Transistors and Transistor Biasing
1 Transistor - 羿ரான்殿ஸ்ட쏍
❖ 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) உமி폍ப்பான்- emits (supplies) charges - always forward biased
Collector(C) ஏ쟍பான்- collects charges- always reverse biased
Base(B) அ羿வாய் -Middle sections which forms 2PN junctions- forward biased
3 Doping and Size
Emitter is heavily Collector (C) is Base is lightly doped doped (inject large moderately doped no. of electrons) • Base is thin • Emitter is moderate • Collector (C) is size 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 -믁ன்ன ா埍கு சார்ꯁ
❖ Collector diode is always Reversed Biased -பின்ன ா埍கு母 சார்ꯁ
❖ EB junction is Forward Biased (FB)- low resistance- குறைந்த மின்தறை
❖ CB junction is Reversed Biased (RB) - High resistance- அதிக 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 - 믁ன்ன ா埍கு சார்ꯁ –VEB
CB - reversed biased - பின்ன ா埍கு母 சார்ꯁ–VCB
VEB < VCB
EB junction - EB சந்தி (heavily doped) - ejects more electrons
Majority charge carriers பப쏁ம்பான்றம 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 ீ翁 2 terminal- output பவளியீ翁 One battery –needed to give biasing
Transistor -3 terminal device 1 terminal – input 2 terminal - output 3 terminal - common (பபா鏁) 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 - பபா鏁 அ羿வாய் அறமப்ꯁ
Base --Common terminal
❖ E and B Forward Biased
❖ C is Reversed Biased
10 Common base configuration
11 Common Emitter configuration - பபா鏁 உமிழ்ப்பான் அறமப்ꯁ
❖ E and B - Forward Biased
❖ C is Reversed Biased 12 Common Emitter configuration
13 Common Collector configuration - பபா鏁 ஏற்பான் அறமப்ꯁ
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 ா翍ைபப쏁埍ககாரணி) (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 ா翍ைத்திற்கா 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) (மின்鏁கள்-鏁றள ம쟁னசர்埍றக).
•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 (சி쟁பான்றம கைத்திகள்)
This is the leakage current கசிퟁ மின்ன ா翍ைம் - Ileakage
• This is due to minority charge carriers and hence very small
•Total collector current ( )= IC=αIE+Ileakage ஏற்பான்மின்ன ா翍ைம் 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 ா翍ைத்திற்கா 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 ா翍ைபப쏁埍ககாரணி) (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 埍கா பதாைர்ꯁ)
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 ா翍ைத்திற்கா 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 பபா鏁 உமிழ்ப்பான் - 羿ரான்சிைர் சிைப்பியல்
29 Input Characteristic Curve - உள்ள ீ翁 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 ீ翁 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翁 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 The output resistance of CE is less than CB 32 Output Characteristic Curve - பவளியீ翁 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 毁ற்ைில் பரிமாற்쟁 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 - ப쿁埍னகா翁 •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 - ப쿁埍னகா翁
•Maximum possible collector current (IC) is a point on the Y-axis - Saturation point (பதவி翍翁 ꯁள்ளி) (A)
•The maximum possible collector emitter voltage VCE is a point on the X-axis- Cutoff point (பவ翍翁 ꯁள்ளி)(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 (பதவி翍翁 ꯁள்ளி) and cut off (பவ翍翁 ꯁள்ளி) points
• 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 - பசயல்பா翍翁 ꯁள்ளி 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-羿ரான்சிைர் சார்ꯁ 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--நிறைப்ப翁த்தல் 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. o •ICBO gets doubled (for every 10 rise) o •VBE decreases by 2.5mv (for every 1 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 ஓ翍ைம்
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- நிறைப்ப翁த்தல் 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 Stabilityfactor.
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
I ICO IC=βIB+(β+1) ICO CBO ~
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 doneby 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 அ羿வாய் மின்தறை சார்ꯁ 믁றை
❖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
+1 S = 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. The circuit is simple.
2. Only one resistor RB is required. 3. Biasing conditions are set easily. 4. No loading effect as no resistor is present at base- emitter junction.
Disadvantages 1. The stabilization is poor as heat development can’t be stopped. 2. The stability factor is very high. So, there are strong chances of thermal run away.
61 Voltage Divider Bias Method மின் 폁த்தப் பகுப்பான் சார்ꯁ 믁றை 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
−VBE−ICRE+V2 IB = RT + RE dIB 0 − RE − 0 = dIc RT + RE β +1 S = RE 1−β(- ) RT + RE
69 β +1 S = If the ratio RT/RE is very small RE 1+ β( ) RT + RE RT/RE can be neglected as (β +1)(RT + RE) S = compared to 1 RT + RE + βRE
(RT + RE) (β +1) Stability factor becomes S = RE S=(β+1)×1/(β+1)=1 (RT + RE + βRE)/RE
RT (β +1) +1 This is the smallest possible RE S = value of S and leads to the RT ( +1) + β maximum possible thermal RE stability. (β +1) S = =1 1+ β
70