TRANSISTOR h PARAMETERS

TRANSISTOR h PARAMETERS Bipolar transistor specifications often include h-parameter values (Se Figure 4 and these values are frequently used in the design and analysis of IT a  circuits Field effect-transistor specifications more often include y-param Cr we as which we- will discuss later in the chapter.Since there are three possible BJT configurations (CE. CC. and CB). there arc three different ways that the input and output can be defined. and therefore three corresponding sets of” parameters. A second subscript is added 10 each It-P Amur-r symbol to show the configuration for which it applies: e fur CF for CB. Thus, the 12″ parameters for a bipolar transistor arc J natch h common-emitter h parameters common-collector parameters common-base”parameters If all of the parameter values in one configuration arc known. then the values corresponding to any other configuration can be determined. The common-emitter values are the ones most often given. The values specified for transistor” parameters are almost always small-signal quantities. Occasionally. a dc value will be given. in which case it is common practice to use capital-letter subscripts, as, for example, “n:. It is important to remember that all small-signal BJT parameter values ate affected by de (quiescent) current, as discussed in Chapter 5. Each small-signal parameter is the ratio of certain ac voltages and currents. and each is valid only in a small region over which there is negligible change in device characteristics. Figure 9-15 shows how common-emitter Il-parametric values can be determined graphically using characteristic curves. In the common-emitter configuration, note that Therefore. from the definitions of the parameters. the computation of and h must be performed with Note that requiring the  quantity V”r tn he () is  the same as requiring that the tic quantity V be held constant. Similarly. the computation of h.; and h,…requires that. O. which is satisfied by requiring that In remain constant. A typical set of It-parameter values for a silicon NPN transistor is  u. Figure 9-10 shows the it-parameter equivalent circuit of a transistor in its common-emitter configuration. Recall that we discussed in Chapter 4 how the output voltage of a transistor feels its input characteristics, and we saw evidence of that fact in the family of curves of 1/1 versus VHf: generated by different values of Vn·. It should now be apparent that this feedback effect is accounted for in the It-parameter equivalent circuit: The voltage source h v in the input side of Figure 9-16 opposes vi: so the greater the value of v the smaller the current  See Figure 4-18. The value of h is a measure of how significant the feedback influence is, and since ,t is usually very small, we have been able to neglect the effect in earlier discussions. The It-parameter circuit provides us with a more accurate analysis model. Of course, the important fact that input current affects output voltage is also reflected in the h-paramedic model. by virtue or the controlled current source h This source is the same one we used in our original model for the transistor; the current source we labeled  See Figure 5-31. Table 9-2 gives conversion equations that can be used to obtain ii-parameter’ values for any configuration, given the values in another configuration. The table also lists typical values of each parameter in each configuration.Using Thole 9-2 and the common-emitter output characteristics shown in Figure 9-17, find approximate values for and 11 when lc = 4.2 O1A and Vo; = 15 V. The transistor whose characteristics are shown has hit = 1500 n and 2 X 10 4 at the operating point. Solution. The operating point where lc = 4.2 mA and Va = 15 V is labeled Q in Figure 9-17. To find hI” we determine Me along the vertical line through Q (a line of constant Vo) and the corresponding value of tll”. As shown in the figure, Me = (5.6 mA) (3.1 mA) = 2.5 mA and M” = (50 ILA) – (30 ILA) = 20 ILA. Therefore.operating.