EDCbijuntiiontransitorlecturesnist1.pptx

Contents
• BJT - Characteristics and Biasing
• Fundamentals of BJT& Operation
• Minority carrier profiles
• I/P and O/P Characteristics CB configurations.
• I/P and O/P Characteristics CE configurations.
• I/P and O/P Characteristics CC configurations.
• Switching characteristics & Problems
• Biasing Methods & Stabilization.
• Fixed Bias, Collector to Base Bias, Voltage Divider Bias in
BJTs & Problems
• Thermal runway.

BJT-Introduction
• Bipolar Junction Transistor is an active device .
• It works as a current-controlled current source.
• It’s basic action is control of current at one terminal by
controlling current applied at other terminal.
• Current conduction is due to two types of charge carriers i.e
due to both Holes and Electrons. (hence the name Bipolar)
• It consists of Input junction and Output junction. It is a Bipolar
Bi junction transistor.
• Transistor means Transfer Resistor.
• Signals transfer from Low resistance path to High Resistance
path (Transfer resistor ).

BJT
• Transistor consists of three terminals they are
Emitter, Base, Collector.
• Emitter is a heavily doped region,
• Base is lightly doped region and
• Collector is moderately doped region.
• Transistors are divided into two types - They are
PNP and NPN transistors.
• Arrow placed on the emitter of the transistor indicates the
direction of current flow.
• The direction of current flow is always from ‘p’region to ‘n’
region.

NPN and PNP Transistor Symbols

BJT
• Transistors can act as either an insulator or a conductor by the
application of a small signal voltage.
• The transistor's ability to change between these two states
enables it to have two basic functions:
"switching" (digital electronics) or
"amplification" (analog electronics).
• It is used as amplifier and oscillator circuits, and as a switch in
digital circuits.
• It has wide applications in computers, satellites and other
modern communication systems.

• The principle of operation of the two transistor types PNP and
NPN, is exactly the same the only difference being in their
biasing and the polarity of the power supply for each type

BJT – Regions of Operation
• Bipolar transistors have the ability to operate within three
different regions:
1. Active Region - the transistor operates as an amplifier and Ic
= β.Ib
2. Saturation Region - the transistor is "fully-ON" operating as a
switch and Ic = I(saturation)
3. Cut-off Region - the transistor is "fully-OFF" operating as a
switch and Ic = 0

•In CB configuration,
Active Region -
VB > VE & VC > VB
Saturation Region –
VB > VE & VB > VC
Cut-off Region –
VE > VB & VC > VB

Need of Configurations
• In order to measure input and output voltages transistor has to
work like a Two-port network.
• Port means two terminals. For a two-port network we require
four terminals, but transistor has three terminals.
• By making one of the transistor terminal as common, three
different configurations are made.
• They are
1. Common Base configuration
2. Common Emitter configuration
3. Common Collector configuration.

Types of Configuration
Note:
1. Base is always input terminal except in Common Base configuration.
2. Collector is always output terminal except in Common Collector
configuration.
Name of the
configuration
Common
Terminal
Input
Terminal
Output
Terminal
Common
Emitter
Emitter Base Collector
Common
Base
Base Emitter Collector
Common
Collector
Collector Base Emitter

Common Base, Common Emitter &
Common Collector Configurations
CB CE CC
E

Transistor Construction
• In an NPN transistor, a layer of P-type material is sandwiched
between two layers of N-type material.
• The BJT consists of a silicon (or germanium) crystal in which
a thin layer of N-type silicon is sandwiched between two
layers of P-type silicon which forms PNP transistor.
• The 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. It passes most of the
injected charge carriers from the emitter into the collector.
• The collector is moderately doped.

Transistor Biasing
• Usually the emitter-base junction is forward biased (F.B.) and
the collector-base junction is reverse biased (R.B.).

Transistor working (NPN)
• The forward bias applied to the emitter base junction of an
NPN transistor causes a lot of electrons from the emitter
region to cross over to the base region.
• As the base is lightly doped with P-type impurity, the number
of holes in the base region is very small and, hence, the
number of electrons that combine with holes in the P-type base
region is also very small.
• Hence, a few electrons recombine with holes to constitute a
base current IB.
• The remaining electrons (more than 95%) cross over into the
collector region to constitute a collector current IC.

Transistor working (NPN)
• Thus, the base and collector current summed up gives the
emitter current, i.e., IE = – (IC + IB).
• In the external circuit of the NPN bipolar junction transistor,
the magnitudes of the emitter current IE, the base current IB,
and the collector current IC are related by IE = IC + IB.

