Switching Power Supplies A - Z

Feedback Loop Analysis and Stability

On the right side of Figure 7-5, we have deliberately made the graph perfectly square in

shape. We have also assigned an equal number of grid divisions on the two axes. In addition,

to keep the x and y scaling identical, we have plotted 20 × log(f) on the x-axis (instead of

just log(f)). Having thus made the x- and y-axes identical in all respects, we realize why the

slope is called “−1” — it really does fall at exactly 45° now (visually too).

Therefore, by plotting 20 × log(gain) versus 20 × log(f), we have obtained a straight line

with a −1 slope. This allows us to do some simple math as shown in Figure 7-5.