Editing Feedback (section)

===Electronic engineering===
[[File:Ideal feedback model.svg|thumb|The simplest form of a feedback amplifier can be represented by the ''ideal block diagram'' made up of [https://www.google.com/search?tbo=p&tbm=bks&q=%22A+unilateral+block+or+network+is+one+in+which+power+may+be+transmitted+in+one+direction+only.%22&num=10&gws_rd=ssl unilateral elements].<ref name="Chen">
{{cite book|title=Circuit Analysis and Feedback Amplifier Theory|author=Wai-Kai Chen|publisher=CRC Press|date=2005|isbn=9781420037272|location=Boca Raton, FL, USA |id=423825181|pages=13.1–13.14|chapter=Chapter 13: General feedback theory|quote=[In a practical amplifier] the forward path may not be strictly unilateral, the feedback path is usually bilateral, and the input and output coupling networks are often complicated.|chapter-url=https://books.google.com/books?id=ZlJM1OLDQx0C&pg=SA13-PA1}}
</ref>|280px|right]]

The use of feedback is widespread in the design of [[electronics|electronic]] components such as [[amplifier]]s, [[oscillator]]s, and stateful [[logic circuit]] elements such as [[flip-flop (electronics)|flip-flop]]s and [[counter (digital)|counter]]s. Electronic feedback systems are also very commonly used to control mechanical, thermal and other physical processes.

If the signal is inverted on its way round the control loop, the system is said to have ''[[negative feedback amplifier|negative feedback]]'';<ref name=KalS>
{{cite book |title=Basic Electronics: Devices, Circuits and IT Fundamentals |author=Santiram Kal |url=https://books.google.com/books?id=_Bw_-ZyGL6YC&q=%22it+is+called+negative+feedback%22+%22if+the+feedback+signal+reduces+the+input+signal%22&pg=PA191 |quote=If the feedback signal reduces the input signal, ''i.e.'' it is out of phase with the input [signal], it is called negative feedback. |isbn=9788120319523 |year=2009 |publisher=PHI Learning Pvt. Ltd |page=191}}
</ref> otherwise, the feedback is said to be ''positive''. Negative feedback is often deliberately introduced to increase the [[BIBO stability|stability]] and accuracy of a system by correcting or reducing the influence of unwanted changes.  This scheme can fail if the input changes faster than the system can respond to it.  When this happens, the lag in arrival of the correcting signal can result in over-correction, causing the output to [[oscillation|oscillate]] or "hunt".<ref>With mechanical devices, hunting can be severe enough to destroy the device.</ref> While often an unwanted consequence of system behaviour, this effect is used deliberately in electronic oscillators.

[[Harry Nyquist]] at [[Bell Labs]] derived the [[Nyquist stability criterion]] for determining the stability of feedback systems. An easier method, but less general, is to use [[Bode plot]]s developed by [[Hendrik Wade Bode|Hendrik Bode]] to determine the [[Gain margin|gain margin and phase margin]]. Design to ensure stability often involves [[frequency compensation]] to control the location of the [[pole (complex analysis)|pole]]s of the amplifier.

Electronic feedback loops are used to control the output of [[electronics|electronic]] devices, such as [[amplifiers]]. A feedback loop is created when all or some portion of the output is fed back to the input. A device is said to be operating ''open loop'' if no output feedback is being employed and ''closed loop'' if feedback is being used.<ref>P. Horowitz & W. Hill, ''The Art of Electronics'', Cambridge University Press (1980), Chapter 3, relating to operational amplifiers.</ref>

When two or more amplifiers are cross-coupled using positive feedback, complex behaviors can be created. These ''[[multivibrator]]s'' are widely used and include:
* astable circuits, which act as oscillators
* monostable circuits, which can be pushed into a state, and will return to the stable state after some time
* bistable circuits, which have two stable states that the circuit can be switched between