Transistor working (PNP)
• The forward bias applied to the emitter-base junction of a PNP
transistor causes a lot of holes from the emitter region to cross
over to the base region as the base is lightly doped with N-type
impurity.
• A few holes combined with electrons to constitute a base
current IB.
• The remaining holes (more than
95%) cross over into the collector
region to constitute a collector
current IC.
• Thus, the collector and base
current when summed up gives the
emitter current, i.e.,
IE = – (IC + IB)

• In an npn transistor current conduction is due to majority
charge carriers i.e electrons.
• Emitter current is a combination of hole current and electron
current.

Current Amplification Factor
• In a transistor amplifier with ac input signal, the ratio of
change in output current to the change in input current is
known as the CurrentAmplification Factor.
• In the CB configuration, the current amplification factor,
𝛼 =
∆𝐼𝐶
∆𝐼𝐸
• In the CE configuration, the current amplification factor,
𝛽 =
∆𝐼𝐶
∆𝐼𝐵
• In the CC configuration, the current amplification factor,
𝛾 =
∆𝐼𝐸
∆𝐼𝐵

Transistor Current Components
• The emitter current consists of two constituents hole current
and electron current. IE = IpE + InE
– Hole current is due to the crossing of holes from emitter (p side) into
base region (n side).
– Electron current is due to the crossing of electrons from base (n side)
into emitter region (p side).
• In transistor emitter is heavily doped as compared to base.
Thus, electron current is negligible as compared to hole
current.
• Thus, emitter current in PNP transistor is mainly due to the
flow of holes from emitter to base.
• All the holes crossing the emitter junction do not reach the
collector junction because some of them combine with
electrons in N type base.

• A few holes (1 to 5%) combined with electrons in the n-type
base.
• Only 95 to 99% of the holes reach the collector region.
• If IpC is the hole current at collector junction then the bulk
recombination current leaving the base lead is IB = IpE - IpC
• Since collector junction is reverse biased, which acts as
reverse biased PN junction diode, a reverse saturation current
ICO flows across this junction.
• In PNP, ICO consists of holes crossing from base to collector
region(IpCO) and electrons crossing from collector to base
region(InCO) in opposite direction.
• Thus, In PNP,
ICO = IpCO + InCO

• Large signal current gain 𝑎 [Common Base]
• It is defined as the ratio of the collector-current increment to
the emitter-current change from cut-off (IE = 0) to IE, i.e.
𝛼 =
𝐼𝐶−𝐼𝐶𝐵𝑂
𝐼𝐸 − 0
• where ICBO (or ICO) is the reverse saturation current flowing through the
reverse biased collector-base junction, i.e. the collector to base leakage
current with emitter open.
also, ICBO <<< IC
𝐼𝐶
𝐼𝐸
• So, 𝛼 =
• Typical value of 𝛼 ranges from 0.96 to 0.995.
• Also, 𝛼 is not a constant but varies with emitter current IE, collector voltage
VCB and temperature.
• Note: α is always positive

• In the active region of the transistor, the emitter is forward
biased and the collector is reverse biased.
• The generalized expression for collector current IC for
collector junction voltage VC and emitter current IE is given by
• If VC is negative and |VC | is very large compared with VT ,
then the above equation reduces to
IC = – 𝛼 IE + ICBO
• If VC, i.e. VCB, is few volts, IC is independent of VC. Hence the
collector current IC is determined only by the fraction 𝛼 of the
current IE flowing in the emitter.
• Since IC and IE are flowing in opposite directions,
IE = – (IC + IB)

• In Common-Emitter (CE) transistor circuit,
• If the base circuit is open, i.e., IB = 0, then a small collector
current flows from the collector to emitter. This is denoted as
as ICEO (collector-to-emitter leakage current).
• In Common-Emitter (CE) transistor circuit,
– Emitter-Base junction is forward-biased
– Collector-Base junction is reverse-biased
• The Collector current, IC = IE + ICEO
IE = IC - ICEO

• Hence, the large-signal current gain (𝖰) is defined as
𝛽 =
∆IC
∆𝐼𝐵
𝛽 =
IC − 𝐼𝐶𝐸𝑂
𝐼𝐵
IC = 𝛽 IB + ICEO
• Relation between IC , IB & ICEO is given as above

• Relation between ICBO & ICEO
• Relationship between the leakage currents of transistor
common-base (CB) and common-emitter (CE) configurations
as
ICEO = (1 +𝛽 ) ICBO
• As ICBO is temperature-dependent, ICEO varies by large amount
when temperature of the junctions changes.
• Expression for emitter-current,
IE = IC + IB
IE = (1 +𝛽 ) ICBO + (1 +𝛽 ) IB
IE 1−𝛼 1−𝛼
=
1
I +
1
I
CBO B

Transistor Configuration
• Common base (CB) configuration,
• Common emitter (CE) configuration,
• Common collector (CC) configuration.

