Common-emitter output characteristics show collector current I( versus collector voltage Vo:, for different fixed values of 11/. These characteristics are often called collector churacteristics. Figure 4-19 shows a typical set of output characteristics for an NPt’J transistor in the CE configuration.
The approximate value of the {3 of the transistor can be determined at any point on the characteristics in Figure 4- t 9 simply by dividing the values le/I/I at the’ point. As illustrated in -the figure, at Vo:, = 5 V and 1/1 = 50 J.tA. the value of Ie is 5 mA, and the value of the (3 at that point is therefore /”///1 = (5 IIIA )/(50 J.tA) = 100. Clearly (3. like a, is no~ constant, but depends on the region of the characteristics where the transistor is operated. The region where the curves arc approximately horizontal is the active region of the. CE configuration. In this region. (3 is essentially constant, but increases somewhat with Vo., as can he deduced from . the rise in each curve as Vo increases to the right.
increase rapidly if Veil is permitted to become large. When 1/1= 0 (base open), the collector-to-emitter current at which breakdown occurs is document Tile value of {{Vou is always less than that of IJVUi(I for a given transistor. IJVou is sometimes called the “sustaining voltage” and denoted L I’CH)’ When interpreting the characteristics of Figure 4-19, it i\ Important to realize that each curve is drawn for a small, essentially constant value of V/IF (about
0,7 V for silicon). Figure 4-20 illustrates this point
Comparing the CB output characteristics in Figure 4-13 with the CE output characteristics in Figure 4-19, we note that the curves rise more steeply to the right in the CE case. This rise simply reflects the fact we have already discussed in connection with the CE input characteristics: The greater the value of VC/;, the smaller the base region, and, consequently, the smaller the base current. But, since
base current is constant along each curve in Figure 4-19, the effect appears as an increase in l.: J n other words, the fact that there are fewer recombinations occurring in the base means that a greater proportion of carriers cross the junction to become collector current
The .DC statement causes VCE to be :.lcppeJ in 5-V increments from 0 through 50 V and ‘/1 to be stepped in lO-,uA increments from 0 through 40 ,uA.
The plot generated by SPICE is shown in Figure 4-22(b). This presentation is somewhat different than that shown in Figure 4-19, since the individual curves corresponding to different values of l« must be displaced vertically by the printer. The curves have been filled in between the plotted points (asterisks) and labeled with the values of IIJ to clarify the presentation. At VCF. = 25 V and III = 30,uA, we see that I(VDUM), which equals Ie, is 3.636 mA Thus,
Note how the curves rise for increasing values of Vcr due to the effect of the Early voltage, meaning that f3 increases with increasing VC/. (and with mcrcasing III)’
