Transistors and Transistor
Biasing
1
Transistor ❖
❖
❖
டிரான்சிஸ்டர்
3 terminal device – 2 back-to-back p-n
junctions
NPN Transistor - p-type sandwiched
between two n-type semiconductors
PNP Transistor - n-type sandwiched
between two p-type semiconductors
2
Emitter(E) உமிழ்ப்பான்- emits (supplies)
charges - always forward biased
Collector(C) ஏற்பான்- collects
charges- always reverse biased
Base(B) அடிவாய் -Middle sections
which forms 2PN junctionsforward biased
3
Doping and Size
Base is lightly doped
• Base is thin
❖
Emitter is heavily
doped (inject large
no. of electrons)
• Emitter is moderate
size
Collector (C) is
moderately doped
• Collector (C) is
wider than E and B
Since the base is thin, most carriers from emitter
injected into the collector
4
Transistor Symbol
Conventional current (arrow) is opposite to electron flow
5
❖
Emitter diode is always Forward Biased -முன்ன
❖
Collector diode is always Reversed Biased
❖
EB junction is Forward Biased (FB)- low resistance-
சார்பு
ாக்கு சார்பு
-பின்ன
ாக்குச்
குறைந்த மின்தறை
❖
CB junction is Reversed Biased (RB) - High resistanceஅதிக மின்தறை
❖
❖
Transistor transfers signal from low resistance to high
resistance
‘Trans’ means transfers; ‘istor’ means family of resistors
6
Working of NPN Transistor
EB - forward biased -
முன்ன
CB - reversed biased - பின்ன
ாக்கு சார்பு
–VEB
ாக்குச் சார்பு–VCB
VEB < VCB
EB junction - EB
சந்தி
(heavily doped) - ejects more electrons
Majority charge carriers பபரும்பான்றம கைத்திகள் from emitter
move towards the base - emitter current IE (100%)
7
Working of NPN Transistor (cont..)
•The electrons enter into the base (lightly doped)
•Combine with the few holes - constitutes the base current IB (5%)
•Reversed bias potential of the collector is high
•Attracts the electrons reaching collector (95%)
•Emitter current is the sum of the collector or the base current
IE=IB +IC
8
Transistor connections
Diode - 2 terminal device
1 terminal –input உள்ள ீடு
2 terminal- output பவளியீ டு
One battery –needed to give biasing
Transistor -3 terminal device
1 terminal – input
2 terminal - output
3 terminal - common (பபாது) for both input and output
Input applied between 1st terminal and common terminal
Output is taken between 2nd terminal and common terminal
Two batteries needed- one in input side; another in output side
9
Common base configuration -
பபாது அடிவாய் அறமப்பு
Base --Common terminal
❖
E and B Forward Biased
❖
C is Reversed Biased
10
Common base configuration
11
Common Emitter configuration -
பபாது உமிழ்ப்பான் அறமப்பு
❖
E and B - Forward Biased
❖
C is Reversed Biased
12
Common Emitter configuration
13
Common Collector configuration -
பபாது ஏற்பான் அறமப்பு
14
Common Collector configuration
15
16
Common Base Configuration (CB)
• Base terminal is common for both input and output of
the transistor
•Emitter –Base junction is forward biased
•Collector –Base junction is reverse biased
•VCB is kept constant
•Input current = Emitter current IE
•Output current = collector current IC
17
CB- Current Amplification Factor
(மின்ன ாட்ைபபருக்ககாரணி) (α)
Current amplification factor = Ratio of output current to the
input current
✓The ratio of change in collector current (ΔIC ) to the
change in emitter current (ΔIE) when collector voltage
VCB is kept constant, is called as Current
amplification factor.
✓It is denoted by α (less than 1)
✓α=ΔIC / ΔIE at constant VCB
18
CB- ஏற்பான்மின்ன
ாட்ைத்திற்கா
னகாறவ
Expression for Collector current in CB mode
•Current at C = Part of emitter current + some amount of base
current IB (which flows through the base terminal due to electron
hole recombination) (மின்துகள்-துறள மறுனசர்க்றக).
