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.

Fig. 1 Example of negative feedback circuit (inverting amplifier circuit)
Fig. 1 Example of negative feedback circuit (inverting amplifier circuit)

Fig. 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.
Fig. 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.
In addition, although the open loop gain in the DC region of a plain operational amplifier (without negative feedback) has a very high gain, there are variations. By applying negative feedback, the gain is fixed at the gain (closed-loop gain) set by an external resistor, making it possible to absorb variations in open-loop gain.

Fig. 2 Bode plot of an op-amp
Fig. 2 Bode plot of an op-amp
Fig. 3 Example of variation in the same product
Fig. 3 Example of variation in the same product
Fig. 4 Absorption of variations in negative feedback circuit
Fig. 4 Absorption of variations in negative feedback circuit

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