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Positive feedback does not necessarily imply instability of an equilibrium, for example stable ''on'' and ''off'' states may exist in positive-feedback architectures.<ref name="ReferenceA">{{cite journal|last1=Lopez-Caamal|first1=Fernando|last2=Middleton|first2=Richard H.|last3=Huber|first3=Heinrich|title=Equilibria and stability of a class of positive feedback loops|journal=Journal of Mathematical Biology|date=February 2014|pages=609–645|doi = 10.1007/s00285-013-0644-z|pmid=23358701|volume=68|issue=3}}</ref>
 
Positive feedback does not necessarily imply instability of an equilibrium, for example stable ''on'' and ''off'' states may exist in positive-feedback architectures.<ref name="ReferenceA">{{cite journal|last1=Lopez-Caamal|first1=Fernando|last2=Middleton|first2=Richard H.|last3=Huber|first3=Heinrich|title=Equilibria and stability of a class of positive feedback loops|journal=Journal of Mathematical Biology|date=February 2014|pages=609–645|doi = 10.1007/s00285-013-0644-z|pmid=23358701|volume=68|issue=3}}</ref>
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正反馈不一定意味着平衡的不稳定性,例如在正反馈架构中可能存在稳定的 "开 "和 "关 "状态。
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正反馈并不一定意味着平衡的不稳定性,例如,在正反馈结构中可能存在稳定的开关状态。
 
      
=== Hysteresis ===
 
=== Hysteresis ===
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::"Positive feed-back increases the gain of the amplifier, negative feed-back reduces it."<ref name=black>
 
::"Positive feed-back increases the gain of the amplifier, negative feed-back reduces it."<ref name=black>
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正反馈会增加放大器的增益,负反馈会降低增益
    
{{Citation |first=H. S. |last=Black |title=Stabilized feed-back amplifiers |journal=Electrical Engineering |volume=53 |issue= |pages=114–120 |date=January 1934 |doi=10.1109/ee.1934.6540374}}</ref>
 
{{Citation |first=H. S. |last=Black |title=Stabilized feed-back amplifiers |journal=Electrical Engineering |volume=53 |issue= |pages=114–120 |date=January 1934 |doi=10.1109/ee.1934.6540374}}</ref>
    
According to {{harvtxt|Mindell|2002}} confusion in the terms arose shortly after this:
 
According to {{harvtxt|Mindell|2002}} confusion in the terms arose shortly after this:
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根据 harvtxt Mindell(2002)的说法,这之后不久就出现了术语上的混乱。
    
::"...Friis and Jensen had made the same distinction Black used between 'positive feed-back' and 'negative feed-back', based not on the sign of the feedback itself but rather on its effect on the amplifier’s gain. In contrast, Nyquist and Bode, when they built on Black’s work, referred to negative feedback as that with the sign reversed. Black had trouble convincing others of the utility of his invention in part because confusion existed over basic matters of definition."<ref name=mindell/>{{rp|page=121}}
 
::"...Friis and Jensen had made the same distinction Black used between 'positive feed-back' and 'negative feed-back', based not on the sign of the feedback itself but rather on its effect on the amplifier’s gain. In contrast, Nyquist and Bode, when they built on Black’s work, referred to negative feedback as that with the sign reversed. Black had trouble convincing others of the utility of his invention in part because confusion existed over basic matters of definition."<ref name=mindell/>{{rp|page=121}}
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“ ... ... Friis和Jensen对Black在 "正反馈 "和 "负反馈 "之间的区分是一样的,不是基于反馈本身的符号,而是基于它对放大器增益的影响。相反,Nyquist和Bode在Black的工作基础上,将负反馈称为符号相反的反馈。Black难以说服其他人相信他的发明的实用性,部分原因是在基本的定义问题上存在混乱。
    
Gunnar Myrdal described a vicious circle of increasing inequalities, and poverty, which is known as "circular cumulative causation".
 
