When a fraction of the output of an amplifier is combined with the input, feedback exists; if the feedback opposes the original signal, it is negative feedback and if it increases the signal it is positive feedback. A negative feedback amplifier, or more commonly simply a feedback amplifier, is an amplifier which uses negative feedback to improve performance (gain stability, linearity, frequency response, step response) and reduce sensitivity to parameter variations due to manufacturing or environmental uncertainties. A single feedback loop with unilateral blocks is shown in Figure 1. Negative feedback is used in this way in many amplifiers and control systems.
Overview[]
Fundamentally, all electronic devices used to provide power gain (e.g. vacuum tubes, bipolar transistors, MOS transistors) are nonlinear. Negative feedback allows gain to be traded for higher linearity (reducing distortion), amongst other things. If not designed correctly amplifiers with negative feedback can become unstable, resulting in unwanted behavior, such as oscillation. The Nyquist stability criterion developed by Harry Nyquist of Bell Laboratories can be 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 distortion (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
- The gain of the amplifier decreases
- The input and output impedance's of the amplifier with feedback (the closed-loop amplifier) become sensitive to the gain of the amplifier without feedback (the open-loop amplifier); that exposes these impedance's to variations in the open loop gain, for example, due to parameter variations or due to nonlinearity of the open-loop gain.
Video[]
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