Electronics and Communication Engineering

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TRANSISTORS 87
BIPOLAR TRANSISTORS
Basically, this name transistor is made up of two terms “transfer-of –resistor”
because its resistance transfers from one value to other value depending on its
bias. In other words the current through transistor is controlled by the bias.
Bipolar means both, holes and electrons take part in carrying the current through
transistor therefore they are called as bipolar transistors.
The transistor also is a P-N device but with two P-N junctions and three
terminals
1) Emitter –that emits electrons
2) Base-which controls this electron flow
3) Collector –that collects these electrons.
There are two basic types of bipolar transistors
i) NPN transistor and ii) PNP transistor. Fig. shows their structure and symbol.
Arrow –out indicates NPN and arrow – in is PNP.
TRANSISTORS

TRANSISTORS 88
Emitter – Base junction is forward biased and Base-Collector junction is
reverse biased. In this method, transistor works as a controlled device. Refer the
fig. it shows this biasing method. Base- Emitter is biased with low voltage batter
 BBV and Base – Collector with high voltage battery CCV . Now observe the battery
connections, negative terminal of the battery  BBV it will repel electrons towards
the Base, they enter in the Base region. Base is P-type material and it is lightly
doped thus few electrons recombine with holes and those number of electrons will
pass into the Base towards positive terminal BBV .
Rests of the electrons are attracted by positive terminal of the battery  CCV through
Collector. The Collector collects most of the electrons; therefore Emitter current is
very close to the Collector current.
E CI = I
But as shown in the fig. we can say
Emitter Current = Base current + Collector current
E B CI I I 
Similarly fig. (b) shows the direction of conventional current from Collector to
Emitter and fig. (a) shows the direction of electron flow.
Alpha and Beta of Transistor
The relation between Emitter current EI , and Collector current  CI is given by
alpha(α) and the relation between Base current  BI and Collector current by  β of
the transistor. „β ‟ of a transistor tells us that small Base current controls a large
Collector current.

TRANSISTORS 89
 
 
 
 
C
E
C
B
Collector current
Alpha α=
Emitter current
I
α= …… I
I
Collector current
Similarly Beeta β=
Basecurrent
I
β= L II
I
Therelation between α and β is determined as follows

 
 
 
E C B
C
C E
E
C
E E B
B E E
B E
I = I + I …… III
I
Sinceα= I =αI
I
Substituting I in equation III
I =αI +I
I = I -αI
I = I 1-α


   
 
C E B E
C E
B E
Nowβis given by equation II and substituting I = αI and I = I 1-α
I αI
β= =
I I 1-α
 
 
α
β=
1-α
β
Andit can beproved that α=
1+β

C E B
C
E
Example 1 : I = 98 mA, I = 100 mA and ,I = 2 mA then find α and β.
Solution :
I 98
α= = =0.98
I 100
That meansαisalwaysless than1
PNP TRANSISTOR
The action of PNP transistor is quite similar to NPN transistor it differs in
battery connection only. It requires opposite battery connections. Biasing method
is same (FR), refer fig. (2.3) Emitter (P-type) is connected to positive terminal while
Base (N-type) to negative and Collector (P-type) to negative terminal of high voltage
battery CCV or Collector supply. Current through PNP transistor is due to holes as

TRANSISTORS 90
it is due to electrons in NPN transistor. Compare the two diagrams fig. (2.2) and
fig.(2.3) to understand the basic difference between biasing connections of NPN
and PNP transistor and the direction of conventional current through the circuit.
TRANSISTOR AMPLIFIER CONFIGURATIONS
1) Common Base Amplifier (CB)
2) Common Emitter Amplifier (CE)
3) Common Collector Amplifier (CC)
1) Common Base Amplifier
In this type Base is common to both input and output. Output is taken from
Collector. Observe the circuit, Input current is EI , output current is CI .
C E
E
a) ,sinceI < I thecurrent gainin CBis<1.
) ,sinceI islarge;inputimpedanceof CBis verylow.
)
C
E
in
E
Output Current I
Currentgain
Input Current I
InputVoltage V
b Input inpedance
Input Current I
OutputVoltag
c Output impedance
 
 

0
C
C
V
sinceI is verylarge;itislow
I
As input voltage increases and since Collector current is equal to Emitter current, it
also increases (IC = IE). To develop a large output voltage, RC is m
e
Output Current

