Let amax 104 and amin 103 and let b 01 we then

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Unformatted text preview: by at least an order of magnitude over a useful range of frequency. Let Amax = 104 and Amin = 103, and let B = 0:1. We then calculate for the corresponding closed-loop gain extremes: 104 Gmax = 1 + 103  101 , 10,3  103 Gmin = 1 + 102  101 , 10,2  Hence, the factor of 10 open-loop gain variation has been reduced to a 1 variation. This is typical of negative feedback. It attenuates errors which appear within the feedback loop, either internal or external to the op-amp proper. In general, the bene ts of negative feedback go as the loop gain factor AB . For most op-amps, A is very large, starting at 105 for f 100 Hz. A large gain G can be achieved with large A and relatively small B , at the expense of somewhat poorer performance relative to a smaller gain, large B choice, which will tend to very good stability and error compensation properties. An extreme example of the latter choice is the op-amp follower" circuit, consisting of a non-inverting ampli er see Fig. 31 with R2 = 0 and R1 removed. In this case, B = 1, giving G = A=1 + A  1. Another interesting feature of negative feedback is one we discussed brie y in class. The qualitative statement is that any signal irregularity which is put into the feedback loop will, in the limit B ! 1, be taken out of the output. This reasoning is as follows. Imagine a small, steady signal vs which is added within the feedback loop. This is returned to the output with the opposite sign after passing through the feedback loop. In the limit B = 1 the output and feedback are identical G = 1 and the cancellation of vs is complete. An example of this is that of placing a push-pull" output stage to the op-amp output in order to boost output current. See text Section 2:15. The push-pull circuits, while boosting current, also exhibit cross-over distortion", as we discussed in class and in the text. However, when the stage is placed within the op-amp negative feedback loop, this distortion can essentially be removed, at least when the loop gain AB is large. 6.7 Compensation in Op-amps Recall that...
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