Feedback Amplifier Trivia Questions & FAQ

Feedback is a mechanism, process or signal that is looped back to control a system within itself. Such a loop is called a feedback loop. In systems containing an input and output, feeding back part of the output to the input.

1)An in-phase feedback signal, where a positive-going wave on the input leads to a positive-going change on the output, will amplify the input signal, leading to more modification. This is known as positive feedback. Positive feedback amplifies possibilities of divergences.
2) A feedback signal which is inverted, where a positive-going change on the input leads to a negative-going change on the output, will dampen the effect of the input signal, leading to less modification. This is known as negative feedback. They exist when the fed-back output signal is out of phase with the input signal. This occurs when the fed-back signal is anywhere from 90? to 270? with respect to the input signal.

The main applications of feedback in electronics are in the designs of amplifiers, oscillators, and logic circuit elements. The processing and control of feedback is engineered into many electronic devices and may also be embedded in other technologies.

A device is said to be operating open loop if no output feedback is being employed and closed loop if feedback is being used. Electronic feedback loops take two forms: negative feedback loops and positive feedback loops.

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.

A negative feedback amplifier is a system of three elements an amplifier with gain AOL, an attenuating feedback network with a constant ? < 1 and a summing 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.

1) Can increase or decrease input impedance (depending on type of feedback)
2) Can increase or decrease output impedance (depending on type of feedback)
3) Reduces distortion (increases linearity)
4) Increases the bandwidth
5) Desensitizes gain to component variations
6) Can control step response of amplifier.

The negative voltage series feedback increases the input impedance and decreases the output impedance of amplifier. Such a change is profitable in practice as the amplifier can then serve the purpose of impedance matching.

To obtain very high i/p and o/p impedance in feedback amplifier, the topology mostly used is current series feedback amplifier.

The negative current shunt feedback increases the bandwidth of amplifier by the factor (1+mI Ai).