0 C C
ade large so we get large output
voltage.ItisV = I ×R

TRANSISTORS 91
Similarly, as (AC) input when decreases at negative half cycle we get same
change in the output. It is indicating that output is in phase with input. Since
output resistance is large, we get amplified large output voltage.
2) Common Collector amplifier
Here the Collector terminal is common to input and output. In common
Collector configuration output is taken from Emitter therefore it is called as an
„Emitter follower‟ and input is applied to the Base. Since CR is zero, Collector is
directly connected to battery therefore it is a Collector grounded circuit because
it is always at DC voltage or it is at AC ground.
The Common Collector configuration is normally used as Emitter follower to get
low output impedance. As shown in the fig. as input increases or decreases Base
current varies and therefore Emitter current varies in the same manner.
E
B E
B
Output Current I
a)Current gain = = =Veryhigh.SinceI isverysmallandI islarge
Input Current I
it'scurrent gainisveryhigh.

TRANSISTORS 92
in
B
B
Input Voltage V
b)Inputimpedance = = ,sinceI is very small input impedance is very high.
Input Current I
0
E
E
Output Voltage V
c)Outputimpedance = = ,sinceI is very large it is low.
Output Current I
 B B E C B E E
E E
V
B B
V
The voltage gain of the amplifier is calculated as follows
Suppose R = 10kΩ,I = 1 mA, I = 10 mA β=10 I =βI = I R =1K
I ×R
VoltageGain A =
I ×R
10mA×1K
=
1mA×10K
10
A = =1
10

Therefore the voltage gain of this amplifier is always 1 or less than one that
means input and output are same in magnitude. The current gain of common
Collector amplifier is large IE/IB but voltage gain is less than or equal to 1. The
advantage of common Collector amplifier is its low output impedance and high
input impedance. The output voltage is in phase with input.
3. Common Emitter Amplifier
This configuration is most popular as an amplifier in almost all circuits,
Emitter is grounded therefore it is called as grounded Emitter amplifier.
In common Emitter amplifier input is applied through the Base and output is
normally taken between Collector and ground. In common Emitter amplifier, we
get large voltage gain but output voltage is 180⁰ out of phase with input voltage.
We will discuss CE amplifier in detail later.

TRANSISTORS 93
C
C B
B
in
B
B
Output Current I
a) Current gain = = ,sinceI is verylargethan I it'scurrent gainishigh.
Input Current I
Input Voltage V
b) Input impedance = = ,sinceI is verysmallitsinputimpedanceis veryhigh.
Input Current I
c)Outpu
O
C
C
Output Voltage V
timpedance= = ,sinceI islargeitislow.
Output Current I
COMPARISON
TRANSISTOR CHARACTERISTICS
C CEI -V Characteristics of a transistor are one of the important characteristics it
tells us how transistor behaves for different Base currents. Fig. shows the circuit
diagram and these characteristic curves.
 
CC CE B
CC,
C CE
Where V =CollectorsupplyV = voltageacrossCollector,I =Basecurrent. By
keeping the Base current constant and varying Collector supply V observation of
Collector current I andV arenoted.These curves can be summarised as follows.
i) When  BI =0 Base current is zero, small Collector current flows this is because
of minority current carriers in the Collector region. This is called as “reverse
leakage current.”
ii) In “active region” CI slowly increases with CEV . In active region the Collector
current depends on the Base current and it is „β ‟ time larger than Base
current.
CB CC CE
1)Voltage Gain Low Less than or = 1 Maximum
2)Current Gain Less than 1 Very High Maximum
3)Input Impedance Lowest (=50 Ω) Highest (300KΩ) Medium (= 1KΩ)
4)Output
Impedance
Low Lowest (= 300Ω) Medium (= 50KΩ)
5) Phase inversion No No Yes (180⁰)
6) Application In RF Amplifier Emitter follower Amplifiers

TRANSISTORS 94
iii) In „saturation region‟ the Collector current depends on the Base current and
CEV is very small below 1 volt and in saturation, Base Collector diode becomes
forward biased.
a) The transistor is OFF in Cutoff region and current through transistor is
zero.
b) The transistor is ON in Active region and shows a linear change between BI
and CI and CEV also between 0 and CCV .
c) The transistor in Saturation is fully conducting current is maximum but
CEV is almost zero.
TRANSISTOR LOAD LINE
The Common Emitter (CE) configuration is commonly used in amplifier circuit.
Transistor with biasing resistor is shown in fig. as a common Emitter amplifier
circuit. There are two resistors BR and CR or LR as Base bias resistor and Collector
load resistor respectively. Proper biasing of transistor is very much important this
is carried out by a load line analysis and calculations.

