it takes him a time finished to react to an entry signal and for this short period, the amplifier is “out of control”. A musical transient of which the duration is of the same order of magnitude that this period coarsely will thus be distorted. And that, even if the amplifier has a low rate of distortion in permanent mode. It is primarily that which explains the existence of the “distortions of Intermodulation S transients” in the amplifiers. This subject was largely discussed at the end of the years 1970 and most of the years 1980 , , .

These arguments were sources of controversies during years, and brought to take into account these phenomena when designing amplifier in order to eliminate them , . In the facts, the majority of the modern amplifiers use strong negative feedbacks, whereas the diagrams used for the top-of-the-range audio amplifiers seek to minimize it.

Whatever the merits of these arguments on the way in which it modifies the distortion, the negative feedback modifies the impedance of exit of the amplifier and consequently, its ratio damping. While simplifying, the damping ratio characterizes the skill of an amplifier to control a speaker. Normally, more the negative feedback is strong, more the output impedance is low and more the damping ratio is large. That has an effect on the performances in low frequencies of much of enclosures which returned one of low irregular if the damping ratio of the amplifier is too weak.

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