Electrical Engineering ⇒ Topic : Re-Transistor Model

r_e-TRANSISTOR MODEL

The bipolar junction transistors are current controlled devices. In the re model, a diode and a current controlled source are used in the transistor equivalent circuit to explain the behaviour.

The common base bipolar junction transistor amplifier is shown in Figure (a) and its re-model is shown in Figure (b). The input impedance is:

          Figure (a) Common base bipolar junction             Figure (b) Equivalent re-rnodel of (of PNP) transistor. Figure (a)

 The value of Z_i is in the range of 0 to 50 Ω. Similarly, the output impedance is given by

                                      Z_O = ∞                                                    ............ (1)

Then

   ........... (2)

                                    ............. (3)

  .......... (4)

   ........... (5)

In the common emitter configuration, the collector current Figure (1) is given by

                                    I_C = βI_B

                  (a) CE mode                                (b) Equivalent circuit using r_e transistor model

                             Figure (1) Common emitter configuration (for N-P-N transistor).

                                       ........... (6)

                                      ........... (7)

                             ............ (8)

                                                   ............(9)

                                                 ............ (10)

In the common emitter configuration, the values of input impedance, Z_i = βr_e, is in the range between a few hundred ohms (minimum value) to a maximum of about 7 to 8 KΩ

In the above model, the output impedance r_e has not been shown. If the value r_o, is available from manufacturer's data sheets, the output circuit can be drawn as shown in Figure (c).

               Figure (C) Output circuit for common emitter configuration.

The output impedance for the circuit is:

                                                           ........... (11)

The voltage gain is:
                                           ......... (12)

The current gain is:

                          ........... (13)