1. Bipolar Junction Transistor
BASIC ELECTRONICS
Bipolar Junction Transistor
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

2. Syllabus Introduction to Bipolar Junction Transistor BJT Operation
BJT Configurations
Tutorials
BJT Biasing
BJT Amplifier
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

3. Reference Books
“Electronic Devices and Circuit Theory” by Boylestad & Nashelsky,
“Integrated Electronics” by Millman & Halkias,
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

4. Introduction
Solid state transistor was invented by a team of scientists at Bell laboratories during
It brought an end to vacuum tube era
Advantages of solid state transistor over vacuum devices:
Smaller size, light weight
No heating elements required
Lower power consumption and operating voltages
Low price
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

5. Introduction Figure showing relative sizes of transistor, IC and LED
Figure showing different transistor packages
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

6. Introduction
Bipolar Junction Transistor (BJT) is a sandwich consisting of three layers of two different types of semiconductor
Two kinds of BJT sandwiches are: NPN and PNP
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

7. Introduction
The three layers of BJT are called Emitter, Base and Collector
Base is very thin compared to the other two layers
Base is lightly doped. Emitter is heavily doped. Collector is moderately doped
NPN – Emitter and Collector are made of N-type semiconductors; Base is P-type
PNP – Emitter and Collector are P-type, Base is N-type
Both types (NPN and PNP) are extensively used, either separately or in the same circuit
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

8. Introduction Transistor symbols:
Note: Arrow direction from P to N (like diode)
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

9. Introduction
BJT has two junctions – Emitter-Base (EB) Junction and Collector-Base (CB) Junction
Analogous to two diodes connected back-to-back:
EB diode and CB diode
The device is called “bipolar junction transistor” because current is due to motion of two types of charge carriers – free electrons & holes
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

10. Transistor Operation
Operation of NPN transistor is discussed here; operation of PNP is similar with roles of free electrons and holes interchanged
For normal operation (amplifier application)
EB junction should be forward biased
CB junction should be reverse biased
Depletion width at EB junction is narrow (forward biased)
Depletion width at CB junction is wide (reverse biased)
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

11. Transistor Operation
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

12. Transistor Operation
Un-biased transistor showing barriers at the junctions
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

13. Transistor Operation
C-B junction is reverse biased – increased barrier height
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

14. Transistor Operation E-B junction is forward biased – aids charge flow
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

15. Transistor Operation
Electron-hole combination – leads to small base current
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

16. Transistor Operation
When EB junction is forward biased, free electrons from emitter region drift towards base region
Some free electrons combine with holes in the base to form small base current
Inside the base region (p-type), free electrons are minority carriers. So most of the free electrons are swept away into the collector region due to reverse biased CB junction
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

17. Transistor Operation Three currents can be identified in BJT
Emitter current
This is due to flow of free electrons from emitter to base
Results in current from base to emitter
Base current
This is due to combination of free electrons and holes in the base region
Small in magnitude (usually in micro amperes)
Collector current
Has two current components:
One is due to injected free electrons flowing from base to collector
Another is due to thermally generated minority carriers
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

18. Transistor Operation C E B IC IE IB
NPN C E B IC IE IB
PNP
Note the current directions in NPN and PNP transistors
For both varieties: ---(1)
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

19. Transistor Operation
As noted earlier, collector current has two components:
One due to injected charge carriers from emitter
Another due to thermally generated minority carriers
Both results in current in the same direction. Hence
--- (2)
αdc is the fraction of charge carriers emitted from emitter, that enter into the collector region
ICBO is the reverse saturation current in CB diode
--- (3)
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

20. Transistor Operation
As approximation, we can neglect ICBO compared to IE and IC
Hence approximate equations are:
Like the reverse saturation current of ordinary diode, ICBO also doubles for every 10o C rise in temperature.
So ICBO cannot be neglected at higher temperatures
The parameter αdc is called common-base dc current gain
Value of αdc is around 0.99
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

21. Transistor Operation We have Substituting for IE, we get --- (4)
Where and
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

22. Transistor Operation
Equations (2) and (4) are two alternate forms of BJT current equation
Since value of αdc is around 0.99, ICEO >> ICBO
However, ICEO is still very small compared to IC
Hence approximation of (4) gives: or
Parameter βdc is called common emitter dc current gain
Values of αdc and βdc vary from transistor to transistor. Both αdc and βdc are sensitive to temperature changes
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

23. Problems
A BJT has alpha (dc) and collector-to-base reverse sat current 1μA. If emitter current is 5mA, calculate the collector and base currents.
(Ans: 4.99 mA, 10 μA)
2. An npn transistor has collector current 4mA and base current 10 μA. Calculate the alpha and beta values of the transistor, neglecting the reverse sat current ICBO
(Ans: , 400)
In a transistor, 99% of the carriers injected into the base cross over to the collector region. If collector current is 4mA and collector leakage current is 6 μA, Calculate emitter and base currents
(Ans: mA, 34 μA)
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