Transistor characteristics
• Input characteristics
• Input characteristics are those
curves which are drawn between
Input Voltage vs Input Current by
keeping output voltage as constant
• Output characteristics
• Output characteristics are those
curves which are drawn between
Output Voltage vs Output Current
by keeping input current as constant

Transistor characteristics
Input characteristics Output characteristics
Name of
the
configur
ation
Input
Voltage
X-axis
Input
Current
Y-axis
Output
voltage
Constant
Output
Voltage
X-axis
Output
Current
Y-axis
Input
Current
Constant
CE VBE IB VCE VCE IC IB
CB VEB IE VCB VBC IC IE
CC
VBC IB VEC VCE IE IB

CB configuration
• Base is common ,
• Emitter is input terminal and
• Collector is output terminal.
• Emitter base junction (I/P) Junction is forward biased.
• Collector base Junction (O/P) junction is reverse biased.

CB - Input characteristics
• Input characteristics:
• Input characteristics represents forward characteristics of
emitter base diode for various collector voltages VCB as
constant.
• The collector base voltage VCB is kept constant at zero volt and
the emitter current IE is increased from zero in suitable equal
steps by increasing VEB.
• This is repeated for higher fixed values of VCB
• A curve is drawn between IE and VEB at constant VCB.

• When VCB = 0 and the emitter base junction is forward biased, the junction
behaves as a forward biased diode so that emitter current IE increases
rapidly with small increase in VEB.
• When VCB is increased keeping VEB constant,the width of the base region
will decrease.
• This effect results in an increase of IE. (as Rate of recombination ↓ & IB is
reduced)
• Therefore, the curves shift towards the left as VCB is increased.
• This curve shows the
relationship between
input voltage (VBE) to
input current (IE) for
various levels of output
voltage (VCB).

• Input Resistance:
• Input Resistance is the reciprocal of the slope of the
characteristic curve.
• Input Resistance is Low in Common Base configuration.
𝑅𝑖 =
∆𝑉𝐵𝐸
∆𝐼𝐸
𝑂ℎ𝑚𝑠

Early effect or Base-width modulation
• As the collector voltage VCC is made to increase the reverse
bias, the space charge width between collector and base tends
to increase, with the result that the effective width of the base
decreases.
• This dependency of base-width on collector-to-emitter voltage
is known as the Early effect.
• As, VCC ↑
• Depletion region ↑
• Base width ↓
• Rate of recombination ↓
• IE ↑

Effects of Base Width Modulation
• Consequences:
1. There is less chance of recombination in the base region.
Hence Current Gain(α) increases.
2. Current due to injected minority carriers at Junction increases.
3. At extremely large collector voltages effective Base width may
be related to zero causing voltage break down in transistor
known as “Punch through”.
4. As Base width reduces, recombination also decreases, as a
result Base current (IB) also decreases.

CB - Output characteristics
• To determine the output characteristics, the emitter current IE
is kept constant at a suitable value by adjusting the emitter-
base voltage VEB.
• Then VCB is increased in suitable equal steps and the collector
current IC is noted for each value of IE. This is repeated for
different fixed values of IE.
• The curves of VCB vs IC are plotted for constant values of IE.

• From the characteristics, it is seen that for a constant value of
IE, IC is independent of VCB and the curves are parallel to the
axis of VCB.
• and IC ≅ 𝐼𝐸
• Also, IC flows even when VCB is equal to zero.
output voltage (VCB) to
output current (IC) for
various levels of input
current (IE).

• As the emitter-base junction is forward biased, the majority
carriers, i.e. electrons, from the emitter are injected into the
base region.
• Due to the action of the internal potential barrier at the reverse
biased collector-base junction, they flow to the collector region
and give rise to IC even when VCB is equal to zero.
• Output Resistance:
• Reciprocal of the slope of the characteristics gives the output
resistance.
• Output Resistance is Very High. Typical Value is around
500Kilo ohms.

Output characteristics
• There are three different regions in transistor Output
characteristics.
1. Saturation Region
2. Active Region
3. Break Down Region.