•The emitter current that reaches the collector terminal is αIE
( α=IC / IE )
•As collector-base junction is reverse biased, there is another current
which flows is due to minority charge carriers (சிறுபான்றம
கைத்திகள்)
This is the leakage current
கசிவு மின்ன
ாட்ைம் -
Ileakage
• This is due to minority charge carriers and hence very small
•Total collector current (ஏற்பான்மின்ன
ாட்ைம்)= IC=αIE+Ileakage
19
Expression for Collector Current in CB mode
If the emitter-base voltage VEB = 0, IB =0 there flows a small leakage
current ICBO (collector to base current with emitter open)
The collector current therefore can be expressed as
IC=α IE+ICBO
(IE=IC+IB)
IC=α(IC+IB)+ICBO
IC(1−α)=α IB+ICBO
IC =
IB+
ICBO
IC=βIB+(β+1)ICBO --Equation for collector current
The value of collector current depends on base current and leakage
current along with the current amplification factor of that transistor
in use.
20
CE- ஏற்பான்மின்ன
ாட்ைத்திற்கா
னகாறவ
Expression for Collector current in CE mode
Emitter – base --- forward biased
Collector is reverse biased
Input current =base current IB
Output current = collector current IC
21
CE -Current Amplification factor
(மின்ன ாட்ைபபருக்ககாரணி) (β)
β= Output current/ Input current
The ratio of change in collector current IC to the
change in base current IB is known as base current
amplification factor(β)
β=ΔIC / ΔIB
IB =5% of the emitter current β is greater than 20.
β = 20 to 500
22
Relation between α and β (α , β
க்கா
பதாைர்பு)
23
Relation between α and β
β= α/(1−α)
β(1- α)=α
β-αβ=α
If β= 98 what is α?
β=α+ αβ
β=α(1+ β)
β/(1+β)=α
24
PNP transistors
CB
CE
CC
25
PNP transistors
Common Base
Common Emitter
Common Collector
26
Expression for Collector Current in CE mode
IE=IB+IC
IC=αIE+ICBO
IC=α(IB+IC)+ICBO
IC(1−α)=αIB+ICBO
IC =
IB+
ICBO
If the base-emitter voltage VBE = 0, base circuit is open, i.e. IB = 0,
there flows a small leakage current, which can be termed as
ICEO (collector to emitter current with base open)
27
Expression for Collector Current in CE mode
CB- ஏற்பான் மின்ன
ாட்ைத்திற்கா
னகாறவ
The collector emitter current with base open is ICEO
ICEO=[1/(1−α)]ICBO
Substituting the value of this in the previous equation, we get
IC=[α/(1−α)]IB+[1/(1−α)]ICBO
IC =[α/(1−α)]IB+ICEO
Since β =
IC=βIB+ICEO
This is the equation for collector current
28
Transistor Characteristics in Common emitter (CE) mode
பபாது உமிழ்ப்பான் - டிரான்சிஸ்ைர் சிைப்பியல்
29
Input Characteristic Curve - உள்ள ீடு
சிைப்பியல்
Graph between – VBE (X axis) and IB (y axis)
VCE = constant
VBE is varied and IB is measured
Repeated for different constant VCE =2V, 6V, 10V
Family of curves are drawn
Curve is similar to a forward diode characteristics
IB increases with the increases in VBE - Sharp increase
Input resistance of the CE is comparatively higher that of CB
30
Input Characteristic Curve -உள்ள ீடு
சிைப்பியல்
Input Resistance(~100 ohms): Ratio of change in base-emitter
voltage VBE to the change in base current ∆IB at
constant collector-emitter voltage VCE ,
31
Output Characteristic Curve - பவளியீ டு சிைப்பியல்
VCE (X axis) and IC (y axis)
IB = constant; VCE is varied and IC is measured
• Repeated for different constant IB = 20,30,40,50,60 μA
•Upto Knee region : (0-1V); IC increases with VCE . This value of
VCE up to which collector current IC changes with VCE is called
the Knee Voltage
•Above Knee region (transistors are operated in this region)
• IC ~ constant ; for a particular VCE, IC ~ βIB
(because β=IC / IB )
•IC is independent of VCE ; depletion layer gets wider
•Small increase in IC, because collector captures electrons before
recombination in base area
•Cut off Region: A small current IC (is not zero), equal to ICEO (due
to minority carriers) flows
32
The output resistance of CE is less than CB
Output Characteristic Curve -
பவளியீ டு சிைப்பியல்
Output Resistance (~50k ohm): The ratio of change in
collector-emitter voltage VCE to the collector current IC at a
constant base current IB
33
Transfer Characteristics for CE Transistor
CE சுற்ைில் பரிமாற்று சிைப்பியல்
•The variation of output current in accordance with the input current,
keeping the output voltage constant. IC and IB increase almost linearly
•The variation of IC with IB keeping VCE as a constant. β=ΔIC / ΔIB
•Current Amplification Factor (β) is the ratio of change in the
collector current (IC) to the change in base current (IB) when the
collector-emitter voltage (VCE) is kept constant.
