10.5 HYBRID OR h-PARAMETERS

If I ₁ and V ₂ are taken as the independent variables, the two-port equations may be written as

The constants h ₁₁, h ₁₂, h ₂₁, and h ₂₂ are called the hybrid (or h) parameters of the two-port network.

If

If

From the definitions of h-parameters, it is seen that h ₁₁ has the dimensions of impedance, h ₁₂ and h ₂₁ are dimensionless, and h ₂₂ has the dimensions of admittance. Thus, these parameters are dimensionally homogeneous. That is why these parameters are called hybrid parameters. These parameters are most suitable for transistor testing because of the following reasons:

1. The output impedance of a transistor is very high and shorting of port 2-2 ? makes it easier to measure h ₁₁ and h ₂₁.

2. The input impedance is low and the open circuit test can be performed makes it conveniently to measure h ₁₂ and h ₂₂.

In transistor applications, the symbols h ₁₁, h ₁₂, h ₂₁, h ₂₂ are replaced by h _i, h _r, hf, h _o, respectively, to denote input, reverse, forward, and output.

The standard Equations (10.7 and 10.8) may be used to determine the equivalent circuit of a two-port network in terms of the h-parameter. This circuit is shown in Figure 10.11.

Figure 10.11: Equivalent circuit of a two-port network in terms of h-parameters