Gunnar Myrdal described a vicious circle of increasing inequalities, and poverty, which is known as "circular cumulative causation".
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[[Regenerative circuit]]s were invented and patented in 1914<ref>{{cite patent |inventor-last=Armstrong |inventor-first=E. H. |country-code=US |patent-number=1113149 |title=Wireless receiving system |date=1914}}</ref> for the amplification and reception of very weak radio signals. Carefully controlled positive feedback around a single [[transistor]] amplifier can multiply its [[Gain (electronics)|gain]] by 1,000 or more.<ref>{{cite web|last=Kitchin|first=Charles|title=A Short Wave Regenerative Receiver Project|url=http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|accessdate=23 September 2010|url-status=live|archiveurl=https://web.archive.org/web/20100710100031/http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|archivedate=10 July 2010}}</ref> Therefore, a signal can be amplified 20,000 or even 100,000 times in one stage, that would normally have a gain of only 20 to 50. The problem with regenerative amplifiers working at these very high gains is that they easily become unstable and start to oscillate. The radio operator has to be prepared to tweak the amount of feedback fairly continuously for good reception. Modern radio receivers use the [[superheterodyne]] design, with many more amplification stages, but much more stable operation and no positive feedback.
 
[[Regenerative circuit]]s were invented and patented in 1914<ref>{{cite patent |inventor-last=Armstrong |inventor-first=E. H. |country-code=US |patent-number=1113149 |title=Wireless receiving system |date=1914}}</ref> for the amplification and reception of very weak radio signals. Carefully controlled positive feedback around a single [[transistor]] amplifier can multiply its [[Gain (electronics)|gain]] by 1,000 or more.<ref>{{cite web|last=Kitchin|first=Charles|title=A Short Wave Regenerative Receiver Project|url=http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|accessdate=23 September 2010|url-status=live|archiveurl=https://web.archive.org/web/20100710100031/http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|archivedate=10 July 2010}}</ref> Therefore, a signal can be amplified 20,000 or even 100,000 times in one stage, that would normally have a gain of only 20 to 50. The problem with regenerative amplifiers working at these very high gains is that they easily become unstable and start to oscillate. The radio operator has to be prepared to tweak the amount of feedback fairly continuously for good reception. Modern radio receivers use the [[superheterodyne]] design, with many more amplification stages, but much more stable operation and no positive feedback.
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"用于放大和接收非常弱的无线电信号。仔细控制单个晶体管放大器周围的正反馈,可以使其增益增加1000倍或更多。因此,一个信号可以在一个阶段被放大20000甚至100000倍,通常情况下,该信号的增益只有20到50。再生放大器在这些非常高的增益下工作的问题是,它们很容易变得不稳定,开始振荡。无线电操作员必须准备相当连续地调整反馈量,以获得良好的接收效果。现代无线电接收机采用超异构设计,多了许多放大级,但工作更稳定,没有正反馈。
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[[Thermal runaway]] occurs in electronic systems because some aspect of a circuit is allowed to pass more current when it gets hotter, then the hotter it gets, the more current it passes, which heats it some more and so it passes yet more current. The effects are usually catastrophic for the device in question. If devices have to be used near to their maximum power-handling capacity, and thermal runaway is possible or likely under certain conditions, improvements can usually be achieved by careful design.<ref>{{cite web|last=Sharma|first=Bijay Kumar|title=Analog Electronics Lecture 4 Part C RC coupled Amplifier Design Procedure|url=http://cnx.org/content/m31058/latest/|accessdate=29 September 2010|year=2009}}</ref>
 
[[Thermal runaway]] occurs in electronic systems because some aspect of a circuit is allowed to pass more current when it gets hotter, then the hotter it gets, the more current it passes, which heats it some more and so it passes yet more current. The effects are usually catastrophic for the device in question. If devices have to be used near to their maximum power-handling capacity, and thermal runaway is possible or likely under certain conditions, improvements can usually be achieved by careful design.<ref>{{cite web|last=Sharma|first=Bijay Kumar|title=Analog Electronics Lecture 4 Part C RC coupled Amplifier Design Procedure|url=http://cnx.org/content/m31058/latest/|accessdate=29 September 2010|year=2009}}</ref>
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热失控在电子系统中发生的原因是,当电路的某些方面变得更热时,允许它通过更多的电流,然后它越热,通过的电流就越多,这就使它更热一些,所以它又通过更多的电流。这种影响对有关器件来说通常是灾难性的。如果设备必须在接近其最大功率处理能力的情况下使用,并且在某些条件下可能或可能出现热失控,通常可以通过精心设计来改进。
       
[[File:Technics SL-1210MK2.jpg|thumb|left|A phonograph turntable is prone to acoustic feedback.]]
 
