C
B
E
Kristin Ackerson, Virginia Tech EE
Spring 2002
Table of Contents
The Bipolar Junction Transistor_______________________________slide 3
BJT Relationships – Equations________________________________slide 4
DC  and DC  _____________________________________________slides 5
BJT Example_______________________________________________slide 6
BJT Transconductance Curve_________________________________slide 7
Modes of Operation_________________________________________slide 8
Three Types of BJT Biasing__________________________________slide 9
Common Base______________________slide 10-11
Common Emitter_____________________slide 12
Common Collector___________________slide 13
Eber-Moll Model__________________________________________slides 14-15
Small Signal BJT Equivalent Circuit__________________________slides 16
The Early Effect___________________________________________slide 17
Early Effect Example_______________________________________slide 18
Breakdown Voltage________________________________________slide 19
Sources__________________________________________________slide 20
Kristin Ackerson, Virginia Tech EE
Spring 2002
The BJT – Bipolar Junction Transistor
 Note: It will be very helpful to go through the Analog Electronics
Diodes Tutorial to get information on doping, n-type and p-type materials.
The Two Types of BJT Transistors:
npn
pnp
E
n
p
n
C
C
Cross Section
B
E
p
n
p
C
C
Cross Section
B
B
Schematic
Symbol
B
E
Schematic
Symbol
• Collector doping is usually ~ 106
• Base doping is slightly higher ~ 107 – 108
• Emitter doping is much higher ~ 1015
E
Kristin Ackerson, Virginia Tech EE
Spring 2002
BJT Relationships - Equations
IE
-
E
IC
VCE +
IE
C
-
VBE
VBC
IB
+
+
E
+
VEC
+
VEB
IC
-
C
+
VCB
IB
-
-
B
B
npn
pnp
IE = IB + IC
IE = IB + IC
VCE = -VBC + VBE
VEC = VEB - VCB
Note: The equations seen above are for the
transistor, not the circuit.
Kristin Ackerson, Virginia Tech EE
Spring 2002
DC  and DC 
 = Common-emitter current gain
 = Common-base current gain
 = IC
 = IC
IB
IE
The relationships between the two parameters are:
=

+1
=

1-
Note:  and  are sometimes referred to as dc and dc
because the relationships being dealt with in the BJT
are DC.
Kristin Ackerson, Virginia Tech EE
Spring 2002
BJT Example
Using Common-Base NPN Circuit Configuration
C
Given: IB = 50  A , IC = 1 mA
VCB
Find:
IE ,  , and 
IB
B
VBE
IC
+
_
Solution:
+
_
IE
IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA
 = IC / IB = 1 mA / 0.05 mA = 20
E
 = IC / IE = 1 mA / 1.05 mA = 0.95238
 could also be calculated using the value of
 with the formula from the previous slide.
=

