A bipolar junction transistor (BJT) consists of two PN junctions formed by sandwiching either a p-type or n-type semiconductor between two opposite types. It has three sections - the emitter, base, and collector. Current flows due to both electrons and holes, making it a bipolar device. The base is lightly doped and very thin to allow charge carriers to easily move from the emitter to the collector. BJTs can be used as amplifiers because the collector current is controlled by the base current.

1. UNIT-3
Bipolar junction Transistor (BJT)

2. A bipolar transistor is a semiconductor device in which electric
current flows due to electrons and holes BOTH,
simultaneously. Thus both types of charges take part in the
conduction of current through it. Hence it is called bipolar
transistor.
A bipolar junction transistor (BJT) consists of two PN
junctions formed by sandwiching either p-type or n-type
semiconductor between a pair of opposite types.
These are two types 1. NPN 2. PNP transistor
The outer layers of the npn or pnp sandwich are called the
emitter and the collector, and the centre layer is termed the
base.
Two pn junctions are formed with depletion regions and barrier
voltages at each junction.

3. Contd…
A transistor (PNP or NPN) has three
sections of doped semiconductors.
It consists of 3 terminals
1.emitter 2. Base 3. collector
1. section on one side that supplies
charge carriers (electrons of holes)
is called emitter. Which is heavily
doped .
2. The middle section which form
two pn-junctions between the
emitter and collector is called the
base

4. Contd…
3. The section on the other side that collects the charges is
called the collector . Collector junction is always reverse
biased. This section is moderately doped.
Where as emitter is heavily doped so that it can inject a large
number of charge carriers into the base.
The base is lightly doped and very thin.
The resistance of emitter diode is small and collector diode is
large.
Therefore the transistor transfers the input signal current from
a low-resistance circuit to a high-resistance circuit.

5. force – voltage/current
water flow – current
- amplification
Understanding of BJT

6. Physical structure of BJT

7. The barrier voltages are negative on the p-side and
positive on the n-side.
The base-emitter junction is forward, so that charge
carriers are emitted into the base.
The collector-base junction is reverse-biased, and its
depletion region penetrates deep into the base.

8. The base section is made as narrow as possible so that charge
carriers can easily move across from emitter to collector.
The base is lightly doped, so that few charge carriers are
available to recombine with the majority charge carriers from
the emitter.
Most charge carriers from the emitter flow to the collector, a
few flow out through the base terminal.

9. Operation of NPN and PNP

10. Contd …

11. NPN and PNP Transistors

12. Current components in BJT

13. Contd..
It can be noted from the diagram the battery VEB forward biases
the EB junction while the battery VCB reverse biases the CB
junction.
As the EB junction is forward biased the holes from emitter
region flow towards the base causing a hole current IPE.
At the same time, the electrons from base region flow towards
the emitter causing an electron current INE. Sum of these two
currents constitute an emitter current IE = IPE +INE.
The ratio of hole current IPE to electron current INE is directly
proportional to the ratio of the conductivity of the p-type
material to that of n-type material.

14. Contd,..
Not all the holes, crossing EB junction reach the CB junction
because some of the them combine with the electrons in the n-type
base.
If IPC is the hole current at (Jc) CB junction. There will be a
recombination current IPE - IPC leaving the base as shown in fig
above
If emitter is open circuited, no charge carriers are injected from
emitter into the base and hence emitter current IE =0.
Under this condition CB junction acts a a reverse biased diode
and therefore the collector current ( IC = ICO) will be equal to te
reverse saturation current.
Therefore when EB junction is forward biased and collector base
junction is reverse biased the total collector current IC = IPC +ICO.

15. Transistor operating regions.

16. Contd...
A transistor can be operated in three different regions as
a) active region
b) saturation region
c) cut-off region
Active region: The transistor is said to be operated in active region
when the emitter-base junction is forward biased and collector –
base junction is reverse biased.
 The collector current is said to have two current components one is
due to the forward biasing of EB junction and the other is due to
reverse biasing of CB junction.
The collector current component due to the reverse biasing of the
collector junction is called reverse saturation current (ICO or ICBO) and
it is very small in magnitude.

