h-parameter Model for Transistor Configurations

Common-emitter configuration

Figure below shows the h-parameter equivalent model for the common-emitter configuration.

h-parameter Model for Transistor Configurations

A second subscript has been added to the nomenclature of the h-parameters so as to distinguish between the h-parameters of the three transistor configurations. The h-parameter equations for the common-emitter configuration are given by the following two equations

Where,
Parameter h11 is denoted as hie and is referred to as the input impedance of the transistor in the common-emitter configuration
Parameter h21 is denoted as hfe and stands for forward current transfer ratio
Parameter h12 is denoted as hre and is referred to as the reverse voltage transfer ratio
Parameter h22 is denoted as hoe and it means the output admittance for the common-emitter configuration Current Ii is the base current Ib
Current Io is the collector current Ic
Voltage Vi is the voltage Vbe
Voltage Vo is the voltage Vce.
The values of the h-parameters are given by the following equations.

The symbol ∆ refers to a small variation around the quiescent point of operation, that is, the h-parameters are determined in the region of operation for the applied signal.

The effect of the parameter h_re is so small on the transistor amplifier that it can be neglected. Therefore, h_re is assumed to be zero. Hence, the magnitude of the voltage source h_reV_ce is also equal to zero. In other words, it results in short-circuit equivalent for the feedback element. Figure below shows the simplified equivalent model for the common-emitter transistor configuration.

Simplified equivalent h-parameter model for the common-emitter transistor

In cases where the value of 1/h_oe is very large as compared to the value of load resistance, it is assumed to be open in comparison with the parallel load to be connected across the output terminals.

Common-collector configuration

Figure below shows the h-parameter equivalent model for the common-collector configuration

h-parameter equivalent model for the common-collector configuration

The parameter hic is referred to as the input impedance of the transistor in the common-collector configuration. Parameter h_fc stands for forward current transfer ratio and parameter h_rc is referred to as the reverse voltage transfer ratio. Parameter hoc is the output admittance in the common-collector configuration. The h-parameters for the common-collector configuration can be determined in a similar fashion as that for the common-emitter configuration.

Figure below shows the simplified equivalent model for the common-collector transistor configuration.

Simplified h-parameter equivalent model for the common collector configuration

Z

Figure below shows the h-parameter equivalent model for the common-base configuration.

h-parameter equivalent model for the common-base configuration

h_ib is the input impedance parameter, h_fb is the forward current transfer ratio, h_rb is the reverse voltage transfer ratio and h_ob is the output admittance parameter.

Figure below shows the simplified equivalent model for the common-base transistor configuration.

Simplified h-parameter equivalent model for the common-base transistor configuration

Table below lists the approximate conversion formulae for the conversion of h-parameters in the three transistor configurations.

Approximate conversion formulae for the h-parameters

Table below lists the typical values of h-parameters in the three transistor configurations.

Typical values of h-parameters for the three transistor configurations