This document summarizes key concepts about biasing BJTs: 1) Biasing involves applying DC voltages to turn on a transistor so it can amplify an AC signal. This establishes an operating point called the Q-point. 2) There are three regions of transistor operation depending on junction biases: active, cutoff, and saturation. 3) Common bias circuits include fixed bias, emitter-stabilized bias, and voltage divider bias. Adding a resistor to the emitter improves stability.

1. Chapter 4
DC Biasing–BJTs

2. Biasing
Biasing
g
g
Biasing:
Biasing: T
The DC voltages applied to a transistor in
order to turn it on so that it can amplify the AC signal.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

3. Operating Point
Operating Point
p g
p g
The DC input
establishes an
establishes an
operating or
quiescent point
called the Q
Q-
-point
point.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

4. The Three States of Operation
The Three States of Operation
p
p
• Active or Linear Region Operation
Active or Linear Region Operation
• Active or Linear Region Operation
Active or Linear Region Operation
Base–Emitter junction is forward biased
Base–Collector junction is reverse biased
j
• Cutoff Region Operation
Cutoff Region Operation
B E itt j ti i bi d
Base–Emitter junction is reverse biased
•
• Saturation Region Operation
Saturation Region Operation
•
• Saturation Region Operation
Saturation Region Operation
Base–Emitter junction is forward biased
Base–Collector junction is forward biased
j
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

5. DC Biasing Circuits
DC Biasing Circuits
g
g
• Fixed-bias circuit
• Fixed-bias circuit
• Emitter-stabilized bias circuit
• Collector emitter loop
• Collector-emitter loop
• Voltage divider bias circuit
• DC bias with voltage feedback
• DC bias with voltage feedback
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

6. Fixed Bias
Fixed Bias
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

7. The Base
The Base-
-Emitter Loop
Emitter Loop
p
p
From Kirchhoff’s voltage
law:
V I R V 0
+VCC – IBRB – VBE = 0
Solving for base current:
BE
CC V
V −
B
BE
CC
B
R
V
V
I
−
=
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

8. Collector
Collector-
-Emitter Loop
Emitter Loop
p
p
Collector current:
From Kirchhoff’s voltage law:
B
I
I C
β
=
From Kirchhoff s voltage law:
C
C
CC
CE R
I
V
V −
= C
C
CC
C
Copyright ©2009 by Pearson Education, Inc.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

9. Saturation
Saturation
Wh th t i t i ti i t ti t
When the transistor is operating in saturation, current
through the transistor is at its maximum possible value.
R
CC
V
Csat
I =
C
R
V
0
CE
V ≅
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

10. Load Line Analysis
Load Line Analysis
I
I
The end points of the load line are:
I
ICsat
Csat
IC
C = VCC
CC / RC
C
VCE
CE = 0 V
V
VCEcutoff
CEcutoff
VCE
CE = VCC
CC
I = 0 mA
IC
C = 0 mA
The Q-point is the operating point:
• where the value of RB
B sets the value of
IB
B
• that sets the values of V and I
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
• that sets the values of VCE
CE and IC
C

11. Circuit Values Affect the Q
Circuit Values Affect the Q-
-Point
Point
Q
Q
more
more …
…
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

12. Circuit Values Affect the Q
Circuit Values Affect the Q-
-Point
Point
Q
Q
more
more …
…
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

13. Circuit Values Affect the Q
Circuit Values Affect the Q-
-Point
Point
Q
Q
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

14. Emitter
Emitter-
-Stabilized Bias Circuit
Stabilized Bias Circuit
Emitter
Emitter-
-Stabilized Bias Circuit
Stabilized Bias Circuit
Adding a resistor
Adding a resistor
(RE) to the emitter
circuit stabilizes
circuit stabilizes
the bias circuit.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

