Reduced Non-linear Distortion:

 The final stage of an OPAMP has non-linear distortion when the signal swings over most of the ac load line. Large swings in current cause the r'_e of a transistor to change during the cycle. In other words, the open loop gain varies throughout the cycle of when a large signal is being applied. It is this changing voltage gain that is a source of the non-linear distortion.

Noninverting voltage feedback reduces non-linear distortion because the feedback stabilizes the closed loop voltage gain, making it almost independent of the changes in open loop voltage gain. As long as loop gain, is much greater than 1, the output voltage equals 1/B times the input voltage. This implies that output will be a more faithful reproduction of the input .

Consider, under large signal conditions, the open loop OPAMP circuit produces a distortion voltage, designated v_dist. It can be represented by connecting a source v_dist in series with Av_d. Without negative feedback all the distortion voltage v_dist appears at the output. But with negative feedback, a fraction of v_dist is feedback to inverting input. This is amplified and arrives at the output with inverted phase almost completely canceling the original distortion produced by the output stage.

The first term is the amplified output voltage. The second term in the distortion that appears at the final output. The distortion voltage is very much, reduced because AB>>1

Bandwidth with Feedback:

The bandwidth of an amplifier is defined as the band of frequencies for which the gain remains constant. Fig. 3, shows the open loop gain vs frequency curve of 741C OPAMP. From this curve for a gain of 2 x 10⁵ the bandwidth is approximately 5Hz. On the other hand, the bandwidth is approximately 1MHz when the gain is unity.

Fig. 3

The frequency at which gain equals 1 is known as the unity gain bandwidth. It is the maximum frequency the OPAMP can be used for.
Furthermore, the gain bandwidth product obtained from the open loop gain vs frequency curve is equal to the unity gain bandwidth of the OPAMP.
Since the gain bandwidth product is constant obviously the higher the gain the smaller the bandwidth and vice versa. If negative feedback is used gain decrease from A to A / (1+AB). Therefore the closed loop bandwidth increases by (1+AB).

Bandwidth with feedback = (1+ A B) x (B.W. without feedback)

f_f= f_o (1+A B)

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