• Saturation Region: In this region both I/P and O/P Junctions
are forward biased.
• A small Change in VCB results a large change in IC.
• Active region: In active region I/P Junction is forward biased
and O/P is reverse biased.
• In this region collector current IC is independent of collector
voltage VCB and Depends only on emitter current.
• Cut-off Region: In this both I/P & O/P Junctions are reverse
biased.
• The Region below IE = 0 is cutoff region.

CB- Conclusions
1. Input Resistance of CB configuration is very Low.
2. Output Resistance of CB Configuration is very High.
3. Current Gain is less than unity.
4. It is used for high frequency circuits.
• In CB configuration current gain varies between 0.9 to 0.985(α).

CE - Configuration
• Emitter is common,
• Base is input terminal and
• Collector is output terminal.

CE – Input Characteristics
• Input Characteristics:
• The collector-to-emitter voltage is kept constant at zero volt,
and the base current IB is increased from zero in equal steps by
increasing VBE.
• The value of VBE is noted for each setting of IB.
• This procedure is repeated for higher fixed values of VCE,
input voltage (VBE) to
input current (IB) for
voltage (VCE).

• When VCE = 0, the emitter-base junction is forward biased and
behaves as a forward biased diode.
• Hence, the input characteristic for VCE = 0 is similar to that of
a forward-biased diode.
• When VCE is increased, the width of the depletion region at
the reverse-biased collector-base junction will increase. Hence,
the effective width of the base will decrease.
• This effect causes a decrease in the base current IB.
• Hence, to get the same value of IB as that for VCE = 0, VBE
should be increased.
• Therefore, the curve shifts to the right as VCE increases.

• Reciprocal of the slope of the curve is Input Resistance and it
is medium.
• Input Resistance is about 750 Ohms.
• Rin=∆ VBE / ∆ IB .

CE – Output Characteristics
• Output Characteristics:
• The base current IB is kept constant at a suitable value by
adjusting the base-emitter voltage, VBE.
• The magnitude of the collector-emitter voltage VCE is
increased in suitable equal steps from zero and the collector
current IC is noted for each setting VCE.
• This is repeated for different fixed values of IB.
output voltage (VCE) to
output current (IC) for
current (IB).

VCE = VCB + VBE
• When IB is kept constant,
• As VCE increases, VCB also increases,
• We can write
• As VCB increases, depletion region of Base Collector junction
increases, effective Base width decreases, IB decreases.
• As VCE increases, IC increases
• We have IC = β IB
• IC not only depends on IB but also VCE

• we have,
• For larger values of VCE, due to early effect, a very small
change in α is reflected in a very large change in β.
1−0.98
• For example, when α = 0.98, β =
0.98
= 49.
• If α increases to 0.985, then β =
0.985
= 66.
1−0.985
• Here, a slight increase in α by about 0.5% results in an
increase in β by about 34%.
• Hence, the output characteristics of CE configuration show a
larger slope when compared with CB configuration.

• In active region of CE configuration transistor will work as an
Amplifier.
• Here Input junction is forward biased and Collector Base
junction is Reverse biased.
• Transistor output current IC responds to the input current IB.
• IC varies with changes in VCE when IB is kept constant.
• If VCE is increased continuously then depletion region in CB
junction increases.
• It increases IC Rapidly and operates the transistor in the active
Region.

• In Cut-off region of CE configuration both Emitter Base junction
and Collector Base junction are reverse biased.
• It is the region below the curve IB = 0.
• Current is due to minority charge carriers, called as Reverse
saturation current.
• The transistor is virtually an open circuit between collector and
emitter.
• In Saturation region of CE configuration both junctions are
forward biased.
• Current is maximum.
• An increase in the base current IB does not cause a corresponding
large change in IC.
• In saturation region, current IC = VCC / RL.

• Output Resistance: Rout=∆ VCE / ∆ IC .
• Reciprocal of the slope of the curve is output Resistance and it is
medium.
• Output Resistance is about 45 Kilo Ohms.
• Conclusion:
• It has medium Input Resistance and medium Output Resistance.
Hence CE configuration is preferred forAmplification purpose.
• Application: CE configuration is used forAmplification
purpose.

CC - Configuration
• Collector is common,
• Base is input terminal and
• Emitter is output terminal.

CC – Input Characteristics
• Input Characteristics: To determine the input characteristics,
VEC is kept at a suitable fixed value.
• The base-collector voltage VBC is increased in equal steps and
the corresponding increase in IB is noted.
• This is repeated for different fixed values of VEC.
input voltage (VBC) to
input current (IB) for
voltage (VEC).