15. Base
Base-
-Emitter Loop
Emitter Loop
Base
Base-
-Emitter Loop
Emitter Loop
From Kirchhoff’s voltage law:
0
R
I
-
V
-
R
I
-
V E
E
BE
E
E
CC =
+
0
R
1)I
(
-
R
I
-
V E
B
B
B
CC =
+
β
Since IE = (β + 1)IB:
)
( E
B
B
B
CC β
BE
CC V
-
V
Solving for IB:
E
B
BE
CC
B
1)R
(
R
V
-
V
I
+
β
+
=
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

16. Collector
Collector-
-Emitter Loop
Emitter Loop
Collector
Collector-
-Emitter Loop
Emitter Loop
From Kirchhoff’s voltage law:
From Kirchhoff’s voltage law:
0
CC
V
C
R
C
I
CE
V
E
R
E
I =
−
+
+
Since IE ≅ IC:
)
R
(R
I
–
V
V E
C
C
CC
CE +
= )
R
(R
I
V
V E
C
C
CC
CE +
=
Also:
R
I
V
E
BE
B
R
CC
B
C
C
CC
E
CE
C
E
E
E
V
V
R
I
–
V
V
R
I
-
V
V
V
V
R
I
V
+
=
=
=
+
=
=
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

17. Improved Biased Stability
Improved Biased Stability
Improved Biased Stability
Improved Biased Stability
Stability refers to a circuit condition in which the currents and voltages
will remain fairly constant over a wide range of temperatures and
transistor Beta (β) values.
Adding RE to the emitter improves the stability of a transistor.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

18. Saturation Level
Saturation Level
Saturation Level
Saturation Level
VCEcutoff:
: ICsat:
The endpoints can be determined from the load line.
mA
0
I
V
V
C
CC
CE
=
=
E
R
C
R
CC
V
C
I
CE V
0
V
+
=
=
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
E
R
C
R +

19. Voltage Divider Bias
Voltage Divider Bias
Voltage Divider Bias
Voltage Divider Bias
This is a very stable
bias circuit.
Th t d
The currents and
voltages are nearly
independent of any
any
independent of any
any
variations in β.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

20. Approximate Analysis
Approximate Analysis
Approximate Analysis
Approximate Analysis
Where IB << I1 and I1 ≅ I2 :
2
1
CC
2
B
R
R
V
R
V
+
=
Where βRE > 10R2:
E
E
V
I =
E
E
R
I
BE
B
E V
V
V −
=
From Kirchhoff’s voltage law:
E
E
C
C
CC
CE R
I
R
I
V
V −
−
=
)
R
(R
I
V
V
I
I
E
C
C
CC
CE
C
E
+
−
=
≅
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

21. Voltage Divider Bias Analysis
Voltage Divider Bias Analysis
Voltage Divider Bias Analysis
Voltage Divider Bias Analysis
Transistor Saturation Level
Transistor Saturation Level
CC
Cmax
Csat
V
I
I =
=
E
C
Cmax
Csat
R
R
I
I
+
Load Line Analysis
Load Line Analysis
Cutoff:
Cutoff: Saturation:
Saturation:
V
mA
0
I
V
V
C
CC
CE
=
=
V
0
VCE
E
R
C
R
CC
V
C
I
=
+
=
CE
Copyright ©2009 by Pearson Education, Inc.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

22. DC Bias with Voltage Feedback
DC Bias with Voltage Feedback
DC Bias with Voltage Feedback
DC Bias with Voltage Feedback
Another way to
improve the stability
improve the stability
of a bias circuit is to
add a feedback path
from collector to
from collector to
base.
I thi bi i it
In this bias circuit
the Q-point is only
slightly dependent on
the transistor beta, β.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