17. Contd…
Saturation Region:
Transistor is said to be operated in saturation region when both
EB junction and CB junction are forward biased as shown.
When transistor is operated in saturation region IC increases
rapidly for a very small change in VC.
Cut-off region:
When both EB junction and CB junction are reverse biased, the
transistor is said to be operated in cut-off region. In this region,
the current in the transistor is very small and thus when a
transistor in this region it is assumed to be in off state.

18. Transistor Model
Large signal Equivalent model of NPN BJT
BJT is used as a current controlled current source

19. BJT is current controlled current source
For a BJT to amplify we give input signal if suppose we use BJT in CE
configuration input is given at Emitter-Base junction and output is taken at
Collector base junction, the input voltage increases or decreases the
forward bias of the E-B junction affecting a change in the base current and
we know that collector current is a function of base current collector
current also varies so by selectively changing the base current we can
effectively change the collector current
Because the operation of the transistor is determined by the current at the
base. the principle equations of BJT operation are: Ic = h*Ib ,and Ie=Ib+Ic.
Thus device operation is controlled by the input current.
Hence BJT is also called as current controlled current source.

20. α is common base current gain
α dc = IC / IE
β is common emitter current gain
β = IC / IB
IC = α dc ( IC + IB)
IC = α dc IB / 1- α dc
IC = β dc IB
β = α dc / 1- α dc

21. COMMON BASE CHARACTERISTICS

22. Common base characteristics
A pnp transistor with its base terminal common to
both the input (emitter-base) terminal and the output
(collector-base) terminal.
The transistor is said to be connected in common-base
configuration.
Voltmeters and ammeters are used to measure the
input and output voltages and currents

24. Common-Base
input characteristics
To investigate the input characteristics, the output
voltage (VCB) is kept constant, and the input voltage
(VEB) is set at several convenient levels.
 At each input voltage, the corresponding input
current (IE) is recorded.
The IE and VEB levels are then plotted to give the
common-base input Characteristics

25. Input Characteristics

26. Common base
Output characteristics
To investigate output characteristics, the input
current (IE) is kept constant, and VCB is adjusted in
convenient steps, and the corresponding values of IC
are recorded.

27. The corresponding IC and VCB levels obtained when
IE was held constant at 1 m A are potted, and the
resultant characteristic is identified as IE=1mA.
Similarly other characteristics are potted for IE equal
to 2mA, 3mA, and so on.

28. Output Characteristics

29. COMMON EMITTER CHARACTERISTICS

30. COMMON-EMITTER CHARACTERISTCS
A pnp transistor with its Emitter terminal common
to both the input (base-emitter) terminal and the
output (collector-emitter) terminal.
The transistor is said to be connected in common-emitter
configuration.
Voltmeters and ammeters are used to measure the
input and output voltages and currents

32. Relation between transistor currents

33. Common-Emitter
Input Characteristics
To investigate the input characteristics, the output
voltage (VCE) is kept constant, and the input voltage
(VBE) is set at several convenient levels.
 At each input voltage, the corresponding input
current (IB) is recorded.
The IB and VBE levels are then plotted to give the
common-emitter input Characteristics

35. Common Emitter
Output characteristics
To investigate output characteristics, the input
current (IB) is kept constant, and VCE is adjusted in
convenient steps, and the corresponding values of IC
are recorded.

36. The corresponding IC and VCE levels obtained when
IB was held constant at 10 μ A are potted, and the
resultant characteristic is identified as IB=10 μ A.
Similarly other characteristics are potted for IB equal
to 20 μ A, 30 μ A, and so on.

