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Concept
Negative-feedback amplifier
Résumé
A negative-feedback amplifier (or feedback amplifier) is an electronic amplifier that subtracts a fraction of its output from its input, so that negative feedback opposes the original signal. The applied negative feedback can improve its performance (gain stability, linearity, frequency response, step response) and reduces sensitivity to parameter variations due to manufacturing or environment. Because of these advantages, many amplifiers and control systems use negative feedback.
An idealized negative-feedback amplifier as shown in the diagram is a system of three elements (see Figure 1):
an amplifier with gain AOL,
a feedback network β, which senses the output signal and possibly transforms it in some way (for example by attenuating or filtering it),
a summing circuit that acts as a subtractor (the circle in the figure), which combines the input and the transformed output.
Fundamentally, all electronic devices that provide power gain (e.g., vacuum tubes, bipolar transistors, MOS transistors) are nonlinear. Negative feedback trades 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:
Pros:
Can increase or decrease input 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.
Source officielle
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