24. Transistor Configurations
BJT has three terminals
For two-port applications, one of the BJT terminals needs to be made common between input and output
Accordingly three configurations exist:
Common Base (CB) configuration
Common Emitter (CE) configuration
Common Collector (CC) configuration
(The last one is not discussed in this course)
Input
Output
2-port device
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

25. Transistor Configurations
Common Base configuration
(Resistors are not shown here for simplicity)
Base is common between input and output
Input voltage: VEB Input current: IE
Output voltage: VCB Output current: IC
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

26. Transistor Configurations
CB Input characteristics
A plot of IE versus VEB for various values of VCB
It is similar to forward biased diode characteristics
As VCB is increased, IE increases only slightly
Note that second letter in the suffix is B (for base)
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

27. Transistor Configurations
Input resistance ri
Voltage amplification factor AV
Both can be determined from the CB input characteristics
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

28. Transistor Configurations
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

29. Transistor Configurations
CB Output characteristics
A plot of IC versus VCB for various values of IE
Three regions are identified: Active, Cutoff, Saturation
Active region:
E-B junction forward biased
C-B junction reverse biased
IC is positive, VCB is positive
IC increases with IE
For given IE, IC is almost constant; increases only slightly with increase in VCB. This is due to base-width modulation (Early effect)
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

30. Transistor Configurations
CB Output characteristics
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

31. Transistor Configurations
Base Width modulation
As the reverse bias voltage VCB is increased, the depletion region width at the C-B junction increases. Part of this depletion region lies in the base layer. So, effective base width decreases. Hence number of electron-hole combination at the base decreases. So base current reduces and collector current increases.
Note that IE = IC + IB
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

32. Transistor Configurations
When IE = 0, IC = ICBO
ICBO is collector to base current with emitter open
Below this line we have cut-off region
Here both junctions are reverse biased
Region to the left of y-axis (VCB negative) is saturation region
Here both junctions are forward biased
IC decreases exponentially, and eventually changes direction
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

33. Transistor Configurations
Output resistance ro
Current amplification factor AI or αac
Both can be measured from output characteristics
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

34. Transistor Configurations
Common Emitter configuration
(Resistors are omitted for simplicity)
Emitter is common between input and output
Input voltage: VBE Input current: IB
Output voltage: VCE Output current: IC
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

35. Transistor Configurations
CE input characteristics
Plot of IB versus VBE for
various values of VCE
Similar to diode
characteristics
As VCE is increased, IB
decreases only slightly
This is due to base-width
modulation
Note that second suffix is E
(for emitter)
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

36. Transistor Configurations
CE output characteristics
A plot of IC versus VCE for various values of IB
Three regions identified: Active, Cut-off, Saturation
Active region:
Linear region in the output characteristics
E-B junction forward biased
C-B junction reverse biased
IC increases with IB
For given IB, IC increases slightly with increase in VCE; this is due to base-width modulation (Early effect)
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

37. Transistor Configurations
CE output characteristics
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

38. Transistor Configurations
Note that VCE = VCB + VBE
So if VCE is increased, effectively VCB also increases
For saturation to take place, C-B junction should be forward biased.
This happens when VCE is approximately 0.3 V (or less) for Si
Note that when VCE= 0.3V, and VBE= 0.6 V, VCB= –0.3V (a forward bias of 0.3 V)
So region to the left of the vertical line VCE=VCE(sat)=0.3V (for silicon) is considered as saturation region
Region below IB=0 line (or IC=ICEO) is cut-off region
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

39. Transistor Configurations
ICEO is much larger than ICBO because of the relation:
Note that value of αdc is around 0.99
The values of αdc & αac, and βdc & βac are almost the same. Hence the subscripts can be omitted for simplicity
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

40. Transistor Configurations
Input resistance ri
Output resistance ro
Voltage gain AV
Current gain AI or βac
All these parameters can be determined from CE characteristics
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

41. Transistor Configurations
Experimental setup for determining CE characteristics:
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

42. Tutorials
A Ge transistor with β = 100 has collector-to-base leakage current of 5 µA. If the transistor is connected in common-emitter operation, find the collector current for base current (a) 0 (b) 40 µA.
Sol: Given that ICBO = 5µA, and β = 100
We know that
When IB = 0, IC = ICEO = (β+1)ICBO = 505 µA
When IB = 40 µA, IC = βIB + ICEO
= (100 × 40 × 10–6) + (505 × 10–6)
= mA
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

43. Tutorials
A Ge Transistor has collector current of 51 mA when the base current is 0.4 mA. If β = 125, then what is its collector cutoff current ICEO?
(Ans: 1 mA)
In a transistor circuit, when the base current is increased from 0.32 mA to 0.48 mA, the emitter current increases from 15 mA to 20 mA. Find αac and βac values.
(Ans: 0.968, 30.25)
A transistor with α = 0.98 and ICBO = 5 µA has IB = 100 µA. Find IC and IE.
(Ans: 5.15 mA, 5.25 mA)
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