38. Common emitter output characteristics of NPN-BJT

39. Early Effect
An increase in magnitude of collector voltage
increases (reverse bias) the space charge width
increase at the output junction diode (C-B junction).
Such action causes the effective base width ‘W’ to
decreases. A phenomenon know as ‘Early Effect’

40. Punch-through
(or)
Reach -through
If an excessive reverse-bias voltage is applied to the
collector-base junction, the device breakdown may
occur

41. Breakdown can also result from the collector-base
depletion region penetrating into the base (as the
reverse bias increases) until it makes contact with
emitter-base depletion region
This condition is known as punch through or reach
through
A Very large currents can flow when it occurs, possible
destroying the device

45. Common collector input
characteristics

47. i/p characteristics

48. o/p characteristics

49. AMPLIFICATION in Bipolar junction transistors
(BJTs)
Discussion of an amplification effect
v = R ´
i
v = R ´
i
i i B
o L C
R v R v
= D << = D
D D
BE CE
i L
i i
B C
B C With i << i
i o v < < v

50. DC Load Line

51. DC Load Line (Contd..)
 Load line:To draw load line, we have to find saturation current and the cutoff
voltage.
 After plotting these values on the vertical and the horizontal axes, a line is drawn
joining these two points, which represents DC load line.
 It represents all possible combinations of the collector current Ic and the collector
.voltage Vc (or Vce) for the given load resistor Rc.
 Saturation point
The point at which the load line intersects the characteristic curve near the collector
current axis is referred to as the saturation point. At this point of time, the current
through the transistor is maximum and the voltage across collector is minimum for a
given value of load.
 Therefore saturation current for the fixed bias circuit,
Ic (sat) =Vcc/Rc
 Cutoff point
The point where the load line intersects the cutoff region of the collector curves is
referred as the cutoff point (i.e. end of load line). At this point, collector current is
approximately zero and emitter is grounded for fixed bias circuit. Therefore,
Vce (cut) = Vc = Vcc

52. Q-Point (Static Operation Point)/Quiescent point
The intersection of the dc bias value of IB with the dc load line determines
the Q-point.
When a circuit is designed to have a centered Q-point, the amplifier is said
to be midpoint biased.
Midpoint biasing allows optimum ac operation of the amplifier.
When the Q-point is centered, IC and VCE can both make the maximum
possible transitions above and below their initial dc values

53. Q-Point (contd..)
 When the Q-point is below
midpoint on the load line, the
input signal may cause the
transistor to cutoff. This can
also cause a portion of the
output signal to be clipped.
 When the Q-point is above the
center on the load line, the
input signal may cause the
transistor to saturate. When
this happens, a part of the
output signal will be clipped
off

54. Cutoff and Saturation Clipping

55. DC Biasing
Purpose of the DC biasing circuit is to turn the device “ON” .
To place it in operation in the region of its characteristic where the device
operates most linearly, i.e. to set up the initial dc values of IB, IC, and VCE .
DC biasing is a static operation since it deals with setting a fixed (steady)
level of current (through the device) with a desired fixed voltage drop
across the device.

56. 56
WHY BIASING?
If the transistor is not biased properly, it would work inefficiently and
produce distortion in output signal.
HOW A TRANSISTOR CAN BE BIASED?
A transistor is biased either with the help of battery or associating a
circuit with the transistor. The later method is more efficient and is
frequently used. The circuit used for transistor biasing is called the
biasing circuit.

57. 57
The Thermal Stability of Operating Point (SIco)
Stability Factor S:- The stability factor S, as the change of
collector current with respect to the reverse saturation current,
keeping β and VBE constant. This can be written as:
The TThheerrmmaall SSttaabbiilliittyy FFaaccttoorr :: SSIIccoo
SSIIccoo == ∂∂IIcc
∂∂IIccoo
TThhiiss eeqquuaattiioonn ssiiggnniiffiieess tthhaatt IIcc CChhaannggeess SSIIccoo ttiimmeess aass ffaasstt aass IIccoo
DDiiffffeerreennttiiaattiinngg tthhee eeqquuaattiioonn ooff CCoolllleeccttoorr CCuurrrreenntt IICC == ((11++ββ))IIccoo++
ββIIbb && rreeaarrrraannggiinngg tthhee tteerrmmss wwee ccaann wwrriittee
SSIIccoo ═ 11++ββ
11-- ββ ((∂∂IIbb//∂∂IICC))
IItt mmaayy bbee nnootteedd tthhaatt LLoowweerr iiss tthhee vvaalluuee ooff SSIIccoo bbeetttteerr iiss tthhee
ssttaabbiilliittyy
VVbbee,, ββ

