Biasing for Amplification

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Three different transistor bias points. The most amplification is obtained when the bias is near the middle of the straight-line portion of the curve.
Because a small change in IB causes a large variation in IC when the bias is just right, a transistor can operate as a current amplifier. If you look at above figure, you’ll see that there are some bias values at which a transistor won’t provide any current amplification. If the transistor is in saturation, the IC versus IBB, in these portions of the curve, causes little or no change in IC. But if the transistor is biased near the middle of the straight-line part of the curve in above figure, the transistor will work as a current amplifier.
 
The same situation holds for the curve in below figure. At some bias points, a small change in EB does not produce much, if any, change in IC; at other points, a small change in EB produces a dramatic change in IC. Whenever we want a transistor to amplify a signal, it’s important that it be biased in such a way that a small change in the base current or voltage will result in a large change in the collector current.
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    Relative collector current (IC) as a function of base voltage (EB) for a hypothetical NPN silicon transistor.

    Static Current Amplification

    Current amplification is often called beta by engineers. It can range from a factor of just a few times up to hundreds of times. The beta of a transistor can be expressed as the static forward current transfer ratio, abbreviated HFE. Mathematically, this is the collector current divided by the base current:
    HFE = IC/IB

    For example, if a base current, IB, of 1 mA results in a collector current, IC, of 35 mA, then HFE = 35/1 = 35. If IB = 0.5 mA and IC = 35 mA, then HFE = 35/0.5 = 70. The HFE specification for a particular transistor represents the greatest amount of current amplification that can be obtained with it.

    Dynamic Current Amplification

    A more practical way to define current amplification is as the ratio of the difference in IC to the difference in IB that occurs when a small signal is applied to the base of a transistor. Abbreviate the words “the difference in” by d. Then, according to this second definition:
    Current amplification = dIC/dIB
    Above Figure is a graph of the collector current as a function of the base current (IC versus IB) for a hypothetical transistor. Three different points are shown, corresponding to three different bias scenarios. 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 above figure is a dashed straight line; the tangent lines for points A and C lie right along the curve and are therefore not shown. The steeper the slope of the line, the greater is dIC/dIB. Point A provides the highest value of dIC/dIB, provided the input signal is not too strong. This value is very close to HFE.
     
    For small-signal amplification, point A in above figure 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, so point B is not as good for small-signal amplification. At point C, dIC/dIB is practically zero. The transistor won’t amplify much, if at all, if it is biased at this point.

    Overdrive

    Even when a transistor is biased for the greatest possible current amplification (at or near point A in above figure), a strong ac input signal can drive it to point B or beyond during part of the signal cycle.

    Then, dIC/dIB is reduced, as shown in following figure. Points X and Y in the graph represent the instantaneous current extremes during the signal cycle in this particular case. When conditions are like those in following figure, a transistor amplifier will cause distortion in the signal. This means that the output wave will not have the same shape as the input wave. This phenomenon is known as non linearity. It can sometimes be tolerated, but often it is undesirable. When the input signal to a transistor amplifier is too strong, the condition is called overdrive, and the amplifier is said to be over driven.
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    Excessive input reduces amplification.
    Overdrive can cause problems other than signal distortion. An overdriven transistor is in or near saturation during part of the input signal cycle. This reduces circuit efficiency, causes excessive collector current, and can overheat the base-collector (B-C) junction. Sometimes overdrive can destroy a transistor.