Experimental Determination of Hybrid Parameters:

Determination of hybrid parameters of a general linear circuit has already been discussed. For determination of hybrid parameters for a CE transistor, consider the circuits given in Fig. 11.8. The rms values will be considered in discussion. Volt-ampere equations for a CE transistor are as given below :

For measurement of h_ie and h_fe parameters, output is ac short circuited [Fig. 11.8 (a)] by making capacitance C₂ deliberately large. The result is that the changing component of collector current flows through C₂ instead of R_C and ac voltage developed across C₂ is zero i.e., V_ce = 0. Here it should be clarified that setting V_ce = 0 does not mean that the dc collector-emitter voltage, V_CE is zero. It simply means that ac output is short circuited.

Substituting V_ce = 0 in Eqs. (11.10) and (11.11), we have

Thus from Eqs. (11.12) and (11.13) h parameters h_ie and h_fe can be determined. It is to be noted here that I_b and I_c are ac rms base and collector currents respectively. Also V_be is the ac rms value of base-emitter voltage.

For measuring hybrid parameters h_oe and h_re the input is ac open circuited, a signal generator is applied across the output and a large inductor L is connected in series with R_B, as illustrated in Fig. 11.8 (b). The inductor L, having dc resistance very small, does not disturb the operating point but does not allow flow of ac current through R_B. Furthermore, a voltmeter of high input impedance is used to measure base-emitter voltage V_be and hence there are no paths connected to the base with any appreciable ac current. Thus base is effectively ac open circuited i.e., I_b = 0.

Substituting I_b = 0 in Eqs. (11.10) and (11.11), we have

Thus by measurement of V_be, V_ce and l_c, the hybrid parameters h_re and h_oe can be determined.