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Feedback Principle

  The Feedback Principle, in electronics, is implied when a measure of the output signal from some system is applied, or fed back, to the input of the same system. By this means the system input can be controlled to compensate for undesired output states or signals, and the system stability is improved. The principle may be considered as a way in which a mechanical or electrical system ‘learns from its errors’.

Feedback is used in a myriad of diverse applications. The concept first appeared as the mechanical governor for a steam engine, where the centrifugal force on rotating weights was used to control the supply of steam to the cylinders. Since the centrifugal force depended on the rotational speed, the engine was controlled to run at a constant speed. The cruise control in motor cars, for example, applies the principle of feedback to maintain a steady cruising speed. If the cruise control detects that the car\'s speed is increasing, for instance because it is going downhill, then the electronic sensors will send a signal back to the control system which will reduce the fuel flow and therefore the power developed by the engine, causing the car to slow down until the required steady speed is achieved.

A great deal of the early work concerned with understanding and controlling the use of feedback was applied to telephony and electronic amplifiers. The early vacuum amplifiers were among the first electronic devices to be commercially used, and had produced significant advances in the area of long-distance telephony and improving the quality of radio reception. However, it had been found that quite severe distortion occurred in some amplifiers, resulting in audible hum or unintelligible speech being reproduced. In 1927, H.S. Black invented the concept of ‘negative feedback’, whereby controlling the amount and phase of the feedback signal greatly reduced the distortion, at the cost of a small reduction in amplification. The invention of the negative feedback amplifier greatly improved telephone communication at this time, and other applications of feedback soon followed.

In 1932, H. Nyquist analysed a generic feedback system and devised a mathematical and graphical method to establish general rules of stability for the system. By this time it was recognized that distortion was due to instability or unwanted oscillations in the output signal, caused in most instances by unsuitable levels of feedback. By investigation of the Nyquist Diagram, as it became known, much improved electronic amplifiers were built, and complex control system behaviour could be modelled and assessed.

Around the 1930s and 1940s, work on oscillators, where positive feedback is used, created effective signal generators which have become standard pieces of electrical communication test equipment, and crystal oscillators. Positive feedback occurs when the output signal is fed back to increase the input signal, which in turn increases the output signal and so on. Oscillators apply this principle but effectively limit the maximum value that can be reached. Crystal oscillators, for example, use positive feedback to produce the very accurate high frequency waveforms, or oscillations, that are used in modern radio tuners to enable demodulation of incoming radio signals. Crystal oscillators are also used in accurate timing circuits and in modern quartz watches.

The development and application of the feedback principle, along with electromagnetism has led to atomic models and theories giving much greater understanding of the materials we use today. Quantum mechanics has led to many other related discoveries, including wave-particle duality of matter and the use of semiconducting materials; it has also helped scientists to understand and harness nuclear energy. The application of dependent technologies, such as semiconductor materials providing silicon ‘chips’ and electronic computers, has led to quantum mechanics becoming the cornerstone of modern physics and has heralded a new generation of science. AC



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