====Negative feedback====
Negative feedback occurs when the fed-back output signal has a relative phase of 180° with respect to the input signal (upside down). This situation is sometimes referred to as being ''out of phase'', but that term also is used to indicate other phase separations, as in "90° out of phase". Negative feedback can be used to correct output errors or to desensitize a system to unwanted fluctuations.<ref name=Bhattacharya>
For an analysis of desensitization in the system pictured, see {{cite book |author=S.K Bhattacharya |title=Linear Control Systems |pages=134–135 |quote=The parameters of a system ... may vary... The primary advantage of using feedback in control systems is to reduce the system's sensitivity to parameter variations. |chapter=§5.3.1 Effect of feedback on parameter variations |isbn=9788131759523 |publisher=Pearson Education India |year=2011 |chapter-url=https://books.google.com/books?id=e5Z1A_6jxAUC&q=%22primary+advantage+of+using+feedback+in+control+system+is+to+reduce+the+system%27s+sensitivity+to+parameter+variations%22&pg=PA135}}
</ref> In feedback amplifiers, this correction is generally for waveform [[distortion]] reduction<ref>{{Cite web |title=Negative Feedback & Distortion |url=https://learnabout-electronics.org/Amplifiers/amplifiers34.php#:~:text=Using%20negative%20feedback%20to%20control,and/or%20cut%20off%20regions. |access-date=2024-06-07 |website=learnabout-electronics.org}}</ref> or to establish a specified [[Gain (electronics)|gain]] level. A general expression for the gain of a negative feedback amplifier is the [[asymptotic gain model]].

====Positive feedback====
Positive feedback occurs when the fed-back signal is in phase with the input signal. Under certain gain conditions, positive feedback reinforces the input signal to the point where the output of the device [[oscillates]] between its maximum and minimum possible states. Positive feedback may also introduce [[hysteresis]] into a circuit. This can cause the circuit to ignore small signals and respond only to large ones. It is sometimes used to eliminate noise from a digital signal. Under some circumstances, positive feedback may cause a device to latch, i.e., to reach a condition in which the output is locked to its maximum or minimum state. This fact is very widely used in digital electronics to make [[Flip-flop (electronics)|bistable]] circuits for volatile storage of information.

The loud squeals that sometimes occurs in [[audio system]]s, [[public address system|PA systems]], and [[rock music]] are known as [[audio feedback]]. If a microphone is in front of a loudspeaker that it is connected to, sound that the microphone picks up comes out of the speaker, and is picked up by the microphone and re-amplified.  If the [[loop gain]] is sufficient, howling or squealing at the maximum power of the amplifier is possible.

====Oscillator====
[[File:OpAmpHystereticOscillator.svg|thumb|A popular [[Relaxation oscillator#Comparator–based electronic relaxation oscillator|op-amp relaxation oscillator]]]]
An [[electronic oscillator]] is an [[electronic circuit]] that produces a periodic, [[oscillation|oscillating]] electronic signal, often a [[sine wave]] or a [[Square wave (waveform)|square wave]].<ref name="Snelgrove">{{cite encyclopedia
  | last = Snelgrove
  | first = Martin
  | title = Oscillator
  | encyclopedia = McGraw-Hill Encyclopedia of Science and Technology, 10th Ed., Science Access online service
  | publisher = McGraw-Hill
  | year = 2011
  | url = http://accessscience.com/abstract.aspx?id=477900&referURL=http%3a%2f%2faccessscience.com%2fcontent.aspx%3fid%3d477900
  | access-date = 1 March 2012
  | archive-url = https://web.archive.org/web/20130719125711/http://accessscience.com/abstract.aspx?id=477900&referURL=http%3A%2F%2Faccessscience.com%2Fcontent.aspx%3Fid%3D477900
  | archive-date = 19 July 2013
  | url-status = dead
  }}</ref><ref name="Chattopadhyay">{{cite book   
  | last = Chattopadhyay
  | first = D. 
  | title = Electronics (fundamentals And Applications)
  | publisher = New Age International
  | year = 2006
  | pages = 224–225
  | url = https://books.google.com/books?id=n0rf9_2ckeYC&q=%22negative+resistance%22&pg=PA224
  | isbn = 978-81-224-1780-7}}</ref> Oscillators convert [[direct current]] (DC) from a power supply to an [[alternating current]] signal.  They are widely used in many electronic devices.  Common examples of signals generated by oscillators include signals broadcast by [[Radio transmitter|radio]] and [[television transmitter]]s, clock signals that regulate computers and [[quartz clock]]s, and the sounds produced by electronic beepers and [[video game]]s.<ref name="Snelgrove" />