[[File:Technics SL-1210MK2.jpg|thumb|left|A phonograph turntable is prone to acoustic feedback.]]
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留声机转盘容易发生声响,因此,在使用过程中要注意以下几点
    
If a chemical reaction causes the release of heat, and the reaction itself happens faster at higher temperatures, then there is a high likelihood of positive feedback. If the heat produced is not removed from the reactants fast enough, thermal runaway can occur and very quickly lead to a chemical explosion.
 
If a chemical reaction causes the release of heat, and the reaction itself happens faster at higher temperatures, then there is a high likelihood of positive feedback. If the heat produced is not removed from the reactants fast enough, thermal runaway can occur and very quickly lead to a chemical explosion.
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[[Sound recording and reproduction|Audio]] and [[video]] systems can demonstrate positive feedback. If a [[microphone]] picks up the amplified sound output of [[loudspeaker]]s in the same circuit, then howling and screeching sounds of [[audio feedback]] (at up to the maximum power capacity of the amplifier) will be heard, as random noise is re-amplified by positive feedback and [[Filter (signal processing)|filtered]] by the characteristics of the audio system and the room.
 
[[Sound recording and reproduction|Audio]] and [[video]] systems can demonstrate positive feedback. If a [[microphone]] picks up the amplified sound output of [[loudspeaker]]s in the same circuit, then howling and screeching sounds of [[audio feedback]] (at up to the maximum power capacity of the amplifier) will be heard, as random noise is re-amplified by positive feedback and [[Filter (signal processing)|filtered]] by the characteristics of the audio system and the room.
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声音记录和重现音频和视频系统可以展示正反馈。如果传声器接收到同一电路中扬声器的放大声音输出,那么音频反馈的嚎叫和尖叫声就会出现。(在放大器的最大功率容量下)会听到,因为随机噪声被正反馈和滤波器(信号处理)过滤的音频系统和房间的特性重新放大。
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[[Audio feedback]] (also known as acoustic feedback, simply as feedback, or the Larsen effect) is a special kind of positive feedback which occurs when a sound loop exists between an audio input (for example, a [[microphone]] or [[guitar pickup]]) and an audio output (for example, a loudly-amplified [[loudspeaker]]). In this example, a signal received by the microphone is [[Amplifier|amplified]] and passed out of the loudspeaker. The sound from the loudspeaker can then be received by the microphone again, amplified further, and then passed out through the loudspeaker again.  The [[frequency]] of the resulting sound is determined by resonance frequencies in the microphone, amplifier, and loudspeaker, the acoustics of the room, the directional pick-up and emission patterns of the microphone and loudspeaker, and the distance between them. For small [[PA system]]s the sound is readily recognized as a loud squeal or screech.
 
[[Audio feedback]] (also known as acoustic feedback, simply as feedback, or the Larsen effect) is a special kind of positive feedback which occurs when a sound loop exists between an audio input (for example, a [[microphone]] or [[guitar pickup]]) and an audio output (for example, a loudly-amplified [[loudspeaker]]). In this example, a signal received by the microphone is [[Amplifier|amplified]] and passed out of the loudspeaker. The sound from the loudspeaker can then be received by the microphone again, amplified further, and then passed out through the loudspeaker again.  The [[frequency]] of the resulting sound is determined by resonance frequencies in the microphone, amplifier, and loudspeaker, the acoustics of the room, the directional pick-up and emission patterns of the microphone and loudspeaker, and the distance between them. For small [[PA system]]s the sound is readily recognized as a loud squeal or screech.
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音频反馈(也称为声反馈,简称为反馈,或拉森效应)是一种特殊的正反馈,当音频输入(例如麦克风或吉他拾音器)和音频输出(例如经过大声放大的扬声器)之间存在声音回路时,就会出现这种反馈。在这个例子中,麦克风接收到的信号被放大器放大并从扬声器中传出。然后,来自扬声器的声音可以再次被传声器接收,进一步放大,然后再次通过扬声器传递出去。 所产生的声音的频率由传声器、放大器和扬声器的共振频率、房间的声学特性、传声器和扬声器的定向拾取和发射模式以及它们之间的距离决定。对于小型扩音系统来说,这种声音很容易被识别为响亮的尖叫声或尖锐的声音。
          
