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Chapter 4
DC Biasing–BJTs
Biasing
Biasing: The DC voltages applied to a transistor in
order to turn it on so that it can amplify the AC signal.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Operating Point
The DC input
establishes an
operating or
quiescent point
called the Q-point.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
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
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
DC Biasing Circuits
•
•
•
•
•
Fixed-bias circuit
Emitter-stabilized bias circuit
Collector-emitter loop
Voltage divider bias circuit
DC bias with voltage feedback
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Fixed Bias
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
The Base-Emitter Loop
From Kirchhoff’s voltage
law:
+VCC – IBRB – VBE = 0
Solving for base current:
IB 
V CC  V BE
RB
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Collector-Emitter Loop
Collector current:
I
C
 IB
From Kirchhoff’s voltage law:
V CE  V CC  I C R C
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Saturation
When the transistor is operating in saturation, current
through the transistor is at its maximum possible value.
I Csat

V CC
RC
V CE  0 V
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Load Line Analysis
The end points of the load line are:
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
• that sets the values of VCE and IC
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Circuit Values Affect the Q-Point
more …
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Circuit Values Affect the Q-Point
more …
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Circuit Values Affect the Q-Point
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Emitter-Stabilized Bias Circuit
Adding a resistor
(RE) to the emitter
circuit stabilizes
the bias circuit.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Base-Emitter Loop
From Kirchhoff’s voltage law:
 V CC - I E R E - V BE - I E R E  0
Since IE = ( + 1)IB:
V CC - I B R B - (   1)I B R E  0
Solving for IB:
IB 
V CC - V BE
R B  (   1)R E
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Collector-Emitter Loop
From Kirchhoff’s voltage law:
I R V
I R V
0
E E
CE
C C
CC
Since IE  IC:
V CE  V CC – I C (R C  R E )
Also:
VE  I E R E
V C  V CE  V E  V CC - I C R C
V B  V CC – I R R B  V BE  V E
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
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 () values.
Adding RE to the emitter improves the stability of a transistor.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Saturation Level
The endpoints can be determined from the load line.
VCEcutoff:
ICsat:
V CE  V CC
I C  0 mA
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
V CE  0 V
IC 
V CC
RC  RE
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Voltage Divider Bias
This is a very stable
bias circuit.
The currents and
voltages are nearly
independent of any
variations in .
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Approximate Analysis
Where IB << I1 and I1  I2 :
VB 
R 2 V CC
R1  R2
Where RE > 10R2:
IE 
VE
RE
V E  V B  V BE
From Kirchhoff’s voltage law:
VCE  VCC  I C R C  I E R E
IE  IC
V CE  V CC  I C (R C  R E )
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Voltage Divider Bias Analysis
Transistor Saturation Level
I Csat  I Cmax

V CC
RC  RE
Load Line Analysis
Cutoff:
Saturation:
V CE  V CC
I C  0 mA
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
I
C

V CC
RC  RE
V CE  0 V
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
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, .
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Base-Emitter Loop
From Kirchhoff’s voltage law:
V CC – I C R C – I B R B – V BE – I E R E  0
Where IB << IC:
I'
C
I
C
I
B
I
C
Knowing IC = IB and IE  IC, the loop
equation becomes:
V CC –  I B R C  I B R B  V BE   I B R E  0
Solving for IB:
IB 
V CC  V BE
R B   (R C  R E )
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Collector-Emitter Loop
Applying Kirchoff’s voltage law:
IE + VCE + I’CRC – VCC = 0
Since IC  IC and IC = IB:
IC(RC + RE) + VCE – VCC =0
Solving for VCE:
VCE = VCC – IC(RC + RE)
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Base-Emitter Bias Analysis
Transistor Saturation Level
I Csat  I Cmax

V CC
RC  RE
Load Line Analysis
Cutoff:
V CE  V CC
I C  0 mA
Saturation:
V
I
C

R
C
CC
R
E
V CE  0 V
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Transistor Switching Networks
Transistors with only the DC source applied can be used
as electronic switches.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Switching Circuit Calculations
Saturation current:
I Csat 
V CC
RC
To ensure saturation:
IB 
I Csat
 dc
Emitter-collector resistance
at saturation and cutoff:
R sat 
R cutoff
V CEsat
I Csat

V CC
I CEO
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Switching Time
Transistor switching times:
t on  t r  t d
t off  t s  t f
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
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.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
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.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.