1. Bipolar Junction Transistors
BJTs B C E

2. 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

3. The BJT – Bipolar Junction Transistor
The Two Types of BJT Transistors:
npn
pnp n p n p n p E C E C C C
Cross Section
Cross Section B B B B
Schematic Symbol
Schematic Symbol E E
Collector doping is usually ~ 106
Base doping is slightly higher ~ 107 – 108
Emitter doping is much higher ~ 1015

4. BJT Relationships - EquationsIE IC IE IC -
VCE + +
VEC - E C E C - - + +
VBE
VBC IB
VEB
VCB IB + + - - B B
npn
IE = IB + IC
VCE = -VBC + VBE
pnp
IE = IB + IC
VEC = VEB - VCB
Note: The equations seen above are for the transistor, not the circuit.

5. DC  and DC   = Common-emitter current gain
 = Common-base current gain
 = IC  = IC
IB IE
The relationships between the two parameters are:
 =   = 
 
Note:  and  are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.

6. Using Common-Base NPN Circuit Configuration
BJT Example
Using Common-Base NPN Circuit Configuration C
Given: IB = 50  A , IC = 1 mA
Find: IE ,  , and 
Solution:
IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA
= IC / IB = 1 mA / 0.05 mA = 20
 = IC / IE = 1 mA / 1.05 mA =
 could also be calculated using the value of  with the formula from the previous slide.
 =  = =  IC
VCB + _ IB B
VBE IE + _ E

7. BJT Transconductance Curve Typical NPN Transistor 1
Collector Current:
IC =  IES eVBE/VT
Transconductance:
(slope of the curve)
gm =  IC /  VBE
IES = The reverse saturation current
of the B-E Junction.
VT = kT/q = 26 mV T=300K)
 = the emission coefficient and is
usually ~1 IC
8 mA
6 mA
4 mA
2 mA
VBE
0.7 V

8. Modes of Operation Active: Saturation: Cutoff:
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.

9. Operation of The npn Transistor Active Mode
Current flow in an npn transistor biased to operate in the active mode, (Reverse current components due to drift of thermally generated minority carriers are not shown.)

10. Operation of The npn Transistor Active Mode
Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor operating in the active mode; vBE  0 and vCB  0.

11. 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

12. Emitter-Current Curves
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 Region IE
Saturation Region
Cutoff
IE = 0
VCB

13. Circuit Diagram: NPN Transistor
Common-Base + _ IC IE IB
VCB
VBE E C B
VCE
Circuit Diagram: NPN Transistor
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
VCE
VBE
VCB
C-B Bias
E-B Bias
Active
IB
=VBE+VCE
~0.7V
 0V
Rev.
Fwd.
Saturation
Max
~0V
-0.7V<VCE<0
Cutoff ~0
 0V
None/Rev.

14. Collector-Current Curves
Common-Emitter
Circuit Diagram
Collector-Current Curves
VCE IC IC + _
Active Region
VCC IB IB
Region of Operation
Description
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

15. Emitter-Current Curves
Common-Collector
Emitter-Current Curves
The Common-Collector 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

16. Eber-Moll BJT Model IE IC E C RIC RIE IF IR IB B
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
RIC
RIE IF IR IB B

17. Eber-Moll BJT Model IC = FIF – IR IB = IE - IC IE = IF - RIR
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.

18. 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 r
iB
r = ( + 1) * VT IE iE E
@  = 1 and T = 25C
r = ( + 1) * 0.026 IE
Recall:
 = IC / IB

19. Circuit whose operation is to be analyzed graphically.

20. Graphical construction for the determination of the dc base current in the previous circuit

21. Fig Graphical construction for determining the dc collector current IC and the collector-to-emmiter voltage VCE

22. Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB

23. Effect of bias-point location on allowable signal swing: Load-line A results in bias point QA with a corresponding VCE which is too close to VCC and thus limits the positive swing of vCE. At the other extreme, load-line B results in an operating point too close to the saturation region, thus limiting the negative swing of vCE.

24. The Early Effect (Early Voltage) Orange = Actual IC (IC’)
Note: Common-Emitter Configuration IB
-VA
VCE
Green = Ideal IC
Orange = Actual IC (IC’)
IC’ = IC VCE + 1 VA

25. 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
VCE IC
= 100 = IC/IB a)
IC = 100 * IB = 100 * (25x10-6 A)
IC = 2.5 mA + _
VCC IB
b) IC’ = IC VCE + 1 = 2.5x = 2.96 mA VA
IC’ = 2.96 mA

26. 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 ~ 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

27. Sources Web Sites http://www.infoplease.com/ce6/sci/A0861609.html
Dailey, Denton. Electronic Devices and Circuits, Discrete and Integrated. Prentice Hall, New Jersey: (pp )
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:
Neamen, Donald. Semiconductor Physics & Devices. Basic Principles. McGraw-Hill, Boston: (pp )
Web Sites