Feedback is almost always considered undesirable when it occurs with a singer's or public speaker's microphone at an event using a [[sound reinforcement system]] or [[PA system]]. [[Audio engineer]]s use various electronic devices, such as equalizers and, since the 1990s, automatic feedback detection devices to prevent these unwanted squeals or screeching sounds, which detract from the audience's enjoyment of the event. On the other hand, since the 1960s, [[electric guitar]] players in [[rock music]] bands using loud [[guitar amplifier]]s and [[distortion (music)|distortion]] effects have intentionally created guitar feedback to create a desirable musical effect.  "I Feel Fine" by the Beatles marks one of the earliest examples of the use of feedback as a recording effect in popular music. It starts with a single, percussive [[audio feedback|feedback]] note produced by plucking the A string on Lennon's guitar. Artists such as the Kinks and the Who had already used feedback live, but Lennon remained proud of the fact that the Beatles were perhaps the first group to deliberately put it on vinyl. In one of his last interviews, he said, "I defy anybody to find a record—unless it's some old blues record in 1922—that uses feedback that way."<ref>{{cite book |last=Sheff |first=David |date=2000 |title=All We Are Saying |location=New York, New York |publisher=St. Martin's Press |page=[https://archive.org/details/allwearesayingla00lenn/page/173 173] |isbn=978-0-312-25464-3 |url=https://archive.org/details/allwearesayingla00lenn/page/173 }}</ref>
 
Feedback is almost always considered undesirable when it occurs with a singer's or public speaker's microphone at an event using a [[sound reinforcement system]] or [[PA system]]. [[Audio engineer]]s use various electronic devices, such as equalizers and, since the 1990s, automatic feedback detection devices to prevent these unwanted squeals or screeching sounds, which detract from the audience's enjoyment of the event. On the other hand, since the 1960s, [[electric guitar]] players in [[rock music]] bands using loud [[guitar amplifier]]s and [[distortion (music)|distortion]] effects have intentionally created guitar feedback to create a desirable musical effect.  "I Feel Fine" by the Beatles marks one of the earliest examples of the use of feedback as a recording effect in popular music. It starts with a single, percussive [[audio feedback|feedback]] note produced by plucking the A string on Lennon's guitar. Artists such as the Kinks and the Who had already used feedback live, but Lennon remained proud of the fact that the Beatles were perhaps the first group to deliberately put it on vinyl. In one of his last interviews, he said, "I defy anybody to find a record—unless it's some old blues record in 1922—that uses feedback that way."<ref>{{cite book |last=Sheff |first=David |date=2000 |title=All We Are Saying |location=New York, New York |publisher=St. Martin's Press |page=[https://archive.org/details/allwearesayingla00lenn/page/173 173] |isbn=978-0-312-25464-3 |url=https://archive.org/details/allwearesayingla00lenn/page/173 }}</ref>
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在使用扩声系统或扩音系统的活动中,当歌手或公众演讲者的麦克风发生反馈时,几乎总是被认为是不受欢迎的。音频工程师使用各种电子设备,如均衡器,自20世纪90年代以来,自动反馈检测设备,以防止这些不需要的尖叫声或尖叫声,这些声音影响了观众对活动的享受。另一方面,自20世纪60年代以来,摇滚乐队中的电吉他手使用大音量的吉他放大器和失真效果,有意制造吉他反馈,以创造理想的音乐效果。 披头士乐队的 "I Feel Fine "标志着流行音乐中最早使用反馈作为录音效果的例子之一。它的开头是由Lennon拨动吉他上的A弦产生的一个单一的、有冲击力的反馈音。像 Kinks 和 Who 等艺术家已经在现场使用了反馈,但是Lennon仍然为披头士乐队可能是第一个特意把它放在黑胶唱片上的乐队而感到骄傲。在他最后的一次采访中,他说,“我认为任何人都不可能找到一张唱片——除非它是1922年的一张旧布鲁斯唱片——那样使用反馈。”
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The principles of audio feedback were first discovered by Danish scientist [[Søren Absalon Larsen]]. Microphones are not the only transducers subject to this effect. [[Phonograph|Record deck]] [[Magnetic cartridge|pickup cartridges]] can do the same, usually in the low frequency range below about 100&nbsp;Hz, manifesting as a low rumble. [[Jimi Hendrix]] was an innovator in the intentional use of guitar feedback in his [[guitar solo]]s to create unique sound effects. He helped develop the controlled and musical use of audio feedback in [[electric guitar]] playing,<ref>{{cite book|last = Shadwick|first = Keith|title = Jimi Hendrix, Musician|publisher = [[Backbeat Books]]|year = 2003|page = 92|isbn = 978-0-87930-764-6}}</ref> and later [[Brian May]] was a famous proponent of the technique.<ref>{{cite web|last=May|first=Brian|title=Burns Brian May Tri-Sonic Pickups|url=http://www.brianmayguitars.co.uk/accessories/19|publisher=House Music & Duck Productions|accessdate=2 February 2011|url-status=live|archiveurl=https://web.archive.org/web/20101120063431/http://brianmayguitars.co.uk/accessories/19|archivedate=20 November 2010}}</ref>
 