34
DC Load Line - பளுக்னகாடு
•To determine collector
current Ic for various
collector emitter voltage
VcE
•Can be determined from
output characteristics
•Convenient method- Load
line method
35
Load Line - பளுக்னகாடு
•Maximum possible collector current (IC) is a point on the Y-axis Saturation point (பதவிட்டு புள்ளி) (A)
•The maximum possible collector emitter voltage VCE is a point on
the X-axis- Cutoff point (பவட்டு புள்ளி)(B)
•A line is drawn joining these two points - Load line
•This is called so as it symbolizes the output at the load.
36
•The load line is drawn by joining the saturation
புள்ளி) and cut off (பவட்டு புள்ளி) points
(பதவிட்டு
• The region that lies between these two is the linear region. A
transistor acts as a good amplifier in this linear region
•DC load line is drawn only when DC biasing is given to the
transistor, but no input signal is applied, then such a load line is
called as DC load line
•No amplification as the signal is absent
37
.
The value of collector emitter voltage
VCE=VCC−ICRC (Y=mX)
VCC and RC are fixed values
First degree equation - a straight line on the output characteristics.
This line is called as D.C. Load line.
To obtain the load line, the two end points (A and B) of the straight
line are to be determined
38
To obtain point A
When collector emitter voltage VCE = 0, the collector current is
maximum and is equal to VCC/RC.
This gives the maximum value of VCE.
VCE=VCC−ICRC
0=VCC−ICRC
IC==VCC/RC
This gives the point A (OA = VCC/RC) on collector current axis
39
To obtain Point B
When the collector current IC = 0
Collector emitter voltage is maximum and will be equal to the VCC.
This gives the maximum value of IC
VCE=VCC−ICRC (AS IC = 0)
VCE=VCC
This gives the point B (OB=VCC) on the collector emitter voltage
axis
Saturation (A) and cutoff point (B) are joined- straight line - DC
load line
40
41
Operating point - பசயல்பாட்டு புள்ளி
Line is drawn joining the saturation and cut off points- Load
line.
This line, when drawn over the output characteristic curve,
intersects at a point called as Operating point.
This operating point is also called as quiescent point or Q-point.
There can be many such intersecting points, but the Q-point is
selected in such a way that irrespective of AC signal swing, the
transistor remains in the active region
Q point- Zero signal values of VCC and IC
42
43
➢The following graph shows how to represent the operating
point.
➢The operating point should not get disturbed as it should
remain stable to achieve faithful amplification.
➢Q-point is the value where the Faithful Amplification
( amplification without distortion) is achieved.
44
Faithful Amplification
45
Transistor Biasing
Biasing is the process of
providing DC voltage which helps
in the functioning of the circuit.
A transistor is biased in order to
make the emitter base junction
forward biased and collector base
junction reverse biased, so that it
maintains in active region, to work
as an amplifier.
A transistor acts as a good
amplifier, if both the input and
output sections are properly
biased.
46
Transistor Biasing-டிரான்சிஸ்ைர் சார்பு
The proper flow of zero signal collector current (Ic) and the
maintenance of proper collector emitter voltage (VCE) during
the passage of signal is known as Transistor Biasing.
The circuit which provides transistor biasing is called
as Biasing Circuit.
If a signal is of very small voltage is given to the input of
transistor, it cannot be amplified.
To amplify a signal, two conditions have to be met.
The input voltage should exceed cut-in voltage for the
transistor to be ON.
47
Transistor should be in the active region, to be operated as
an amplifier.
If appropriate DC voltages and currents are given by external
sources, so that BJT (Bipolar Junction Transistor) operates in
active region
Superimposing the AC signals to be amplified will not create
problems
The given DC voltage and currents are so chosen that the
transistor remains in active region for entire input AC cycle
Hence DC biasing is needed.
