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Dynamic current amplification
Another way of specifying current amplification is as the ratio of the difference in IC to the difference in IB. Abbreviate the words “the difference in” by the letter d. Then, according to this second definition:
Current amplification = dIC/dIB
A graph of collector current versus base current (IC vs IB) for a hypothetical transistor is shown in Fig. 1. Except that current, rather than voltage, is on the horizontal scale. Three different points are shown, corresponding to various bias values.
Fig. 1: Three different transistor bias points. See text for discussion.
The ratio dIC/dIB is different for each of the points in this graph. Geometrically, dIC/dIB at a given point is the slope of a line tangent to the curve at that point. The tangent line for point B in Fig. 1 is a dotted, straight line; the tangent lines for points A and C lie right along the curve. The steeper the slope of the line, the greater is dIC/dIB. Point A provides the highest dIC/dIB , as long as the input signal is small. This value is very close to HFE. For small-signal amplification, point A represents a good bias level. Engineers would say that it’s a good operating point.
At point B, dIC/dIB is smaller than at point A. (It might actually be less than 1.) At point C, dIC/dIB is practically zero. Transistors are rarely biased at these points.
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