• In Common Collector, Input voltage VCB is largely determined
by output voltage VCE.
VCE = VCB + VBE
VBE = VCE - VCB
• Since
• We can write,
• When VCB increases, at constant VCE,
• VBE decreases and hence IB decreases.

• Input Resistance: Rin=∆ VCB / ∆ IB .
• Reciprocal of the slope of the curve is input resistance and it is
very high about 750 Kilo ohms.

CC – Output Characteristics
• Output Characteristics:
• The base current IB is kept constant at a suitable value by
adjusting the base-collector voltage, VCE.
• The magnitude of the collector-emitter voltage VCE is
increased in suitable equal steps from zero and the collector
current IE is noted for each setting VEC.
• This is repeated for different fixed values of IB.
output voltage (VEC) to
output current (IE) for
current (IB).

• For all practical purposes, the output characteristics of the CC
configuration are the same as for the CE configuration.
• The input current, IB, is the same for both the CE and CC
characteristics.
• IC of the CE characteristics is replaced by IE for the CC
characteristics (since α ≈ 1).
• Output Resistance: Rout=∆ VCE / ∆ IE
• Reciprocal of the slope of the curve is output Resistance
• Output Resistance is very low of the order of 25 ohms.

• Application : It can be used as a buffer amplifier.
• Also called Emitter-Follower (EF).
• The CC configuration is used primarily for impedance-
matching purposes since it has a high input impedance and low
output impedance, opposite to that of the CB and CE
configurations.

Comparison of CB,CE and CC Configurations
Property
Common Base
configuration
Common Emitter
configuration
Common Collector
configuration
Input
Resistance
Low (Around
100Ω)
Medium
(Around 750 Ω)
High (750
kilo Ω )
Output
Resistance
High Medium Very low
Current Gain <1 High High
Voltage Gain Around 150 Around 500 <1
Phase shift
between input
and output
No phase shift 1800
00 or 3600
No phase
shift
Applications
For high
frequency
circuits
For Audio
amplification
For impedance
matching

CE – 180o Phase shift
• In a Common Emitter Amplifier, when base voltage increase,
base current increases.
• It also causes an increase in the collector current which in turn
causes a voltage drop in the collector resistor.
• Because the output is situated below the collector resistance
(with reference to VCC), the output voltage will decrease as
voltage drop across collector resistor increase.
• Thus it produces a 180 phase shift. (input positive, output
negative and vice versa).
• When you apply Kirchoff's law in CE side, we get
VCC=ICRC+Vout
• Here, When IC increases, Vout must decrease because VCC is
constant.

Relation between α& β
∆ IC = 𝛼 ∆ IE
∆𝐼𝐶 ∆𝐼𝐶
• We know that ∆ IE = ∆ IC + ∆ IB
• Since 𝛼 =
∆𝐼𝐶
=>
∆𝐼𝐸
• ∆ IE = 𝛼 ∆ IE+ ∆ IB
• ∆ IB = ∆ IE (1 − 𝛼 )
• Dividing both sides by ∆IC, we get
• ∆𝐼𝐵
=
∆𝐼𝐸
(1 − 𝛼 )
• Therefore,
1
=
𝛽 𝛼
1
(1 − 𝛼)
• β =
𝛼
(1−𝛼)
α =
𝛽
(1+𝛽 ) 𝛼
𝛽
or
1
−
1
= 1
•
• The CE configuration is used for almost all transistor applications because
of its high current gain, β.
From this relationship, it is clear that as 𝛼
approaches unity, β approaches infinity.

Relation between α
, β& γ
• In CC amplifier circuit, γ =
• ∆ IB = ∆ IE - ∆ IC
• γ =
∆𝐼𝐸
∆𝐼𝐵
∆𝐼𝐸
∆ IE − ∆ IC
• γ =
1
1 − ∆𝐼𝐶
∆𝐼𝐸
=
1
1−𝛼
• Therefore, γ =
1
= (𝛽 + 1)
1−𝛼
• Also, α < 𝛽 < γ

Need for Biasing?
• In order to produce distortion-free output in amplifier circuits,
the supply voltages and resistances in the circuit must be
suitably chosen.
• The voltages and currents that are required to operate the
transistor in the active region are called Quiescent values
which determine the operating point or Q-point for the
transistor.
• The process of giving proper supply voltages and resistances
for obtaining the desired Q-point is called Biasing.
• The circuits used for getting the desired and proper operating
point are known as Biasing circuits.

