Bjts

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 α _____________________________________________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 E p n p C

Cross Section C Cross Section C
B B
B B
Schematic Schematic
Symbol Symbol
E E

• Collector doping is usually ~ 106
• Base doping is slightly higher ~ 107 – 108
• Emitter doping is much higher ~ 1015 Kristin Ackerson, Virginia Tech EE
Spring 2002

BJT Relationships - Equations
IE IC IE IC
- VCE + + VEC -
E C E C
- -
+ +
VBE IB VBC VEB VCB
IB
+ + - -
B B

npn pnp
IE = I B + I C IE = I B + I C
VCE = -VBC + VBE VEC = VEB - VCB

Note: The equations seen above are for the
transistor, not the circuit.
Spring 2002

DC  and DC α
 = Common-emitter current gain
α = Common-base current gain
 = IC α = IC
IB IE

The relationships between the two parameters are:
α=  = α
+1 1-α

Note: α and  are sometimes referred to as α dc and  dc
because the relationships being dealt with in the BJT
are DC.

Spring 2002

BJT Example
Using Common-Base NPN Circuit Configuration
C
Given: IB = 50 µ A , IC = 1 mA

VCB +
_ IC Find: IE ,  , and α

IB
B
Solution:

VBE +
_ IE IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA

 = IC / IB = 1 mA / 0.05 mA = 20
E
α = IC / IE = 1 mA / 1.05 mA = 0.95238

α could also be calculated using the value of
 with the formula from the previous slide.

α=  = 20 = 0.95238
+1 21 Kristin Ackerson, Virginia Tech EE
Spring 2002

BJT Transconductance Curve
Typical NPN Transistor 1

Collector Current:
IC IC = α IES eVBE/η VT
8 mA Transconductance:
(slope of the curve)

6 mA
gm =  IC /  VBE

4 mA
IES = The reverse saturation current
of the B-E Junction.
2 mA VT = kT/q = 26 mV (@ T=300K)
η = the emission coefficient and is

0.7 V VBE usually ~1
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

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

IC

Active
Saturation Region

Region
IE

Cutoff
IE = 0

VCB
Spring 2002

Common-Base
VCE
IC IE
Circuit Diagram: NPN Transistor C E

VCB VBE
The Table Below lists assumptions
IB
that can be made for the attributes
of the common-base biased circuit
in the different regions of

_
+

+
_
operation. Given for a Silicon NPN B
VCB VBE
transistor.
Region of IC VCE VBE VCB C-B E-B
Operation Bias Bias

Active  IB =VBE+VCE ~0.7V  0V Rev. Fwd.

Saturation Max ~0V ~0.7V -0.7V<VCE<0 Fwd. Fwd.

=VBE+VCE  0V None
Cutoff ~0  0V Rev.
/Rev.
Spring 2002

Common-Emitter
Circuit Diagram
VCE Collector-Current Curves
IC
IC

+ Active
VCC _ IB
Region

IB
Region of Description
Operation

Active Small base current VCE
controls a large
collector current Saturation Region
Cutoff Region
Saturation VCE(sat) ~ 0.2V, VCE
IB = 0
increases with IC

Cutoff Achieved by reducing
IB to 0, Ideally, IC will
also equal 0. Kristin Ackerson, Virginia Tech EE
Spring 2002

Common-Collector
Emitter-Current Curves
The Common- IE
Collector biasing
circuit is basically
equivalent to the Active
Region
common-emitter
biased circuit except
instead of looking at
IB
IC as a function of VCE
and IB we are looking
at IE.
VCE
Also, since α ~ 1, and
α = IC/IE that means Saturation Region
IC~IE Cutoff Region
IB = 0

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:

IE IC
E C

α RIC α RI E

IF IR

IB

B Kristin Ackerson, Virginia Tech EE
Spring 2002

Eber-Moll BJT Model
α R = Common-base current gain (in forward active mode)
α 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

I C = α FI F – I R IB = I E - I C
IE = I F - α 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 Tech EE
Kristin Ackerson, Virginia
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 C

rπ  iB

rπ = ( + 1) * η VT iE
IE
@ η = 1 and T = 25° C E

rπ = ( + 1) * 0.026 Recall:
IE  = IC / IB
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
Spring 2002

Early Effect Example
Given: The common-emitter circuit below with IB = 25µ A,
VCC = 15V,  = 100 and VA = 80.
Find: a) The ideal collector current
b) The actual collector current
Circuit Diagram
VCE
IC  = 100 = IC/IB

a)
+
VCC _ IB IC = 100 * IB = 100 * (25x10-6 A)
IC = 2.5 mA

b) IC’ = IC VCE + 1 = 2.5x10-3 15 + 1 = 2.96 mA
VA 80
IC’ = 2.96 mA
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

Spring 2002

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