Dynamic current amplification

Chapter Dynamic current amplification

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Dynamic current amplificationAnother way of specifying current amplification is as the ratio of the difference in IC tothe difference in IB. Abbreviate the words “the difference in” by the letter d. Then, ac-cording to this second definition:Current amplification dIC/dIBA graph of collector current versus base current (IC vs IB) for a hypothetical tran-sistor is shown in Fig. 22-6. This graph resembles Fig. 22-4, except that current, ratherthan voltage, is on the horizontal scale. Three different points are shown, correspond-ing to various bias values.406 The bipolar transistor22-6Three different transistorbias points. See text fordiscussion.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 tan-gent line for point B in Fig. 22-6 is a dotted, straight line; the tangent lines for points Aand 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 valueis 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.) Atpoint C, dIC/dIB is practically zero. Transistors are rarely biased at these points.OverdriveEven when a transistor is biased for best operation (near point A in Fig. 22-6), a stronginput signal can drive it to point B or beyond during part of the cycle. Then, dIC/dIB is