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.
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
H-Parameter Model
Common-Emitter
h ie = re
h fe = ac
Common-Base
h ib = re
h fb = − − 1
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
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
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
Common-Emitter Fixed-Bias Configuration
AC equivalent
re model
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
Common-Emitter Fixed-Bias Calculations
Input impedance:
Z i = R B || re
Z i re R E 10 re
Output impedance:
Z o = R C || rO
Z o R C ro 10R C
Voltage gain: Current gain:
Vo (R || r )
Av = =− C o I R B ro
Vi re Ai = o =
I i (ro + R C )(R B + re )
RC
Av = − ro 10R C
re A i ro 10R C , R B 10 re
Current gain from voltage gain:
Zi
Ai = −A v
RC
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
Common-Emitter Voltage-Divider Bias
re model requires you to determine , re, and ro.
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
Common-Emitter Voltage-Divider Bias
Calculations
Input impedance:
R = R 1 || R 2
Z i = R || re
Output impedance:
Z o = R C || ro Current gain:
I R ro
Z o R C ro 10R C Ai = o =
I i (ro + R C )(R + re )
I R
Voltage gain: Ai = o r 10R C
Ii R + re o
Vo − R C || ro I
Av = = A i = o ro 10R C , R 10 re
Vi re Ii
Vo R
Av = − C ro 10R C
Vi re Current gain from voltage gain:
Z
Ai = −A v i
RC
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
Common-Emitter Emitter-Bias
Configuration
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
Impedance Calculations
Input impedance:
Z i = R B || Z b
Z b = re + ( + 1)R E
Z b (re + R E )
Z b R E
Output impedance:
Zo = R C
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
Gain Calculations
Voltage gain:
Vo R C
Av = =−
Vi Zb
V RC
Av = o = −
Vi re + R E Z b = (re + R E )
Vo R
Av = − C Z b R E
Vi RE
Current gain: Current gain from voltage gain:
I R B Zi
Ai = o = Ai = −A v
Ii R B + Zb RC
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
Emitter-Follower Configuration
• This is also known as the common-collector configuration.
• The input is applied to the base and the output is taken from the
emitter.
• There is no phase shift between input and output.
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
Impedance Calculations
Input impedance:
Z i = R B || Z b
Z b = re + ( + 1)R E
Z b (re + R E )
Z b R E
Output impedance:
Z o = R E || re
Z o re R E re
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
Gain Calculations
Voltage gain:
Vo RE
Av = =
Vi R E + re
V
A v = o 1 R E re , R E + re R E
Vi
Current gain:
R B
Ai −
R B + Zb
Current gain from voltage gain:
Z
Ai = −A v i
RE
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
Common-Base Configuration
• The input is applied to the
emitter.
• The output is taken from the
collector.
• Low input impedance.
• High output impedance.
• Current gain less than unity.
• Very high voltage gain.
• No phase shift between input
and output.
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
Calculations
Input impedance:
Z i = R E || re
Output impedance:
Zo = R C
Voltage gain:
Vo R C R C
Av = =
Vi re re
Current gain:
I
A i = o = − −1
Ii
Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc.
Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved.