Oscillators are often characterized by the [[frequency]] of their output signal:
* A [[low frequency oscillation|low-frequency oscillator]] (LFO) is an electronic oscillator that generates a frequency below ≈20&nbsp;Hz.  This term is typically used in the field of audio [[synthesizers]], to distinguish it from an audio frequency oscillator.
* An audio oscillator<!--Please don't link – circular reference--> produces frequencies in the [[audio frequency|audio]] range, about 16&nbsp;Hz to 20&nbsp;kHz.<ref name="Chattopadhyay" />
* An RF oscillator produces signals in the [[radio frequency]] (RF) range of about 100&nbsp;kHz to 100&nbsp;GHz.<ref name="Chattopadhyay" />

Oscillators designed to produce a high-power AC output from a DC supply are usually called [[Inverter (electrical)|inverters]].

There are two main types of electronic oscillator: the linear or harmonic oscillator and the nonlinear or [[relaxation oscillator]].<ref name="Chattopadhyay" /><ref name="Garg">{{cite book   
  | last = Garg
  | first = Rakesh Kumar  
  | author2=Ashish Dixit |author3=Pavan Yadav
  | title = Basic Electronics
  | publisher = Firewall Media
  | year = 2008
  | pages = 280
  | url = https://books.google.com/books?id=9SOdnsHA2IYC&pg=PA280
  | isbn = 978-8131803028}}</ref>

====Latches and flip-flops====
[[File:JohnsonCounter2.png|thumb|A 4-bit [[ring counter]] using [[Flip-flop (electronics)#D flip-flop|D-type flip flops]]]]

A latch or a [[Flip-flop (electronics)|flip-flop]] is a [[electronic circuit|circuit]] that has two stable states and can be used to store state information. They typically constructed using feedback that crosses over between two arms of the circuit, to provide the circuit with a state. The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs. It is the basic storage element in [[sequential logic]]. Latches and flip-flops are fundamental building blocks of [[digital electronics]] systems used in computers, communications, and many other types of systems.

Latches and flip-flops are used as data storage elements. Such data storage can be used for storage of ''[[state (computer science)|state]]'', and such a circuit is described as [[sequential logic]]. When used in a [[finite-state machine]], the output and next state depend not only on its current input, but also on its current state (and hence, previous inputs). It can also be used for counting of pulses, and for synchronizing variably-timed input signals to some reference timing signal.

Flip-flops can be either simple (transparent or opaque) or [[clock signal|clock]]ed (synchronous or edge-triggered).  Although the term flip-flop has historically referred generically to both simple and clocked circuits, in modern usage it is common to reserve the term ''flip-flop'' exclusively for discussing clocked circuits; the simple ones are commonly called ''latches''.<ref name="pedroni">
{{cite book| author = Volnei A. Pedroni| title = Digital electronics and design with VHDL| url = https://books.google.com/books?id=-ZAccwyQeXMC| year = 2008| publisher = Morgan Kaufmann| isbn = 978-0-12-374270-4| page = 329 }}</ref><ref name="ee42">[http://rfic.eecs.berkeley.edu/ee100/pdf/lect24.pdf Latches and Flip Flops] {{Webarchive|url=https://web.archive.org/web/20161005192018/http://rfic.eecs.berkeley.edu/ee100/pdf/lect24.pdf |date=5 October 2016 }} (EE 42/100 Lecture 24 from Berkeley)  ''"...Sometimes the terms flip-flop and latch are used interchangeably..."''</ref>

Using this terminology, a latch is level-sensitive, whereas a flip-flop is edge-sensitive. That is, when a latch is enabled it becomes transparent, while a flip flop's output only changes on a single type (positive going or negative going) of clock edge.