48
STABILIZATION--நிறைப்படுத்தல்
For a transistor to be operated as a faithful amplifier, the operating
point should be stabilized
Factors affecting the operating point
The main factor that affect the operating point is the temperature
and parameters of transistor ( β= Ic/IB, VBE )
IC=βIB+ICEO
=βIB+(β+1)ICBO
As temperature increases, the values of IC, β, VBE gets affected.
•ICBO gets doubled (for every 10o rise)
•VBE decreases by 2.5mv (for every 1o rise)
Operating point should be made independent of the temperature
To achieve this, biasing circuits are introduced.
49
Stabilization
The process of making the operating point independent of
temperature changes or variations in transistor parameters is
known as Stabilization
Once the stabilization is achieved, the values of IC and
VCE become independent of temperature variations or
replacement of transistor.
A good biasing circuit helps in the stabilization of operating
point.
50
Need for Stabilization
Stabilization of the operating point has to be achieved due to
the following reasons.
•Temperature dependence of IC
•Individual variations
Thermal runaway
Individual Variations
As the value of β and the value of VBE are not same for every
transistor, whenever a transistor is replaced---change operating
point. (IC=βIB+ICEO=βIB+(β+1)ICBO)
51
Temperature Dependence of IC
As the expression for collector current IC is
IC=βIB+ICEO
=βIB+(β+1)ICBO
The collector leakage current ICBO is greatly influenced by
temperature variations
The biasing conditions are set so that zero signal collector current
IC = 1 mA.
The operating point needs to be stabilized i.e. it is necessary to
keep IC constant.
52
Thermal Runaway - பவப்ப
ஓட்ைம்
As the expression for collector current IC is
IC=βIB+ICEO
=βIB+(β+1)ICBO
The flow of collector current and also the collector leakage
current causes heat dissipation. If the operating point is not
stabilized, there occurs a cumulative effect which increases this
heat dissipation.
The self-destruction of such an unstabilized transistor is known
as Thermal run away.
In order to avoid thermal runaway and the destruction of
transistor, it is necessary to stabilize the operating point, i.e., to
keep IC constant.
53
Stability Factor- நிறைப்படுத்தல் காரணி
It is understood that IC should be kept constant inspite of variations of
ICBO or ICO.
The extent to which a biasing circuit is successful in maintaining operating
point constant is measured by Stability factor. It denoted by S.
By definition, the rate of change of collector current IC with respect to the
collector leakage current ICO at constant β and IB is called Stability factor.
S=dIC / dICO at constant IB and β
Hence we can understand that any change in collector leakage current (ICO)
changes the collector current (Ic) to a great extent.
The stability factor should be as low as possible so that the collector current
doesn’t get affected.
S=1 is the ideal value.
54
The general expression of stability factor for a CE configuration:
IC= [α/(1−α)]IB +ICEO
IC=βIB +[1/(1−α)]ICBO
IC=βIB+(β+1) ICO
ICBO ~ ICO
Differentiating above expression with respect to IC, we get
.
Hence the stability factor S depends
on β, IB and IC
55
Types of transistor biasing
The biasing in transistor circuits is done by using two DC sources VBB and VCC.
It is economical to minimize the DC source to one supply instead of two which
also makes the circuit simple.
The commonly used methods of transistor biasing are
•Base Resistor method
•Collector to Base bias
•Biasing with Collector feedback resistor
•Voltage-divider bias
All of these methods have the same basic principle of obtaining the required
value of IB and IC from VCC in the zero signal conditions.
56
Base Resistor bias Method
அடிவாய் மின்தறை சார்பு முறை
❖In this method, a resistor RB of high resistance is connected to the base
❖The required zero signal base current is provided by VCC which flows through RB.
❖The base emitter junction is forward biased
57
The required value of zero signal base current and hence the collector current (as IC = βIB)
can be made to flow by selecting the proper value of base resistor RB.
Hence the value of RB is to be known.
Let IC be the required zero signal collector current.
Therefore,
β =Ic / IB
IB=IC / β
Considering the closed circuit from VCC, base, emitter and ground, while applying the
Kirchhoff’s voltage law, we get,
VCC=IBRB+VBE
IBRB=VCC−VBE
58
IBRB=VCC−VBE
Therefore
RB= (VCC−VBE) / IB
Since VBE is generally quite small as compared to VCC it can be neglected
Then,
RB=VCC /I B
VCC is a fixed known quantity and IB is chosen at some suitable value
As RB can be found directly, this method is called as fixed bias method
Hence, this method is rarely employed.