Need for Biasing?
• Biasing is needed in the transistors due to the following reasons.
 To achieve faithful amplification &
 To keep operating point as stable.
• Toachieve faithful amplification Input junction must be properly Forward
biased and Output junction must be Reverse biased.

Biasing
• There are three types of Biasing circuits.
 Fixed bias or Base Bias
 Collector to Base Bias or Biasing with Feed back resistor.
 Self bias or Voltage Divider Bias or Universal Bias.

DC - Load Line
• DC Load line is a line on the output characteristics of a transistor
• It gives the values of VCE, IC corresponding to Quiescent conditions. (Zero
signal Conditions).
Circuit arrangement
DC Load Line

DC - Load Line
VCC = VCE + IC RC
VCE = VCC – IC RC
• We have,
• Load line can be plotted in the following manner.
• Case-I
• If IC = 0, then VCE max = VCC
• This is point B on load line on X axis. (VCC ,0 )
𝑅𝐶
• Case-II
• If VCE = 0 ; ICmax = VCC/RC is point Aon the load line
• and the coordinates are (0,
𝑉𝐶𝐶
)

DC - Load Line
• Thus, the d.c. load line AB can be drawn if the values of RC and VCC are
known.
• The optimum Q-point is located at the midpoint of the d.c. load lineAB
between the saturation and cut-off regions, i.e. Q is exactly midway
betweenAand B.
• In order to get faithful amplification, the Q-point must be well within the
active region of the transistor.
• It is very important to ensure that the operating point remains stable where
it is originally fixed.
• If the Q-point shifts nearer to eitherAor B, the outputvoltage and current
get clipped, thereby output signal is distorted.

DC - Load Line
• Collector current changes due to ICO (Reverse saturation current),
β and VBE.
• In practice, the Q-point tends to shift its position due to any or all of the
following three main factors:
(i)Reverse saturation current, ICO, which doubles for every 10°C increase in
temperature.
(ii) Base-emitter voltage, VBE, decreases by 2.5 mV per °C.
(iii) Transistor current gain, β, i.e., hFE which increases with temperature.

Stability Factor
• To keep operating point as stable, IC must be stable.
• Collector current IC changes due to Ico, β, and VBE.
• There are three types of stability factors.
• S, SV, Sβ
• Maintenance of the operating point is specified by S, which indicates the
degree of change in operating point due to change in temperature.
• Stability factor (S):
• The rate of change of collector current IC with respect to the
collector–base leakage current ICO, keeping both the current IB
and the current gain β constant

Stability Factor
• Stability factor (S’) (SV)
• During calculation of SV, change in Ico and change in β are
kept constant.
• Stability factor (S’ ) (Sβ)
• During calculation of Sβ, change in Ico and change in VBE are
kept constant.

Stability Factor
• The collector current for a CE amplifier is given by
• IC = 𝛽 IB + (𝛽 +1) ICO
• Differentiating the above equation with respect to IC, we get
𝜕𝐼𝐵
depends on the
𝜕𝐼𝐶
type of biasing
• From this equation, it is clear that this factor S should be as
small as possible to have better thermal stability.

Fixed bias or Base bias
• Figure shows a Fixed bias circuit.
• Here input junction is forward
biased and output Junction is
Reverse biased through Base
Resistor
• VCC , VBE , RB all are fixed values.
• By writing KVL to the i/p loop:
From the o/p loop:

C
• Since this equation is independent of current I ,
• Stability factor in
𝜕𝐼𝐵
= 0
𝜕𝐼𝐶
reduces to
• S = 1+β (Fixed Bias)
• If β = 100, S = 1+100
• Change in collector current is 101 times of reverse saturation
current. Which is undesirable.
• Since β is a large quantity, this is a very poor bias stable circuit.
• Therefore, in practice, this circuit is not used for biasing.

• In this method β, VCC are fixed values, hence it is known as
fixed bias method.
• Calculation of Base Resistor in fixed bias:

• The advantage of this method are:
– (a) simplicity,
– (b) small number of components required, and
– (c) if the supply voltage is very large as compared to VBE
of the transistor, then the base current becomes largely
independent of the voltage VBE.
• Disadvantage :
– There is no Stability.
– There is no protection for the Transistor against
temperature variations.

Collector to Base Bias
• Collector to Base Bias is shown in the
circuit with common emitter mode.
• Input junction is forward biased and output
junction is reverse biased through feed
back resistor RB.
• Feed back is through RB . Output collector
is connected to Base (input).
• This circuit provides stability against
temperature variations.