58. 58
Various Biasing Circuits
 Fixed Bias Circuit
 Fixed Bias with Emitter Resistor
 Collector to Base Bias Circuit
 Potential Divider Bias Circuit

59. 59
The Fixed Bias Circuit
15 V
C
E
B
15 V
200 k 1 k
The TThheerrmmaall SSttaabbiilliittyy FFaaccttoorr :: SSIIccoo
SSIIccoo == ∂∂IIcc
VVbbee,, ββ
∂∂IIccoo
GGeenneerraall EEqquuaattiioonn ooff SSIIccoo CCoommeess oouutt ttoo
bbee
SSIIccoo ═ 11 ++ ββ
11-- ββ ((∂∂IIbb//∂∂IICC))
AAppppllyyiinngg KKVVLL tthhrroouugghh BBaassee CCiirrccuuiitt wwee
ccaann wwrriittee,, IIbb RRbb++ VVbbee== VVcccc
DDiiffff ww.. rr.. tt.. IICC,, wwee ggeett ((∂∂IIbb // ∂∂IIcc)) == 00
SSIIccoo== ((11++ββ)) iiss vveerryy llaarrggee
IInnddiiccaattiinngg hhiigghh uunn--ssttaabbiilliittyy
RRbb
II
bb
RRCC
RRCC

60. 60
Fixed bias with emitter
resistor
The fixed bias circuit is
modified by attaching an
external resistor to the
emitter. This resistor
introduces negative
feedback that stabilizes the
Q-point.

61. 61
The Collector to Base Bias Circuit
VCC
RC
C
E
B
RF
IIcc
IIbb
VVBBEE
++
-- II
EE
This configuration employs negative
feedback to prevent thermal runaway
and stabilize the operating point. In
this form of biasing, the base resistor
RF is connected to the collector instead
of connecting it to the DC source Vcc.
So any thermal runaway will induce a
voltage drop across the Rc resistor that
will throttle the transistor's base
current.

62. 62
Applying KKVVLL tthhrroouugghh bbaassee cciirrccuuiitt
wwee ccaann wwrriittee ((IIbb++ IICC)) RRCC ++ IIbb RRff++ VVbbee== VVcccc
DDiiffff.. ww.. rr.. tt.. IICC wwee ggeett
((∂∂IIbb // ∂∂IIcc)) == -- RRCC // ((RRff ++ RRCC))
TThheerreeffoorree,, SSIIccoo ═ ((11++ ββ))
11++ [[ββRRCC//((RRCC++ RRff))]]
WWhhiicchh iiss lleessss tthhaann ((11++ββ)),, ssiiggnniiffyyiinngg bbeetttteerr tthheerrmmaall
ssttaabbiilliittyy

63. This is the most commonly used arrangement for biasing as it provide good
bias stability. In this arrangement the emitter resistance ‘RREE’’ provides
stabilization. The resistance ‘RREE’’ cause a voltage drop in a direction so as to
reverse bias the emitter junction. Since the emitter-base junction is to be
forward biased, the base voltage is obtained from R1-R2 network. The net
forward bias across the emitter base junction is equal to VB- dc voltage drop
across ‘RREE’’. The base voltage is set by Vcc and R1 and R2. The dc bias circuit is
independent of transistor current gain. In case of amplifier, to avoid the loss
of ac signal, a capacitor of large capacitance is connected across RE. The
capacitor offers a very small reactance to ac signal and so it passes through the
condensor.
63
The Potential Divider Bias Circuit