= 20 = 0.95238
+1
21
Kristin Ackerson, Virginia Tech EE
Spring 2002
BJT Transconductance Curve
Typical NPN Transistor 1
Collector Current:
IC =  IES eVBE/VT
IC
Transconductance:
(slope of the curve)
8 mA
gm =  IC /  VBE
6 mA
IES = The reverse saturation current
of the B-E Junction.
4 mA
VT = kT/q = 26 mV (@ T=300K)
2 mA
 = the emission coefficient and is
usually ~1
0.7 V
VBE
Kristin Ackerson, Virginia Tech EE
Spring 2002
Modes of Operation
Active:
• Most important mode of operation
• Central to amplifier operation
• The region where current curves are practically flat
Saturation: • Barrier potential of the junctions cancel each other out
causing a virtual short
Cutoff:
• Current reduced to zero
• Ideal transistor behaves like an open switch
* Note: There is also a mode of operation
called inverse active, but it is rarely used.
Kristin Ackerson, Virginia Tech EE
Spring 2002
Three Types of BJT Biasing
Biasing the transistor refers to applying voltage to get the
transistor to achieve certain operating conditions.
Common-Base Biasing (CB) :
input
= VEB & IE
output = VCB & IC
Common-Emitter Biasing (CE):
input
= VBE & IB
output = VCE & IC
Common-Collector Biasing (CC):
input
= VBC & IB
output = VEC & IE
Kristin Ackerson, Virginia Tech EE
Spring 2002
Common-Base
Although the Common-Base configuration is not the most
common biasing type, it is often helpful in the understanding of
how the BJT works.
Emitter-Current Curves
Saturation Region
IC
Active
Region
IE
Cutoff
IE = 0
VCB
Kristin Ackerson, Virginia Tech EE
Spring 2002
Common-Base
Circuit Diagram: NPN Transistor
C
VCE
IC
VCB
The Table Below lists assumptions
that can be made for the attributes
of the common-base biased circuit
in the different regions of
operation. Given for a Silicon NPN
transistor.
Region of
Operation
IC
Active
IB
Saturation
Max
Cutoff
~0
VCE
E
VBE
+
_
+
_
IB
B
VCB
VBE
=VBE+VCE ~0.7V
~0V
IE
VBE
VCB
 0V
C-B
Bias
E-B
Bias
Rev. Fwd.
~0.7V -0.7V<VCE<0 Fwd. Fwd.
=VBE+VCE  0V
 0V
Rev.
None
/Rev.
Kristin Ackerson, Virginia Tech EE
Spring 2002
Common-Emitter
Circuit Diagram
VCE
IC
VC
+
_
Collector-Current Curves
IC
IB
C
Active
Region
IB
Region of Description
Operation
Active
Small base current
controls a large
collector current
Saturation VCE(sat) ~ 0.2V, VCE
increases with IC
Cutoff
Achieved by reducing
IB to 0, Ideally, IC will
also equal 0.
VCE
Saturation Region
Cutoff Region
IB = 0
Kristin Ackerson, Virginia Tech EE
Spring 2002
Common-Collector
Emitter-Current Curves
The CommonCollector biasing
circuit is basically
equivalent to the
common-emitter
biased circuit except
instead of looking at
IC as a function of VCE
and IB we are looking
at IE.
Also, since  ~ 1, and
 = IC/IE that means
IC~IE
IE
Active
Region
IB
VCE
Saturation Region
Cutoff Region
IB = 0
Kristin Ackerson, Virginia Tech EE
Spring 2002
Eber-Moll BJT Model
The Eber-Moll Model for BJTs is fairly complex, but it is
valid in all regions of BJT operation. The circuit diagram
below shows all the components of the Eber-Moll Model:
E
IE
IC
RIC
C
RIE
IF
IR
IB
B
Kristin Ackerson, Virginia Tech EE
Spring 2002
Eber-Moll BJT Model
R = Common-base current gain (in forward active mode)
F = Common-base current gain (in inverse active mode)
IES = Reverse-Saturation Current of B-E Junction
ICS = Reverse-Saturation Current of B-C Junction
IC = FIF – IR
IB = IE - IC
IE = IF - RIR
IF = IES [exp(qVBE/kT) – 1]
IR = IC [exp(qVBC/kT) – 1]
 If IES & ICS are not given, they can be determined using various
BJT parameters.
Kristin Ackerson, Virginia Tech EE
Spring 2002
Small Signal BJT Equivalent Circuit
The small-signal model can be used when the BJT is in the active region.
The small-signal active-region model for a CB circuit is shown below:
iB
iC
B
iB
r
r = ( + 1) * VT
IE
iE
E
@  = 1 and T = 25C
r = ( + 1) * 0.026
IE
C
Recall:
 = IC / IB
Kristin Ackerson, Virginia Tech EE
Spring 2002
The Early Effect (Early Voltage)
IC
Note: Common-Emitter
Configuration
IB
-VA
VCE
Green = Ideal IC
Orange = Actual IC (IC’)
IC’ = IC
VCE + 1
VA
Kristin Ackerson, Virginia Tech EE
Spring 2002
Early Effect Example
Given: The common-emitter circuit below with IB = 25A,
VCC = 15V,  = 100 and VA = 80.
Find:
a) The ideal collector current
b) The actual collector current
Circuit Diagram
IC
VCE
 = 100 = IC/IB
a)
VCC
+
_
IC = 100 * IB = 100 * (25x10-6 A)
IB
IC = 2.5 mA
b)
IC’ = IC
VCE + 1
VA
= 2.5x10-3
15 + 1
= 2.96 mA
80
IC’ = 2.96 mA
Kristin Ackerson, Virginia Tech EE
Spring 2002
Breakdown Voltage
The maximum voltage that the BJT can withstand.
BVCEO =
The breakdown voltage for a common-emitter
biased circuit. This breakdown voltage usually
ranges from ~20-1000 Volts.
BVCBO =
The breakdown voltage for a common-base biased
circuit. This breakdown voltage is usually much
higher than BVCEO and has a minimum value of ~60
Volts.
Breakdown Voltage is Determined By:
•
•
The Base Width
Material Being Used
•
Doping Levels
•
Biasing Voltage
Kristin Ackerson, Virginia Tech EE
Spring 2002
Sources
Dailey, Denton. Electronic Devices and Circuits, Discrete and Integrated. Prentice Hall, New
Jersey: 2001. (pp 84-153)
1
Figure 3.7, Transconductance curve for a typical npn transistor, pg 90.
Liou, J.J. and Yuan, J.S. Semiconductor Device Physics and Simulation. Plenum Press,
New York: 1998.
Neamen, Donald. Semiconductor Physics & Devices. Basic Principles. McGraw-Hill,
Boston: 1997. (pp 351-409)
Web Sites
http://www.infoplease.com/ce6/sci/A0861609.html
Kristin Ackerson, Virginia Tech EE
Spring 2002