• From I/P loop,
VCC = (IC + IB) RC + IBRB + VBE
𝐼𝐵
=
VCC – VBE – ICRC
RC + RB
• Differentiating w.r.t IC
• This value of the stability factor is smaller
than the value obtained by fixed bias
circuit.

• S can be made small and the stability can
be improved by making RB small or RC
large.
• If RC is very small, then S = (β + 1), i.e.
stability is very poor.
• Hence, the value of RC must be quite large
for good stabilization.

• Advantage: The circuit provides stability against temperature
variations.
• Disadvantage:
• Gain decreases, Output is connected to the input (i.e to the base).
Gain is decreasing which is the Prime requirement of an
amplifier.

Self bias or Voltage Divider Bias or
Universal Bias
• Figure shows Self Bias circuit with
two biasing resistors R1and R2.
• RE is Emitter resistor and RC is
Collector resistor.
• Input junction is Forward biased
and Output junction is Reverse
biased through biasing resistors R1
and R2.
• R1 and R2 forms a potential divider
across VCC.
• To achieve best Stability R1 and R2
must be small.

Universal Bias
• For the transistor to remain in the
active region, the base-emitter
junction has to be forward biased.
• The required base bias is obtained
from the power supply through the
potential divider network of the
resistance R1 and R2.

Universal Bias
Use of Self Bias circuit as constant current circuit
• If IC tends to increase, say, due to
increase in ICO with temperature,
the current in RE increases.
If ICO ↑ , IC ↑ IE ↑
• Hence, the voltage drop across RE
increases thereby decreasing the
base current.
VRE ↑ VR2 ↑ IB ↓
• As a result, IC is maintained almost
constant.
IC maintained Constant

Self bias or Voltage Divider Bias

• Applying Thevenin’s Theorem,
• The loop equation can be written as,
• Differentiating w.r.t IC,
• Stability Factor S,

• The value of S is equal to one if the ratio RB /RE is very small
as compared to 1.
• As this ratio becomes comparable to unity, and beyond
towards infinity, the value of the stability factor goes on
increasing till S = 1 + β.

• Conclusions:
• Stability factor for self bias circuit is equal to one.
• This can be achieved with proper designing.
• Advantages : Good Stability.
• Disadvantage : Output voltage is getting coupled with input
through emitter Resistor.
• There is a possibility for the reduction of Gain.
• To overcome reduction of gain in universal or self bias circuit
emitter bypass capacitor is connected across RE.

• To overcome reduction of gain
in universal or self bias circuit
emitter bypass capacitor is
connected across RE to bypass
ac signals to ground.

Name of the
bias
Advantage Disadvantage
Fixed bias Easy to design Poor stability
Collector to
base bias
Good stability can
be achieved with
proper design
Reduction in Gain.
Voltage divider
Bias
Excellent stability.
Stability factor =1
Good gain
By keeping emitter bypass
capacitor Reduction of gain
can be eliminated

Transistor as a Switch
• Transistor can be used as a Switch
• For obtaining switching characteristics: Saturation and Cutoff
regions are taken into consideration.
• Saturation region is known as ON condition of a switch.
• Cutoff region is known as OFF condition of a switch.

Transistor - Cut-off Region
• Applying KVL to the output loop,
• Case 1:
• Cutoff condition of a transistor is given by,
• When the base of the transistor is given negative, the transistor
goes to cut off state. There is no collector current. Hence IC= 0.
• The voltage VCC applied at the collector, appears across the
collector resistor RC. Therefore,

Cut-off Characteristics
• The input and Base are grounded ( 0v )
• Base-Emitter voltage VBE < 0.7v
• Base-Emitter junction is reverse biased
• Base-Collector junction is reverse
biased
• Transistor is “fully-OFF” ( Cut-off
region )
• No Collector current flows ( IC = 0 )
• VOUT = VCE = VCC = “1”
• Transistor operates as an “open switch”

Transistor - Saturation Region
• Case 2:
• Saturation Region of transistor,
• Base emitter junction is forward
biased.
• When the base voltage is positive
and transistor goes into saturation,
IC flows through RC.
• There exists base current and hence
collector current.
• Then VCC drops across RC. The
output will be zero.

Transistor - Saturation Region
• Here the transistor will be biased so
that the maximum amount of base
current is applied,
• resulting in maximum collector
current,
• resulting in the minimum collector
emitter voltage drop,
• which results in the depletion layer
being as small as possible and
• maximum current flowing through
the transistor.
• Therefore the transistor is switched
“Fully-ON”.

