Electronic devices-and-circuit-theory-10th-ed-boylestad-chapter-4

Biasing
Biasing: The DC voltages applied to a transistor in
order to turn it on so that it can amplify the AC signal.
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

Operating Point
The DC input
establishes an
operating or
quiescent point
called the Q-point.

The Three States of Operation
•• Active or Linear Region Operation
Base–Emitter junction is forward biased
Base–Collector junction is reverse biased
• Cutoff Region Operation
Base–Emitter junction is reverse biased
•• Saturation Region Operation
Base–Emitter junction is forward biased
Base–Collector junction is forward biased

DC Biasing Circuits
•• Fixed-bias circuit
• Emitter-stabilized bias circuit
•• Collector-emitter loop
• Voltage divider bias circuit
•• DC bias with voltage feedback

Fixed Bias

The Base-Emitter Loop
From Kirchhoff’s voltage
law:
+VCC – IBRB – VBE = 0
Solving for base current:
V CC VCC −
V
BE
VBE
B
B R
I
=

Collector-Emitter Loop
Collector current:
I IB C = β
From Kirchhoff Kirchhoff’s s voltage law:
VCE = VCC − ICRC

Saturation
When the transistor is operating in saturation, current
through the transistor is at its maximum possible value.
VCC
R
I Csat =
C
VCE ≅ 0 V

Load Line Analysis
The end points of the load line are:
I
ICsat
IC = VCC / RC
VCE = 0 V
VCEcutoff
VCE = VCC
IC = 0 mA
The Q-point is the operating point:
• where the value of RB sets the value of
IB
• sets the V and I
that values of VCE IC

Circuit Values Affect the Q-Point
more ……

Circuit Values Affect the Q-Point

Emitter-Stabilized Bias Circuit
Adding a resistor
(RE) to the emitter
circuit stabilizes
the bias circuit.

Base-Emitter Loop
From Kirchhoff’s voltage law:
+ VCC - IERE - VBE - IERE = 0
Since IE = (β + 1)IB:
VCC CC - B B IBRB - (β + ) 1)B E
IBRE = 0
VCC - VBE
Solving for IB:
V V
CC BE
B R ( 1)R
B E
I
+ β +
=

From Kirchhoff’’s voltage law:
IERE + VCE + ICRC −VCC = 0
Since IE ≅ IC:
VCE == VCC – IC(RC ++ RE )
Also:
V =
I R
E E E
V = V + V =
V - I R
C CE E CC C C
V = V – I R = V +
V
B CC R B BE E

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.

Saturation Level
The endpoints can be determined from the load line.
VCEcutoff: ICsat:
V =
V
C
CE CC
=
I 0mA
VCE 0 V
VCC
RC RE
IC
+
=
=

Voltage Divider Bias
This is a very stable
bias circuit.
The currents and
voltages are nearly
independent of any
variations in β.

Approximate Analysis
Where IB << I1 and I1 ≅ I2 :
R V
2 CC
B R R
1 2
V
+
=
Where βRE > 10R2:
E
E
V
I
=
E
R
VE = VB − VBE
From Kirchhoff’s voltage law:
CE CC C C E E V = V − I R − I R
I ≅
I
E C
V = V − I (R +
R )
CE CC C C E

Voltage Divider Bias Analysis
Transistor Saturation Level
CC
Csat Cmax
V
I = I
=
R +
R
C E
Load Line Analysis
Cutoff: Saturation:
V
V =
V
C
CE CC
=
I 0mA
VCC
RC RE
IC
VCE =
0V
+
=
CE

DC Bias with Voltage Feedback
Another way to
improve the stability
of a bias circuit is to
add a feedback path
from collector to
base.
In this bias circuit
the Q-point is only
slightly dependent on
the transistor beta, β.

Base-Emitter Loop
From Kirchhoff’’s voltage law:
VCC – I′CRC – IBRB – VBE – IERE = 0
Where IB << IC:
I'C = IC + IB ≅ IC
Knowing IC = ββIB and IE ≅≅ IC, the loop
equation becomes:
VCC – βIBRC − IBRB − VBE − βIBRE = 0
V V
Solving for IB:
−
CC BE
B + β +
R (R R )
I
B C E
=

Applying Kirchoff’’s voltage law:
IE + VCE + I’’CRC – VCC = 0
Since I′′C ≅ IC and IC = βIB:
IC(RC + RE) + VCE – VCC =0
Solving for VCE:
VCE = VCC – IC(RC + RE)

Base-Emitter Bias Analysis
Transistor Saturation Level
CC
V
Csat Cmax R R
C E
I I
+
= =
Load Line Analysis
Cutoff: Saturation:
V V
CE CC
= VCC
I
I C
=
0mA
RC RE C
V CE
=
0 V +
=

Transistor Switching Networks
Transistors with only the DC source applied can be used
as electronic switches.

Switching Circuit Calculations
Saturation current:
V
CC
C
I =
Csat R
To saturation:
Csat
dc
B
I
I
β
>
ensure Emitter-collector resistance
at saturation and cutoff:
V
CEsat
C t
R =
sat I
ICsat
V
CC
CEO
R =
cutoff I

Switching Time
Transistor switching times:
ton = tr + td
toff = ts + tf

Troubleshooting Hints
• Approximate voltages
– 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.
• Note that the load or the next stage affects the transistor operation.

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

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