Negative Feedback Amplifiers

Negative feedback amplifier
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Figure 1: Ideal negative feedback model
A negative feedback amplifier (or more commonly simply a feedback amplifier) is an amplifier a fraction of the output of which is combined with the input so that a negative feedback opposes the original signal. The applied negative feedback improves performance (gain stability,linearity, frequency response, step response) and reduces sensitivity to parameter variations due to manufacturing or environment. Because of these advantages, negative feedback is used in this way in many amplifiers and control systems.[1]
A negative feedback amplifier is a system of three elements (see Figure 1): an amplifier with gain AOL, an attenuating feedback network with a constant β < 1 and asumming circuit acting as a subtractor (the circle in the figure). The amplifier is the only obligatory; the other elements may be omitted in some cases. For example, in a voltage (emitter, source, op-amp) follower the feedback network and the summing circuit are not necessary.

Contents [hide]
1 Overview
2 History
3 Classical feedback
3.1 Gain reduction
3.2 Bandwidth extension
3.3 Multiplepoles
4 Asymptotic gain model
5 Feedback and amplifier type
6 Two-port analysis of feedback
6.1 Replacement of the feedback network with a two-port
6.2 Small-signal circuit
6.3 Loaded open-loop gain
6.4 Gain with feedback
6.5 Input and output resistances
6.5.1 Background on resistance determination
6.5.2 Application to the example amplifier
6.6 Load voltage and load current
6.7 Is themain amplifier block a two port?
7 See also
8 References and notes
[edit]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 canbecome 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:[2]
Pros:
Can increase or decrease input impedance (depending on type of feedback)
Can increase or decrease output impedance (depending on type offeedback)
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 impedances of the amplifier with feedback (the closed-loop amplifier) become sensitive to the gain of the amplifierwithout feedback (the open-loop amplifier); that exposes these impedances to variations in the open loop gain, for example, due to parameter variations or due to nonlinearity of the open-loop gain
[edit]History

The negative feedback amplifier was invented by Harold Stephen Black (US patent 2,102,671 (issued in 1937)[3] ) while a passenger on the Lackawanna Ferry (from Hoboken Terminal to Manhattan)on his way to work at Bell Laboratories (historically located in Manhattan instead of New Jersey in 1927) on August 2, 1927. Black had been toiling at reducing distortion in repeater amplifiers used for telephone transmission. On a blank space in his copy of The New York Times,[4] he recorded the diagram found in Figure 1, and the equations derived below.[5] Black submitted his invention to the U.S. Patent Office on August 8, 1928, and it took more than nine years for the patent to be issued. Black later wrote: "One reason for the delay was that the concept was so contrary to established beliefs that the Patent Office initially did not believe it would work."[6]
[edit]Classical feedback

[edit]Gain reduction
Below, the voltage gain of the amplifier with feedback, the closed-loop...