-VE Feed back amplifier

which uses a A negative feedback amplifier, or more commonly simply a feedback amplifier, is an amplifiernegative feedback network, generally for improving performance (gain stability, linearity, frequency response etc.). Feedback can be negative or positive, but must be negative for overall stability of the system.
of The negative feedback amplifier was invented by Harold Stephen Black (US patent 2,102,671 (issued in 1937) [1] ) at Bell Laboratories in 1927. The idea of this invention is simple: If the loop gain is large, overall response of the whole system can be very close to inverted feedback network response. Fundamentally, all electronic devices (e.g. vacuum tubes, bipolar transistors, MOS transistors) are nonlinear devices. Negative feedback corrects this by trading unused gain for higher linearity (lower distortion). Though much more accurate, amplifiers with negative feedback can become unstable if not designed correctly, causing them to oscillate. Harry NyquistBell Laboratories managed to work out a theory regarding how to make it stable.
Negative feedback is used in this way in many types of amplification systems to stabilize and improve their operating characteristics (see e.g., operational amplifiers). Note that negative feedback is used to "stabilize" the system, not in amplifying the signal, whereas positive feedback is used to amplify the signal itself (which may lead to instability).
Contents[hide]
1 Classical model
1.1 Voltage amplifiers
1.1.1 Advantages
1.1.2 Disadvantages
2 Asymptotic gain model
3 See also
4 Notes
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[] Classical model

Figure 1: Ideal negative feedback model

[edit] Voltage amplifiers
Consider a voltage amplifier (other systems are similar). Without feedback, the output voltage , where the amplification AO (also known as the open-loop gain) may in general be a function of both frequency and voltage.
The open-loop gain AO is given as

Suppose we have a feedback loop so that a fraction of the output is added to the input. β is known as the feedback factor and is determined by the feedback network that is connected around the amplifier. For an operational amplifier two resistors may be used for the feedback network to set an arbitrary closed-loop gain. This network may be modified using reactive elements like capacitors or inductors to (a) give frequency dependent closed-loop gain as in equalization/tone-control circuits or (b) construct oscillators.
The input to the amplifier is now V'in, where

The closed-loop gain AC is given by,

Substituting for ,

Dividing numerator and denominator by ,

But since
,
then

If , then and the effective amplification (or closed-loop gain) AC is set by the characteristics of the feedback constant β, thus making linearizing and stabilizing the amplification characteristics straightforward.
Note also that if there are conditions where , the amplifier has infinite amplification - it has become an oscillator, and the system is unstable.
The stability characteristics of the gain feedback product are often displayed and investigated on a Nyquist plot (a polar plot of the gain/phase shift as a parametric function of frequency).

[] Advantages
Increases input impedance
Decreases output impedance
Reduces distortion
Increases the bandwidth
Desensitizes gain to component variations

[] Disadvantages
May lead to instability if not designed carefully
The gain of the amplifier decreases.

[] Asymptotic gain model
Main article: asymptotic gain model
A more complete way to model negative feedback amplifiers is with the asymptotic gain model.

[] See also
Operational amplifier

[edit] Notes
^ http://eepatents.com/patents/2102671.pdf. Retrieved on October 24, 2005.
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