BMET Wiki
Advertisement

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.

If the signal is inverted on its way round the control loop, the system is said to have negative feedback; otherwise, the feedback is said to be positive. Negative feedback is often deliberately introduced to increase the stability and accuracy of a system by correcting unwanted changes. This scheme can fail if the input changes faster than the system can respond to it. When this happens, the lag in arrival of the correcting signal results in unintended positive feedback, causing the output to oscillate or hunt Oscillation is usually an unwanted consequence of system behavior.

Harry Nyquist contributed the Nyquist plot for assessing the stability of feedback systems. An easier assessment, but less general, is based upon gain margin and phase margin using Bode plots (contributed by Hendrik Bode). Design to insure stability often involves frequency compensation, one method of compensation being pole splitting.

The high-pitched squeal that sometimes occurs in audio systems, PA systems and rock music is known as audio feedback. If a microphone is in front of a speaker that it is connected to, the noise put into the mic will come out of the speaker. Since the mic is in front of the speaker, the original sound (now coming from the speaker) goes back into the mic. This happens over and over, getting louder each time. This process produces the squeal.

Uses[]

uses negative feedback to improve performance ...

  • gain stability
  • linearity
  • frequency response
  • step response

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 impedances 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 impedances to variations in the open loop gain, for example, due to parameter variations or due to nonlinearity of the open-loop gain
Advertisement