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

This preview shows page 1. Sign up to view the full content.

This is the end of the preview. Sign up to access the rest of the document.

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  101 , 10,3  103 Gmin = 1 + 102  101 , 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...
View Full Document

{[ snackBarMessage ]}

Ask a homework question - tutors are online