Variations of Hybrid Parameters of a Transistor:

The variation of h parameters depends upon junction temperature, collector current and collector-to-emitter voltage V_CE. 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 I_C = 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 h_ie varies with operating value of collector current l_C as shown in Fig. 11.11 (a) i.e., h_ie decreases with the increase in the operating value of I_C.

(b) The parameter h_re varies with the operating value of I_C, as depicted in Fig. 11.11 (b). From the curve shown in Fig. 11.11(b), it is obvious that h_re first decreases with the increase in the operating value of I_C, attains a minimum value at a certain value of I_C and thereafter increases with the increase in l_C.

(c) The parameter h_fe varies with I_C as shown in Fig. 11.11 (c). The graph shown in Fig. 11.11 (c) indicates that h_fe first increases with I_C, attains a maximum value at a particular value of I_C and then decreases slightly with I_C. The value of h_fe is temperature dependent and increases with the rise in temperature.

(d) The parameter h_oe varies with I_C as indicated in Fig. 11.11 (d), i.e., h_oe increases with the increase in I_C but not linearly.