23. Base
Base-
-Emitter Loop
Emitter Loop
Base
Base-
-Emitter Loop
Emitter Loop
From Kirchhoff’s voltage law:
From Kirchhoff’s voltage law:
0
R
I
–
V
–
R
I
–
R
I
–
V E
E
BE
B
B
C
C
CC =
′
Where I
Where I << I
<< I :
:
Where I
Where IB
B << I
<< IC
C:
:
C
I
B
I
C
I
C
I' ≅
+
=
Knowing I
Knowing I β
βI
I and I
and I ≅
≅ I
I the loop
the loop
Knowing I
Knowing IC
C =
= β
βI
IB
B and I
and IE
E ≅
≅ I
IC
C, the loop
, the loop
equation becomes:
equation becomes:
0
R
I
V
R
I
R
I
–
V E
B
BE
B
B
C
B
CC =
β
−
−
−
β
V
V
0
R
I
V
R
I
R
I
V E
B
BE
B
B
C
B
CC β
β
Solving for I
Solving for IB
B:
:
)
R
(R
R
V
V
I
E
C
B
BE
CC
B
+
β
+
−
=
Copyright ©2009 by Pearson Education, Inc.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

24. Collector
Collector-
-Emitter Loop
Emitter Loop
Collector
Collector-
-Emitter Loop
Emitter Loop
Applying Kirchoff’s voltage law:
Applying Kirchoff’s voltage law:
I V I’ R V 0
IE + VCE + I’CRC – VCC = 0
Since I
Since I′
′C
C ≅
≅ I
IC
C and I
and IC
C =
= β
βI
IB
B:
:
IC(RC + RE) + VCE – VCC =0
Solving for V
Solving for VCE
CE:
:
g
g CE
CE
VCE = VCC – IC(RC + RE)
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

25. Base
Base-
-Emitter Bias Analysis
Emitter Bias Analysis
Base
Base Emitter Bias Analysis
Emitter Bias Analysis
Transistor Saturation Level
Transistor Saturation Level
Transistor Saturation Level
Transistor Saturation Level
E
C
CC
Cmax
Csat
R
R
V
I
I
+
=
=
Load Line Analysis
Load Line Analysis
Cutoff:
Cutoff: Saturation:
Saturation:
V
V CC
CE = CC
V
I
mA
0
I
V
V
C
CC
CE
=
V
0
VCE
E
R
C
R
C
I
=
+
=
Copyright ©2009 by Pearson Education, Inc.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

26. Transistor Switching Networks
Transistor Switching Networks
Transistor Switching Networks
Transistor Switching Networks
Transistors with only the DC source applied can be used
Transistors with only the DC source applied can be used
as electronic switches.
as electronic switches.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

27. Switching Circuit Calculations
Switching Circuit Calculations
g
g
V
Saturation current:
Saturation current:
C
CC
Csat
R
V
I =
To ensure saturation:
To ensure saturation:
dc
Csat
B
I
I
β
>
To ensure saturation:
To ensure saturation:
dc
β
Emitter
Emitter-
-collector resistance
collector resistance
at saturation and cutoff:
at saturation and cutoff:
C t
CEsat
sat
I
V
R =
at saturation and cutoff:
at saturation and cutoff:
Csat
I
CEO
CC
cutoff
I
V
R =
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

28. Switching Time
Switching Time
Switching Time
Switching Time
Transistor switching times:
Transistor switching times:
d
r
on t
t
t +
=
f
s
off t
t
t +
=
Copyright ©2009 by Pearson Education, Inc.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

29. Troubleshooting Hints
Troubleshooting Hints
Troubleshooting Hints
Troubleshooting Hints
• Approximate voltages
pp g
– VBE ≅ .7 V for silicon transistors
– VCE ≅ 25% to 75% of VCC
• Test for opens and shorts with an ohmmeter.
• Test the solder joints.
• Test the transistor with a transistor tester or a curve tracer
• Test the transistor with a transistor tester or a curve tracer.
• Note that the load or the next stage affects the transistor operation.
Copyright ©2009 by Pearson Education, Inc.
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Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky

30. PNP Transistors
PNP Transistors
PNP Transistors
PNP Transistors
The analysis for pnp transistor biasing circuits is the same
as that for npn transistor circuits. The only difference is that
h fl i i h i di i
the currents are flowing in the opposite direction.
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