44. Transistor Biasing
One of the most common applications of transistor is in amplifiers.
E-B junction should be forward biased; C-B junction should be reverse biased (active region)
For faithful amplification we require that transistor be operated in active region throughout the duration of input signal.
To ensure this, proper dc voltages should be applied. This is called Biasing.
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

45. Operating point
When no input signal is applied to transistor circuit, and only dc voltages are supplied, currents IC, IB and voltage VCE will have certain values.
If these values are plotted over the transistor output characteristics, the point we get is called ‘Operating point’. It is also called ‘Quiescent point’ or just Q-point.
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

46. In above figure, currents IBQ, ICQ and voltage VCEQ are plotted as point Q. In practice, we have to choose Q-point according to our requirement. If we want to operate in the middle of active region, we may choose Q as Q-point.
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

47. If we want to operate near saturation, we may choose Q’ as Q-point.
If we want to operate near cutoff, we may choose Q’’ as Q-point.
Note that if no biasing is used, Q-point will be in the origin of graph.
So, biasing is used to fix the Q-point according to our need.
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

48. Type of Biasing in Transistor
Fixed bias Voltage divider bias
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

49. Fixed Bias
Base resistor RB is connected to Vcc (Instead of VBB). Negative terminal of Vcc is not shown. It is assumed to be at ground.
Applying KVL to the input,
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

50. Applying KVL to output side we get:
Vcc is constant, VBE is almost constant (0.7V for silicon). So by selecting proper RB, we can fix IB as required.
Applying KVL to output side we get:
IC is related to IB by β
So, VCE can be fixed by selecting proper RC.
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

51. Load Line
We have, This is an equation of straight line with VCE and IC as two variables. This straight line is called load line. Now, output characteristic is also a function of same two variables. If RB and RC are held constant and VCC is varied, then load line shifts, maintaining same slope.
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

52. Department of Electronics and Communication School of Engineering, Manipal University Jaipur

53. if RB is decreased, transistor goes towards saturation.
If RB and RC are held constant and VCC is varied, then load line shifts, maintaining same slope. From these graphs we infer that:
with everything else held constant, if RB is increased, transistor goes towards cutoff.
if RB is decreased, transistor goes towards saturation.
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

54. if RC is decreased, transistor goes towards active region.
With everything else held constant, if RC is increased, transistor goes towards saturation.
if RC is decreased, transistor goes towards active region.
With everything else held constant, if VCC is increased, transistor goes towards active region.
if VCC is decreased, transistor goes towards saturation.
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

55. Advantages of fixed bias Simple to analyze and design
Uses very few circuit components
Disadvantages of fixed bias
Q-point is not stable. i.e., if temperature varies, β will vary, hence IC will vary.
If transistor is replaced by another transistor having different β then Q-point will shift.
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

56. Voltage divider bias or Self bias
Uses two resistors R1 and R2 instead of RB. RE is connected between emitter and ground.
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

57. Input side of the above circuit is redrawn below,
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

58. VTH is the open circuit voltage between points A & B in fig (a) given by,
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

59. Applying KVL to the input loop,
RTH is the resistance between A & B with VCC replaced by short circuit.
Applying KVL to the input loop,
Substituting and rearranging, we get
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

60. Applying KVL to the output loop, we get
Rearranging, we get
Also,
Where, VC is voltage from collector to ground.
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

61. Also, Now, Where, VE is voltage from emitter to ground.
Since β>> 1, we have (β+1) ≈ β. If βRE >> RTH, then equation of IB reduces to,
Now,
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

62. Advantages of voltage divider bias
Since equation for IC does not contain β, we say that IC is independent of temperature variation and transistor replacement.
Advantages of voltage divider bias
. Q-point is stable against variation in temperature and replacement of transistor.
Disadvantages of voltage divider bias
Analysis and design are complex
More circuit components required
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

63. Problem
Draw the DC load line and mark the Q- point on fixed bias cct. Assume Beta DC=100 and neglect Base –Emitter voltage. (Vcc=30V,RB=1.5 Mohm, RC=5 Kohm).
Ans: VCE,max=30v,VCEQ=20v,ICQ=2mA
2. In a fixed bias cct. Find the base current required to establish VCE=6v, also find RB & IE, (VBE=0.7v, Beta DC=120, VCC=12v,RC=2.2.Kohm).
Ans: IB=22.75 uA, RB=497 Kohm
3. Determine the region in which the transistor operates. (VBE=0.2v,RB=120kohm,RC=1kohm,VCC=15v,Beta DC=120).
Department of Electronics and Communication School of Engineering, Manipal University Jaipur

64. End of Module 3
Department of Electronics and Communication School of Engineering, Manipal University Jaipur