Module: Electronics I
Module Number: 610/650221-222
Electronic Devices and Circuit Theory, 9th ed., Boylestad and Nashelsky
Philadelphia University
Faculty of Engineering
Communication and Electronics Engineering
Bipolar Junction Transistor
AC Analysis:
•
•
•
A model is an equivalent circuit that represents the AC characteristics of the
transistor.
A model uses circuit elements that approximate the behavior of the transistor.
There are two models commonly used in small signal AC analysis of a
transistor:
– r e model
– Hybrid equivalent model
R
R
The r e Transistor Model:
R
R
BJTs are basically current-controlled devices, therefore the re model uses a diode and a
current source to duplicate the behavior of the transistor. One disadvantage to this model is its
sensitivity to the DC level. This model is designed for specific circuit conditions.
Common Base Configuration
Fig. 5.6
Fig. 5.7
(a) Common-base BJT transistor; (b) re model for the configuration of (a).
Common-base re equivalent circuit.
Lecturer: Dr. Omar Daoud
Fig. 5.9
Defining Av = Vo/Vi for the commonbase configuration.
PartII-II
Module: Electronics I
Module Number: 610/650221-222
Electronic Devices and Circuit Theory, 9th ed., Boylestad and Nashelsky
Common Emitter Configuration
Fig. 5.11
Fig. 5.12
(a) Common-emitter BJT transistor; (b) approximate model for the configuration of a).
Determining Zi using the approximate
Fig. 5.16 Determining the voltage and current gain for the
common-emitter transistor amplifier.
Fig. 5.17 re model for the common-emitter transistor
configuration.
Common Collector Configuration
Use the common-emitter model for the common-collector configuration.
Lecturer: Dr. Omar Daoud
PartII-II
Module: Electronics I
Module Number: 610/650221-222
Electronic Devices and Circuit Theory, 9th ed., Boylestad and Nashelsky
The Hybrid Equivalent Model:
The following hybrid parameters are developed and used for modeling the transistor.
These parameters can be found in a specification sheet for a transistor:
• h i = input resistance
• h r = reverse transfer voltage ratio (Vi/Vo) ≅ 0
• h f = forward transfer current ratio (Io/Ii)
• h o = output conductance ≅ ∞
R
R
R
R
R
R
R
R
Fig. 5.22
Complete hybrid equivalent circuit.
Fig. 5.23 Common-emitter configuration: (a) graphical symbol; (b) hybrid equivalent
circuit
Fig. 5.24 Common-base configuration: (a) graphical symbol; (b) hybrid equivalent
circuit.
Lecturer: Dr. Omar Daoud
PartII-II
Module: Electronics I
Module Number: 610/650221-222
Electronic Devices and Circuit Theory, 9th ed., Boylestad and Nashelsky
Fig. 5.25
Effect of removing hre and hoe from the hybird equivalent circuit.
Fig. 5.26 Approximate hybrid equivalent model.
Common-Emitter re vs. h-Parameter Model
Fig. 5.27
Hybrid versus re model: (a) common-emitter configuration; (b) common-base configuration.
Lecturer: Dr. Omar Daoud
PartII-II