59
Stability factor
S=
 +1
 dIB 
1−  

 dIc 
In fixed-bias method of biasing, IB is independent of IC so that,
 dIB 

=0
 dIc 
Substituting the above value in the previous equation,
Stability factor, S=β+1
Thus the stability factor in a fixed bias is (β+1) which means that
IC changes (β+1) times as ICO.
60
Advantages
1.
2.
3.
4.
The circuit is simple.
Only one resistor RB is required.
Biasing conditions are set easily.
No loading effect as no resistor is present at baseemitter junction.
Disadvantages
1.
2.
The stabilization is poor as heat development
can’t be stopped.
The stability factor is very high. So, there are
strong chances of thermal run away.
61
மின்
Voltage Divider Bias Method
ழுத்தப் பகுப்பான் சார்பு முறை
Among all the methods of providing biasing and stabilization,
the voltage divider bias method is the most prominent one.
Here, two resistors R1 and R2 are employed, which are connected to
VCC and provide biasing.
The resistor RE employed in the emitter provides stabilization.
The name voltage divider comes from the voltage divider formed by
R1 and R2.
The figure below shows the circuit of voltage divider bias method.
62
➢The
voltage
drop
across
R2 forward biases the base-emitter
junction
➢This causes the base current and
hence collector current flow in the
zero signal conditions.
➢Suppose that the current flowing
through resistance R1 is I1.
➢As base current IB is very small,
therefore, it can be assumed with
reasonable accuracy that current
flowing through R2 is also I1.
63
To derive the expressions for collector current and collector
voltage
Collector Current, IC
From the circuit, it is evident that,
I1=VCC/(R1+R2)
Therefore, the voltage across resistance R2 is
V2=(VCC/(R1+R2)R2
64
Applying Kirchhoff’s voltage law to the base circuit,
V2=VBE+VE
V2=VBE+REIE
IE=(V2−VBE ) / RE
Since IE ≈ IC,
IC=(V2−VBE ) / RE
From the above expression, it is evident that IC doesn’t depend
upon β.
VBE is very small that IC doesn’t get affected by VBE at all.
Thus IC in this circuit is almost independent of transistor
parameters and hence good stabilization is achieved.
65
Collector-Emitter Voltage, VCE
Applying Kirchhoff’s voltage law to the collector side,
VCC=ICRC+VCE+IERE
Since IE ≅ IC
VCC=ICRC+VCE+ICRE
=IC(RC+RE)+VCE
Therefore,
VCE=VCC−IC(RC+RE)
RE provides excellent stabilization in this circuit.
V2=VBE+IERE
66
V2=VBE+ICRE
V2= R2VCC/(R1+R2)
Suppose there is a rise in temperature, then the collector
current IC increases
This causes the voltage drop across RE to increase.
As the voltage drop across R2 is V2, which is independent of
IC, the value of VBE decreases.
The reduced value of IB tends to restore IC to the original
value.
67
Stability Factor
To get the equation for Stability factor of
this circuit draw the equivalent circuit
using thevenin theorem
RT=R1R2/(R1+R2)
68
Apply Kirchoff’s law to the B-E circuit
IBRT + VBE + IERE = V2
IBRT + VBE + (IC + IB)RE − V2 = 0
IB(RT + RE) + VBE + ICRE − V2 = 0
IB = − VBE − ICRE + V2
RT + RE
dIB 0 − RE − 0
=
dIc
RT + RE
β +1
S=
RE
1 − β()
RT + RE
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β +1
RE
1 + β(
)
RT + RE
(β + 1)(RT + RE)
S=
RT + RE + βRE
(RT + RE)
(β + 1)
RE
S=
(RT + RE + βRE)/RE
S=
 RT

(β + 1)
+ 1
 RE

S=
RT
(
+ 1) + β
RE
(β + 1)
S=
=1
1+ β
If the ratio RT/RE is very small
RT/RE can be neglected as
compared to 1
Stability factor becomes
S=(β+1)×1/(β+1)=1
This is the smallest possible
value of S and leads to the
maximum possible thermal
stability.
70