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Biasing for current amplification
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Because a small change in the base current, IB, results in a large collector-current (IC) variation when the bias is just right, a transistor can operate as a current amplifier. It might be more technically accurate to say that it is a current-fluctuation amplifier, because it s the magnification of current variations, not the absolute current, that s important.
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22-5 At A, simple PNP circuit using dual-diode modeling. At B, the actual transistor circuit.
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If you look at Fig. 22-4 closely, you ll see that there are some bias values at which a transistor won t give current amplification. If the E-B junction is not conducting, or if the transistor is in saturation, the curve is horizontal. A small change (to the left and right) of the base voltage, EB, in these portions of the curve, will cause little or no up-and-down variation of IC. But if the transistor is biased near the middle of the straight-line part of the curve in Fig. 22-4, the transistor will work as a current amplifier.
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Current amplification is often called beta by engineers. It can range from a factor of just a few times up to hundreds of times. One method of expressing the beta of a transistor is as the static forward current transfer ratio, abbreviated HFE. Mathematically, this is HFE IC /IB
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Thus, if a base current, IB, of 1 mA results in a collector current, IC, of 35 mA, HFE 35/1 35. If IB 0.5 mA yields IC 35 mA, then HFE 35/0.5 70. This definition represents the greatest current amplification possible with a given transistor.
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406 The bipolar transistor
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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. 22-6. This graph resembles Fig. 22-4, except that current, rather than voltage, is on the horizontal scale. Three different points are shown, corresponding to various bias values.
22-6 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. 22-6 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.
Overdrive
Even when a transistor is biased for best operation (near point A in Fig. 22-6), a strong input signal can drive it to point B or beyond during part of the cycle. Then, dIC /dIB is
Gain versus frequency 407 reduced, as shown in Fig. 22-7. Points X and Y in the graph represent the instantaneous current extremes during the signal cycle.
22-7 Excessive input reduces amplification.
When conditions are like those in Fig. 22-7, there will be distortion in a transistor amplifier. The output waveform will not have the same shape as the input waveform. This nonlinearity can sometimes be tolerated; sometimes it cannot. The more serious trouble with overdrive is the fact that the transistor is in or near saturation during part of the cycle. When this happens, you re getting no bang for the buck. The transistor is doing futile work for a portion of every wave cycle. This reduces circuit efficiency, causes excessive collector current, and can overheat the base-collector (B-C) junction. Sometimes overdrive can actually destroy a transistor.
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