TRANSISTORS 95
A load line is normally drawn on C CEI - V characteristics curves for the transistor
used in amplifier circuit. When transistor is used as an amplifier two conditions
must be verified.
i) B-E junction must be forward biased and B-C junction reverse biased.
ii) Transistor should not go into saturation and cutoff, it must be in active region.
As shown in the fig. the first condition is satisfied but second condition is obtained
by load line analysis and selecting the operating point „Q‟
The load line is drawn by taking two extreme conditions in to consideration as
follows
a) Saturation –
At this point Collector current CI is maximum it is calculated by equation
CE CC C L
CC CC
C(sat) CE CC L CC CC
L L
V = V - I R
V V
Put I = V = V - R =V -V =0
R R
CC
L
CC
C(sat) CE
L
V
Thus Co-ordinates at saturation are (0, )
R
V 12V
For the circuit shown in the fig. I = = =12mAand V is zero (0.12mA)
R 1K
 
 
C CE CC C L
CE CC CC
CC,
CE C
CC
CC
L
) – At this point put I = 0 in equation V = V - I R
V = V -0=V
ThusCo-ordinateat cutoff is V 0
Thus we get two points V ,I
V
1)A 0, 2)B(V ,0)
R
Refer the calculations for given circu
 
 
 
b Cutoff
it shown left side of the fig. (2.10) it is
A (0, 12mA) and B(12V,0)
Now after drawing a load line between these two points an operating point „Q‟ is
located at the center of the load line, refer fig.
At operating point „Q‟ we get a fixed DC Base current when input (AC) signal is
zero. When AC signal varies, Base current varies above and below this operating
point.

TRANSISTORS 96
Effect of Q point
Fig. shows the effect of incorrect biasing (incorrect position of Q point) and it
shows what happens to the output signal if operating point is not properly
selected. If „Q‟ point is not correct then we get distorted output which is
undesired. As shown in fig. (a) and (b) where operating point is near cut off or
saturation. Transistor goes into cut off and saturation, which produces distorted
output. When it is near the center the output signal is undistorted as shown in
fig. (c)
BIASING METHODS
Method I) Fixed Bias
In this method as shown in fig. (a) two separate batteries BBV (low voltage) and CCV
(high voltage) are connected through BR and CR resistors to get Base current BI and
Collector current C BI = β× I . But instead of connecting two separate batteries we can
take only battery CCV and by connecting a high value of resistor BR we can get low
voltage as Base bias voltage refer fig. (b)

TRANSISTORS 97
Method II) Collector – Base bias (Self Bias)
This is a modified method from fixed bias circuit. Here Base bias is taken from
Collector instead of CCV as shown in fig (a). Now when temperature increases the
collector current increases due to increase in leakage current. But Collector voltage
CEV decreases because more drop takes place across CR . Therefore Base bias
voltage decreases; which reduces the Base current. Thus ultimately Collector
current is maintained constant.
This feedback from Collector to Base and this action is shown in fig (a),  shows
increase and  shows decrease.
C CE CE CC C C B B C C1) When temp , 2) I , 3) V (V = V - I R ), 4) V , 5) I , 6) I ; thus I is
maintained.
     
Method III) Emitter feedback Bias
As shown in fig (2.13b) Emitter resistor ER is introduced to develop voltage across it
to control Base current. When temperature increases, Collector current as well as