Saturation Characteristics
• The input and Base are connected to VCC
• Base-Emitter voltage VBE > 0.7v
• Base-Emitter junction is forward biased
• Base-Collector junction is forward
biased
• Transistor is “fully-ON” ( saturation
region )
• Max Collector current flows
( IC = VCC/RL )
• VCE = 0 ( ideal saturation )
• VOUT = VCE = ”0”
• Transistor operates as a “closed switch”

Transistor switching timings
• Consider a transistor switch driven by pulse waveform.
• This pulse makes transitions between two voltage levels V2
and V1
• At V2 transistor is at cutoff and at V1 transistor is in saturation.
• Input Vi is applied between Base and Emitter.

• Transistor switching timings are delay time (td), Rise time (tr),
Turn on time (ton), Turn off time (toff), Storage time (ts), Fall
time (tf).
• (ton) = (td)+ (tr)
• (toff)= (ts)+ (tf)

• Let the input pulse duration = T
• When the input pulse is applied the collector current takes
some time to reach the steady state value, due to the stray
capacitances.
• Delay Time (td) − The time taken by the collector current to
reach from its initial value to 10% of its final value after IB has
commenced is called as the Time Delay.
– It is due to mainly the time needed to charge the EB junction depletion
capacitance to the forward-bias voltage VBE.
• Rise Time(tr) − The time taken for the collector current to
reach from 10% of its initial value to 90% of its final value is
called as the Rise Time.
• Turn-on time (TON) − The sum of time delay (td) and rise time
(tr) is called as Turn-on time.
TON = td + tr

• Storage time (ts) − The time interval between the trailing edge
of the input pulse to the 90% of the maximum value of the
output, is called as the Storage time.
– due to the fact that the CB junction is Forward Biased in saturation. –
excess minority charge carriers, stored in the depletion region must be
withdrawn or recombine before begins IC to fall.
• Fall time (tf) − The time taken for the collector current to
reach from 90% of its maximum value to 10% of its initial
value is called as the Fall Time.
• Turn-off time (TOFF) − The sum of storage time (ts) and fall
time (tf) is defined as the Turn-off time.
TOFF = ts + tf

• Three factors contribute to the delay time
1. When the driving signal is applied to the transistor input,
a non zero time required to charge up the emitter junction
transition capacitance so that transistor brought from
cutoff to active.
2. Even when the transistor has been brought to the point
where minority carriers have begun to cross the emitter
junction into the base, a time interval is required before
these carriers can cross the base region to the collector
junction and be recorded as collector current
3. Some time is required for the collector current to rise to
10 percent of its maximum.

• The failure of the transistor to respond to the trailing edge of
the driving pulse results from the fact that a transistor in
saturation has excess minority charge carriers stored in the
base.
• The transistor cannot respond until this excess minority charge
has been removed

Thermal Runaway
• The maximum power dissipation may lie in the range of few
milli watts to 200 watts .
• This maximum power is limited by the collector junction that
it can with stand.
 For Silicon transistor it is in the range of 150 degree centigrade to 225
degree centigrade,
 For Germanium it is between 60 to 100 degree centigrade.
• The Junction temperature may rise either because of the
ambient temperature or because of self heating.
• As the junction temperature increases, ICO increases which will
further increase IC. Again heat dissipates causing the
temperature to increase.
• It is a cumulative process, which will cause the transistor to
burn out.

Thermal Runaway
• The collector current for the CE circuit of is given by
IC = β IB + (1 + β ) ICO.
• The three variables β, IB, and ICO increase with rise in temperature.
• In particular, the reverse saturation current or leakage current ICO
changes greatly with temperature. Specifically, it doubles for every
10 °C rise in temperature.
• The collector current IC causes the collector-base junction
temperature to rise which, in turn, increase ICO, as a result IC will
increase still further, which will further raise the temperature at the
collector-base junction.
• This process will become cumulative leading to Thermal
Runaway.
• Consequently, the ratings of the transistor are exceeded which may
destroy the transistor itself.

Thermal Runaway
• The collector is normally made larger in size than the emitter in
order to help dissipate the heat developed at the collector junction.
• However, if the circuit is designed such that the base current IB is
made to decrease automatically with rise in temperature then the
decrease in β IB will compensate for the increase in (1 + β)ICO,
keeping IC almost constant. [Voltage Divider Biasing]
• In power transistors, the heat developed at the collector junction
may be removed by the use of a heat sink, which is a metal sheet
fitted to the collector and whose surface radiates heat quickly.

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