Negative Feedback in Electronics

Lead Image Novacek

A Look at the Opposite Side

Besides closed-loop control systems, negative feedback is found in many electronic circuits—especially in amplifiers. And just like positive feedback, negative feedback can significantly change or modify a circuit’s performance.

By George Novacek

Following last month’s discussion of positive feedback, let’s now take a look at its opposite: the negative feedback. Besides closed-loop control systems, it is found in many electronic circuits, especially in amplifiers. As we have already seen, feedback significantly changes or modifies a circuit’s performance. The end of the 19th century and the beginning of the 20th century was the era of introduction of the telephone. For long distance calls, amplifiers were needed along the telephone lines to make up for their transmission losses.

Vacuum tube amplifiers of the day suffered from many ailments: drift, high distortion and generally poor performance, making the long-distance voice communications nearly unintelligible. Harold Stephen Black, an AT&T engineer, was one of many working to solve this problem. Eventually—because he was familiar with the effects of negative feedback in mechanical systems—he tried to apply it to a vacuum tube amplifier. The result was astonishing and amplifiers with negative feedback have been with us ever since.

FIGURE 1 Transfer function of an operational amplifier with negative feedback

Transfer function of an operational amplifier with negative feedback

The op amp is the epitome of feedback application in electronic circuits. Because its comprehension is valid for all electronic feedback circuits, let’s take a closer look at the op amp. To analyze the negative feedback mathematically, we’ll consider an amplifier as a combination of two functional blocks: The open loop gain (OLG) block with transfer function A(s) and the feedback block with transfer function β(s). With monolithic amplifiers, the feedback is usually applied externally. The overall transfer function follows the principle shown in Figure 1.. …

Read the full article in the November 328 issue of Circuit Cellar

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