64. 64
The Potential Divider Bias Circuit
VCC
RC
C
E
B
VCC
R1
IICC
IIEE
IIbb
R2 RE
TToo ffiinndd tthhee ssttaabbiilliittyy ooff tthhiiss cciirrccuuiitt wwee
hhaavvee ttoo ccoonnvveerrtt tthhiiss cciirrccuuiitt iinnttoo iittss
TThheevveenniinn’’ss EEqquuiivvaalleenntt cciirrccuuiitt
RRtthh == RR11**RR22 && VVtthh == VVcccc RR22
RR11++RR22 RR11++RR22

65. 65
The Potential Divider Bias Circuit
Applying KVL through iinnppuutt bbaassee cciirrccuuiitt
wwee ccaann wwrriittee IIbbRRTThh ++ IIEE RREE++ VVbbee== VVTThh
TThheerreeffoorree,, IIbbRRTThh ++ ((IICC++ IIbb)) RREE++ VVBBEE== VVTThh
DDiiffff.. ww.. rr.. tt.. IICC && rreeaarrrraannggiinngg wwee ggeett
((∂∂IIbb // ∂∂IIcc)) == -- RREE // ((RRTThh ++ RREE))
TThheerreeffoorree,,
ù
= +
é
R
E
E
TThhiiss sshhoowwss tthhaatt SSIIccoo iiss iinnvveerrsseellyy
pprrooppoorrttiioonnaall ttoo RREE aanndd IItt iiss lleessss tthhaann ((11++ββ)),,
ssiiggnniiffyyiinngg bbeetttteerr tthheerrmmaall ssttaabbiilliittyy
TThheevveenniinn
EEqquuiivvaalleenntt
CCkktt
úû
êë
+
+
R RTh
SIco
b
b
1
1
VCC
RC
C
E
B
RE
RTh
+
_ VTh
TThheevveenniinn
EEqquuiivvaalleenntt
VVoollttaaggee
IIEE
SSeellff--bbiiaass RReessiissttoorr
IIbb
IICC

66. This is the most commonly used arrangement for biasing as it provide good
bias stability. In this arrangement the emitter resistance ‘RREE’’ provides
stabilization. The resistance ‘RREE’’ cause a voltage drop in a direction so as to
reverse bias the emitter junction. Since the emitter-base junction is to be
forward biased, the base voltage is obtained from R1-R2 network. The net
forward bias across the emitter base junction is equal to VB- dc voltage drop
across ‘RREE’’. The base voltage is set by Vcc and R1 and R2. The dc bias circuit is
independent of transistor current gain. In case of amplifier, to avoid the loss
of ac signal, a capacitor of large capacitance is connected across RE. The
capacitor offers a very small reactance to ac signal and so it passes through the
condensor.
66
The Potential Divider Bias Circuit

67. 67
Merits:
• Operating point is almost independent of β variation.
• Operating point stabilized against shift in temperature.
Demerits:
• As β-value is fixed for a given transistor, this relation can be satisfied either
by keeping RE fairly large, or making R1||R2 very low.
 If RE is of large value, high VCC is necessary. This increases cost as well
as precautions necessary while handling.
 If R1 || R2 is low, either R1 is low, or R2 is low, or both are low. A low R1
raises VB closer to VC, reducing the available swing in collector voltage, and
limiting how large RC can be made without driving the transistor out of active
mode. A low R2 lowers Vbe, reducing the allowed collector current. Lowering
both resistor values draws more current from the power supply and lowers
the input resistance of the amplifier as seen from the base.
 AC as well as DC feedback is caused by RE, which reduces the AC voltage
gain of the amplifier. A method to avoid AC feedback while retaining DC
feedback is discussed below.
Usage:
The circuit's stability and merits as above make it widely used for linear circuits.

68. Summary
The Q-point is the best point for operation of a
transistor for a given collector current.
The purpose of biasing is to establish a stable
operating point (Q-point).
The linear region of a transistor is the region of
operation within saturation and cutoff.
Out of all the biasing circuits, potential divider
bias circuit provides highest stability to operating
point.
68