The principles of audio feedback were first discovered by Danish scientist [[Søren Absalon Larsen]]. Microphones are not the only transducers subject to this effect. [[Phonograph|Record deck]] [[Magnetic cartridge|pickup cartridges]] can do the same, usually in the low frequency range below about 100&nbsp;Hz, manifesting as a low rumble. [[Jimi Hendrix]] was an innovator in the intentional use of guitar feedback in his [[guitar solo]]s to create unique sound effects. He helped develop the controlled and musical use of audio feedback in [[electric guitar]] playing,<ref>{{cite book|last = Shadwick|first = Keith|title = Jimi Hendrix, Musician|publisher = [[Backbeat Books]]|year = 2003|page = 92|isbn = 978-0-87930-764-6}}</ref> and later [[Brian May]] was a famous proponent of the technique.<ref>{{cite web|last=May|first=Brian|title=Burns Brian May Tri-Sonic Pickups|url=http://www.brianmayguitars.co.uk/accessories/19|publisher=House Music & Duck Productions|accessdate=2 February 2011|url-status=live|archiveurl=https://web.archive.org/web/20101120063431/http://brianmayguitars.co.uk/accessories/19|archivedate=20 November 2010}}</ref>
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音频反馈的原理是由丹麦科学家Søren Absalon Larsen首先发现的。麦克风并不是唯一受此影响的换能器。录音板拾音器也可以做到这一点,通常在100赫兹以下的低频范围内,表现为低沉的轰鸣声。Jimi Hendrix是一个创新者,在他的吉他独奏中有意使用吉他反馈来创造独特的声音效果。他帮助开发了电吉他演奏中音频反馈的可控性和音乐性,后来Brian May也是这种技术的著名支持者。
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Similarly, if a [[video camera]] is pointed at a [[Video monitor|monitor]] screen that is displaying the camera's own signal, then repeating patterns can be formed on the screen by positive feedback. This video feedback effect was used in the opening sequences to the [[Doctor Who (season 1)|first]] [[Doctor Who (season 10)|ten]] series of the television program ''[[Doctor Who]]''.
 
Similarly, if a [[video camera]] is pointed at a [[Video monitor|monitor]] screen that is displaying the camera's own signal, then repeating patterns can be formed on the screen by positive feedback. This video feedback effect was used in the opening sequences to the [[Doctor Who (season 1)|first]] [[Doctor Who (season 10)|ten]] series of the television program ''[[Doctor Who]]''.
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同样,如果一台摄像机对准一个正在显示摄像机自身信号的监控屏幕,那么通过正反馈就可以在屏幕上形成重复的图案。这种视频反馈效果在电视节目《神秘博士》前十季的开场白中就被使用了。
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In [[electrical switch]]es, including [[bimetallic strip]] based thermostats, the switch usually has hysteresis in the switching action. In these cases hysteresis is mechanically achieved via positive feedback within a tipping point mechanism. The positive feedback action minimises the length of time arcing occurs for during the switching and also holds the contacts in an open or closed state.<ref>{{cite web|title=Positive Feedback and Bistable Systems|url=http://sbw.kgi.edu/sbwwiki/_media/sysbio/labmembers/hsauro/bistablesystems.pdf|publisher=University of Washington|quote=* Non-Hysteretic Switches, Memoryless Switches: These systems have no memory, that is, once the input signal is removed, the system returns to its original state. * Hysteretic Switches, Bistability: Bistable systems, in contrast, have memory. That is, when switched to one state or another, these systems remain in that state unless forced to change back. The light switch is a common example of a bistable system from everyday life. All bistable systems are based around some form of positive feedback loop.|url-status=live|archiveurl=https://web.archive.org/web/20150413020657/http://sbw.kgi.edu/sbwwiki/_media/sysbio/labmembers/hsauro/bistablesystems.pdf|archivedate=2015-04-13}}</ref>
 
