Variations of Hybrid Parameters of a Transistor:
The variation of h parameters depends upon junction temperature, collector current and collector-to-emitter voltage VCE. Among these factors, the variations of Hybrid Parameters of a Transistor due to junction temperature and collector current are significant and thus discussed here.
Variation of h Parameters Due to Temperature Variation:
From the above discussion it is obvious that all the four h parameters of any transistor configuration (viz. CB, CE and CC) can be determined from the slopes and spacing between curves at the quiescent point Q if it is specified. Since the characteristic curves, in general, are neither straight lines nor equally spaced for equal variations in collector-emitter voltage or base current, the values of h parameters depend upon the position of the Q-point on the curves. It is also known that the shape and actual numerical values of the characteristic curves depend on the junction temperature. Thus h parameters of a transistor depends upon the junction temperature. Most transistor specification sheets include curves of the variation of the h parameters with the quiescent point and temperature. Usually IC = 1 mA is taken as the reference collector current and collector junction temperature of 25°C is taken as the reference temperature.
Variations of h Parameters Due to Variation in Collector Current:
(a) The parameter hie varies with operating value of collector current lC as shown in Fig. 11.11 (a) i.e., hie decreases with the increase in the operating value of IC.
(b) The parameter hre varies with the operating value of IC, as depicted in Fig. 11.11 (b). From the curve shown in Fig. 11.11(b), it is obvious that hre first decreases with the increase in the operating value of IC, attains a minimum value at a certain value of IC and thereafter increases with the increase in lC.
(c) The parameter hfe varies with IC as shown in Fig. 11.11 (c). The graph shown in Fig. 11.11 (c) indicates that hfe first increases with IC, attains a maximum value at a particular value of IC and then decreases slightly with IC. The value of hfe is temperature dependent and increases with the rise in temperature.
(d) The parameter hoe varies with IC as indicated in Fig. 11.11 (d), i.e., hoe increases with the increase in IC but not linearly.