![]() In normal operation, transistors function in the active region, the area that the characteristic curve is a segment of an almost horizontal straight line. When saturated, a transistor cannot operate as expected. The collector current increases only if the V CE increases, for instance, to 4 V, for which the operating point moves from A to B.įigure 3 Typical set of characteristic curves for a transistor. This implies that if the base current is increased to I B2, but the V CE is still 2 V, the collector current does not change. ![]() On both curves the corresponding point is A. ![]() Suppose that the collector-emitter voltage is 2 V. Two characteristic curves, corresponding to two base currents I B1 and I B2 are shown. The meaning of transistor saturation is better demonstrated in Figure 3, in which the scale of the horizontal axis is augmented so that the line segments with sharp slopes can be better displayed. Saturation (in a transistor): The state of a transistor at which the collector current has reached its maximum value for the present collector-emitter voltage, and cannot increase further by only increasing the base current I B. In contrast, an increase in I B can move point N to N′, both corresponding to a collector-emitter voltage V N.įigure 2 Collector current versus collector voltage characteristic curve of a transistor. For this point, I C has reached its maximum and cannot be increased by increasing I B. For example, consider point M corresponding to V CE = V M in Figure 2b. Saturation implies that the collector current has reached its maximum value for that collector-emitter voltage and cannot increase further by increasing the base current I B. The area around this abrupt change in I C also shaded in Figure 2a, corresponds to when a transistor is in saturation. A transistor starts conducting and the collector current increases rapidly when V CE > 0. Saturation Regionįor each nonzero value of I B, the collector current starts from zero when the collector-emitter voltage is zero. The area under the curve corresponding to I B = 0, shaded in Figure 2a, represents the region where a transistor is cut off and is not conducting. In both Figure 2a and b, the curve corresponding to I B = 0 is exaggerated for clarity. When I B is zero, a transistor is cut off, and it does not conduct no matter how much voltage is applied to the collector any collector current is due to leaks, is very small, and is negligible. Each individual curve depicts the variation of I C versus the value of collector-emitter voltage (V CE) for a fixed value of base current I B. Because there are two parameters that affect I C, a set of individual curves shown together denote various operating conditions.Ī typical curve is shown in Figure 2a, and a set of these curves are depicted in Figure 2b. The characteristic curves of a transistor provide the relationship between collector-emitter voltage and collector current for different values of the base current. The base current can be varied by the variable resistor R B.įigure 1 Simple circuit for a transistor operation. ![]() This current can be varied either by changing the base current I B or the collector-emitter voltage V CE (the voltage between the collector C and the emitter E). We are interested in determining the variation of the collector current I C. Figure: Transistor Working with Two Power Suppliesįig u re 1 shows a simple circuit of a transistor, in which the 1.5 V battery and the resistance R B determine the base current I B, and the 24 V battery together with R C define the collector current I C.
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