In [[electrical switch]]es, including [[bimetallic strip]] based thermostats, the switch usually has hysteresis in the switching action. In these cases hysteresis is mechanically achieved via positive feedback within a tipping point mechanism. The positive feedback action minimises the length of time arcing occurs for during the switching and also holds the contacts in an open or closed state.<ref>{{cite web|title=Positive Feedback and Bistable Systems|url=http://sbw.kgi.edu/sbwwiki/_media/sysbio/labmembers/hsauro/bistablesystems.pdf|publisher=University of Washington|quote=* Non-Hysteretic Switches, Memoryless Switches: These systems have no memory, that is, once the input signal is removed, the system returns to its original state. * Hysteretic Switches, Bistability: Bistable systems, in contrast, have memory. That is, when switched to one state or another, these systems remain in that state unless forced to change back. The light switch is a common example of a bistable system from everyday life. All bistable systems are based around some form of positive feedback loop.|url-status=live|archiveurl=https://web.archive.org/web/20150413020657/http://sbw.kgi.edu/sbwwiki/_media/sysbio/labmembers/hsauro/bistablesystems.pdf|archivedate=2015-04-13}}</ref>
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在电气开关中,包括双金属条型恒温器,开关通常在开关动作中具有滞后性。在这些情况下,滞后是通过一个临界点机构内的正反馈来实现的。正反馈作用可最大限度地减少开关过程中发生电弧的时间,并使触点保持在断开或闭合状态。
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开关,无记忆开关。这些系统没有记忆,也就是说,一旦输入信号被移除,系统就会回到原来的状态。* 滞性开关,双稳态:与此相反,双稳态系统是有记忆的。也就是说,当切换到一个状态或另一个状态时,这些系统会保持在该状态,除非被迫变回来。电灯开关是日常生活中常见的双稳态系统的例子。所有的双稳态系统都是围绕着某种形式的正反馈环路。
    
=== In biology ===
 
=== In biology ===
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[[File:Positive Feedback- Childbirth (1).svg|thumb|Positive feedback is the amplification of a body's response to a stimulus. For example, in childbirth, when the head of the fetus pushes up against the cervix (1) it stimulates a nerve impulse from the cervix to the brain (2). When the brain is notified, it signals the pituitary gland to release a hormone called [[oxytocin]](3). Oxytocin is then carried via the bloodstream to the [[uterus]] (4) causing contractions, pushing the fetus towards the cervix eventually inducing childbirth.]]
 
[[File:Positive Feedback- Childbirth (1).svg|thumb|Positive feedback is the amplification of a body's response to a stimulus. For example, in childbirth, when the head of the fetus pushes up against the cervix (1) it stimulates a nerve impulse from the cervix to the brain (2). When the brain is notified, it signals the pituitary gland to release a hormone called [[oxytocin]](3). Oxytocin is then carried via the bloodstream to the [[uterus]] (4) causing contractions, pushing the fetus towards the cervix eventually inducing childbirth.]]
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正反馈是指身体对刺激的反应的放大。例如,在分娩过程中,当胎儿的头顶到子宫颈时(1),会刺激神经冲动从子宫颈到大脑(2)。大脑接到通知后,会向脑垂体发出信号,释放一种叫做[催产素]]的激素(3)。 催产素然后通过血液流向[[子宫]]。(4)引起宫缩,将胎儿推向宫颈,最终分娩。
     
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