h-parameter Model for Emitter-Follower Configuration

Figure below shows the circuit diagram of the emitter-follower configuration

Circuit diagram of emitter-follower configuration

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

Simplified h-parameter equivalent model for the emitter-follower configuration

Input impedance: Input impedance is determined in a similar manner as that for the emitter-bias configuration with unbypassed emitter resistor. The input impedance looking into the network to the right of resistor RB is given by

The overall input impedance Zi is given by parallel combination of resistor RB and impedance Zi'.

Output impedance:
The base current I_b is given by

The emitter current Ie is then given as

Substituting the value of Zi in the above equation we get

Figure below shows the network defined by the above equation.

Equivalent network

The output impedance Z_o is defined by setting the input voltage V_i equal to zero. The output impedance Z_o is a parallel combination of resistor R_E and impedance defined by h_ie/(h_fe + 1).

Voltage gain: Output voltage V_o is expressed in terms of the input voltage V_i as

The voltage gain Av is then given by

Current gain: The current gain Ai is given by

The output current Io is given by

Base current I_b is given by

Therefore, Ai is given by