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Why is feedback used in op-amps?

Typical op-amps have an open-loop gain on the order of 105 (100 dB). Without feedback, op-amps make circuit design difficult because of high gain sensitivity. Negative feedback makes it possible to set the gain and cut-off frequency to the desired values, thereby improving their stability and reducing performance variation, part-to-part variation, and sensitivity to temperature and other environmental parameters.

Figure 1 shows an inverting amplifier. The closed-loop gain (Gv) of this negative feedback circuit can be simply expressed as:

Gv = -R2/R1

This equation indicates that the closed-loop gain is solely determined by the ratio of external resistor values and is not affected by environmental factors.

Figure 2 shows the Bode plot of a typical op-amp. The open-loop gain, i.e., the gain without negative feedback, decreases at a rate of 6 dB/oct (= 20 dB/dec). In contrast, the closed loop gain has a constant gain from DC to 10 kHz in this figure (which depends on the frequency response of the op-amp). As indicated by this example, using feedback provides an amplifier with bandwidth.

Figure 1 Negative feedback circuit (inverting amplifier)
Figure 1 Negative feedback circuit (inverting amplifier)

Also see the application notes on op-amps:

Figure 2 Bode plot of an op-amp
Figure 2 Bode plot of an op-amp

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