Editing Negative-feedback amplifier (section)

== Overview ==
Fundamentally, all electronic devices that provide power gain (e.g., [[vacuum tube]]s, [[BJT|bipolar transistors]], [[FET|MOS transistors]]) are [[Electrical element|nonlinear]]. [[Negative feedback]] trades [[Gain (electronics)|gain]] for higher linearity (reducing [[distortion]]) and can provide other benefits. If not designed correctly, amplifiers with negative feedback can under some circumstances become unstable due to the feedback becoming positive, resulting in unwanted behavior such as [[oscillation]]. The [[Nyquist stability criterion]] developed by [[Harry Nyquist]] of [[Bell Laboratories]] is used to study the stability of feedback amplifiers.

Feedback amplifiers share these properties:<ref name=Palumbo1>
{{cite book 
|author1=Palumbo, Gaetano  |author2=Salvatore Pennisi
|name-list-style=amp |title=Feedback amplifiers: theory and design
|page=64
|year= 2002
|publisher=Kluwer Academic 
|location=Boston/Dordrecht/London
|isbn=0-7923-7643-9 
|url=http://worldcat.org/isbn/0792376439}}
</ref>

Pros:
* Can increase or decrease input [[Electrical impedance|impedance]] (depending on type of feedback).
* Can increase or decrease output impedance (depending on type of feedback).
* Reduces total distortion if sufficiently applied (increases linearity).
* Increases the bandwidth.
* Desensitizes gain to component variations.
* Can control [[step response]] of amplifier.

Cons:
* May lead to instability if not designed carefully.
* Amplifier gain decreases.
* Input and output impedances of a negative-feedback amplifier (''closed-loop amplifier'') become sensitive to the gain of an amplifier without feedback (''open-loop amplifier'')—that exposes these impedances to variations in the open-loop gain, for example, due to parameter variations or nonlinearity of the open-loop gain.
*Changes the composition of the distortion (increasing audibility) if insufficiently applied.