Feedback Fundamentals: Basic Concepts and Circuit Topologies

F eedback is an important concept in circuit design, where a signal or voltage derived from the output is superimposed on the input. This output-to-input path can be used for several purposes— control output voltage, control gain, reduce distortion, improve stability, or create instability , as in an oscillator. This short tutorial reviews feedback, with emphasis on the classic negative feedback amplifier. Notes are also included on the methods for sampling the output and injecting the control signal at the input. For a classic concept like feedback, classic reference texts are a good source of instruction. This section was written with the aid of three of those references [1, 2, 3]. A feedback control system consists of the building blocks shown in Figure 1. Although this is not the traditional control theory textbook diagram, it includes the necessary elements: the signal path, a means of sampling the output, processing of the feedback signal, and a means of reintroducing the error signal at the input. In the simplest feedback systems, the feedback signal processing may be one or two passive components, with direct connections to the through circuit at the output and input. The diagram can also become much more complex, with extensive signal processing and the addition of reference signals, com-parators, delay lines and even multiple feedback loops. Intuitively, feedback control loops are used to achieve a specific performance objective. In a simple solid state voltage regulator, the output sample is compared to a known reference, and error signal is used to adjust the conduction of the pass transistor until equilibrium is achieved—a constant output voltage. A radio automatic gain control (AGC) system works similarly. In this case, a portion of the output signal is amplified, then rectified to create the DC voltage that can be compared to a reference. The error signal controls the gain Feedback is used to control gain and reduce distortion, as well as provide other important functions in modern electronic designs Figure 1 · The basic functions required to achieve feedback in an electronic system.