Negative Feedback Amplifier - Overview

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 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