Operational Amplifier Feedback

Feedback is a process whereby a fraction of the output signal is sensed (V_F) and compared with the input (V_IN), generating a very small feedback error term (V_E= V_IN – V_F), thereby making the output an accurate replica of the input.

Example:

The typical non-inverting closed-loop configuration. It is instructive to identify four elements in the feedback system.

• The feed-forward amplifier (A)
• A means of sensing the output
• The feedback network (β)
• A means of generating the feedback error (Σ)

These elements exist in every feedback system. The accompanying image describes a general feedback system showing both negative and positive feedback configurations.

Negative and Positive Feedback

There are two kinds of feedback, negative feedback and positive feedback. The block diagrams show that, if the sensed feedback voltage V_F has the same phase as the input voltage, then we say it is negative feedback. Otherwise, it is positive feedback.

Negative feedback allows high-precision signal processing. Positive feedback makes it possible to build oscillators.

Before proceeding to the analysis of feedback circuits, we'll first describe the benefits of negative feedback.

• Provides gain desensitization. For example, the closed-loop gain is much less sensitive to device parameters than the open-loop gain is.
• Controls the input/output impedance; that is, raises or lowers the input and output impedances by the selection of an appropriate feedback topology.
• Increases the amplifier bandwidth.
• Reduces the noise; that is, minimizes the contribution to the output of unwanted electric signals generated by the circuit components themselves as well as extraneous sources.
• Reduces nonlinear distortion in an amplifier; that is, makes the output proportional to the input.