Conversion of Hybrid Parameters in Transistor Three Configurations:

Some transistor manufacturers provide only the four CE hybrid parameters while others provide CB hybrid parameters. Sometimes, for a specific purpose, it becomes necessary to convert one set of h parameters in one configuration to another set in another configuration. The conversion formulas can be obtained using the definitions of the parameters involved and Kirchhoff’s laws.

For example, let us convert common-emitter h parameters to the common-base h parameters. For this purpose first the CB hybrid model is drawn as shown in Fig. 11.12 (a) and then it is redrawn in CE configuration, as shown in Fig. 11.12 (b). The latter corresponds in every detail to the former except that the emitter terminal E is made common to the input and output ports.

By definitions

If I_b = 0, then I_c = -I_e, and the current I in h_ob in Fig. 11.12 (b) is I = (1 + h_fb) I_e = h_ob V_bc

Applying Kirchhoff’s voltage law to the output mesh of Fig. 11.12 (b) we have

Combining the above two equations we have

Thus

This is an exact expression. The simpler approximate formula is obtained by noting that, for typical values given in Table 11.3 h_rb << 1 and h_obh_ib << (1 + h_fb)

So

By definition

If terminals C and E are connected together in Fig. 11.12 (b), we have circuit shown in Fig. 11.13.

From Fig. 11.13, we see that V_cb = -V_bc = – V_be

Applying Kirchhoff’s voltage law to the left-hand mesh, we have

Combining the above two equations we have

Applying Kirchhoff’s current law to node B in Fig. 11.13, we have

Thus

This is an exact expression. Since h_rb << 1 and h_ob h_ib << (1 + h_fb), so the above equation is reduced to

Approximate conversion formulas for transistor parameters are given in Table 11.4.