This document summarizes key concepts about BJT AC analysis from Chapter 5 of the textbook. It discusses two common models used for small signal AC analysis: the re model and hybrid equivalent model. The re model represents the BJT as a diode and current source and is designed for specific circuit conditions. It then provides details on analyzing several common BJT configurations, including common-emitter, using the re model, discussing input impedance, output impedance, voltage gain and current gain. It also briefly introduces the feedback pair and current mirror circuits.

1. Chapter 5:
BJT AC Analysis

2. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
BJT Transistor Modeling
• 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:
– re model
– Hybrid equivalent model

3. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
The re Transistor Model
• 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.

4. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
The re Transistor Model
Common-Emitter Configuration
• The equivalent circuit of Fig above will be used throughout the
analysis to follow for the common-emitter configuration.

5. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Emitter Fixed-Bias Configuration
• The input is applied to the base
• The output is from the collector
• High input impedance
• Low output impedance
• High voltage and current gain
• Phase shift between input and
output is 180

6. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Emitter Fixed-Bias Configuration
AC equivalent
re model

7. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Emitter Fixed-Bias Calculations
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8. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Emitter Voltage-Divider Bias
re model requires you to determine , re, and ro.

9. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Emitter Voltage-Divider Bias Cal.
Current gain from voltage gain:
Input impedance:
Output impedance:
Voltage gain:
Current gain:
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10. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Common-Emitter Emitter-Bias Config.

11. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Impedance Calculations
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12. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Gain Calculations
Current gain from voltage gain:
Voltage gain:
Current gain:
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13. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Feedback Pair
This is a two-transistor circuit that operates like a
Darlington pair, but it is not a Darlington pair.
It has similar characteristics:
• High current gain
• Voltage gain near unity
• Low output impedance
• High input impedance
The difference is that a Darlington
uses a pair of like transistors,
whereas the feedback-pair
configuration uses complementary
transistors.

14. Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Current Mirror Circuits
Current mirror circuits
provide constant current
in integrated circuits.
Example 4.26: Calculate the mirrored current I in the above circuit
given, Rx = 1.1 kΩ and +Vcc = 12 V.