TRANSISTORS 98
Emitter current  EI increases which increases Emitter voltage EV and thus BEV
Base to Emitter potential difference decreases. As BEV decreases Base current gets
reduced as well as collector current. Thus voltage across Emitter resistor ER is
used to get negative feedback, this circuit is known as Emitter feedback bias.
Method IV) Voltage Divider – Bias (Stabilized Bias)
This method is most popular in amplifier circuits because stabilization of Q point
and temperature effect both are controlled. Here 1R and 2R are used to get Base
biasing voltage divider type circuit. Similarly a capacitor across EC is shown in fig
(a) Since 1 2R R form a voltage divider from CCV it is called as voltage divider bias.
When temperature increases Collector current increases and since C EI = I Emitter
current also increases. This EI flows through ER it increases voltage drop across ER
i.e.  E EI R but BE B EV =V -V therefore BEV gets reduced. This lowers the Base current.
Thus increase in unwanted current is compensated; this sequence of operation is
shown in fig (b).
Similarly to avoid AC feedback a Emitter bypass capacitor is connected across ER to
bypass AC voltage hence EC is known as emitter bypass capacitor. If there is no EC
it would produce AC voltage and reduce the voltage gain.
UNIPOLAR TRANSISTORS
As we have studied transistors were bipolar transistors since their current carriers
were both electrons and holes hence called bipolar.

TRANSISTORS 99
i) FET (Field – Effect – Transistor)
This type of transistor is called as unipolar device only one type of carriers either
electrons or holes take part in carrying the currents. Bipolar transistor like NPN
is operated with Base current but in case of FET, it is operated with gate voltage.
It has three terminals.
1) Source just like Emitter
2) Drain like Collector
3) Gate like Base of bipolar transistor
Below Fig. shows its construction, symbol and both types N-channel and P-
channel. Since there is a junction it is known as junction field effect transistor
or JFET. They are manufactured in two types N-channel JFET and P-channel
JFET.
N-channel JFET has a N-bar in which two P-regions are attached to form a gate
terminal. Here source terminal acts as a source of electrons.
Gate, which controls this flow through N-channel from terminal source to drain.
Drain is normally connected to high positive voltage called as DDV , it drains
electrons. The gate voltage is normally in reverse bias with respect to source
called as GSV .
I-V Characteristics of FET (Operation)
Let us see how FET controls the drain current  DI by gate voltages GSV . Fig. shows
the circuit diagram and curves of DI verses DSV for different values of GSV .

TRANSISTORS 100
Refer fig (a) current flows (conventional) from drain to source but electrons are
attracted by drain voltage DDV . This electron flows is opposed by reverse bias gate
voltage because of reverse voltage, opposing field or a depletion region is developed
therefore small number of electrons flow from source to drain. Fig. (b) shows if GSV
is more negative then N channel becomes very narrow which stops electron flow
this is called as „Pinch – off region‟.
The I-V characteristic shows, as GSV voltage becomes more and more negative drain
current becomes low. After particular drain voltage, JFET goes into breakdown and
then current suddenly increases to a high value. In case of JFET Gate current is
almost zero hence it is called as a voltage operated device. It has very high input
impedance since gate current is zero, practically a small leakage current flows.
ii)MOSFET (Metal Oxide Semiconductor FET)
In junction FET due to leakage current small gate current floes; that can be
reduced by insulating gate from channel by using insulating material silicon
dioxide therefore it is called „Metal-Oxide Semiconductor‟ or MOSFET sometimes
MOSFETs are called as „IGFET‟ Insulated gate FET.
MOSFET can be of two types:
i) Depletion type MOSFET (N-channel and P-channel)
ii) Enhancement type MOSFET (N-channel and P-channel)
Fig. illustrates the structure and symbol of depletion MOSFET in both types N-
channel and P-channel.

TRANSISTORS 101
MOSFET has four terminals drain, source, insulated Gate with Silicon dioxide and
a substrate. Normally substrate is connected to the source as it is shown in
symbol. Arrow “IN” indicates N-channel and arrow “OUT” indicates P-channel
MOSFET.
In enhancement MOSFET construction is different as it is shown by fig. there is a
region between source and drain. P-substrate separates these two N-regions.
Operation
MOSFET is operated with gate voltage ( GSV ), when GSV is zero or negative then in
case of N-channel MOSFET, it is cut off because there is a P-region in between
drain and source. But when GSV is made sufficient positive called as threshold
voltage a N-type inversion layer connects source to drain, current flows through
this layer. Due to silicon dioxide insulator, gate – SiO2 – P – region form a
capacitance as shown in fig. When positive voltage is applied to the gate, electrons
can be accumulated on other side as shown with N-layer.

TRANSISTORS 102
Thus, more is the GSV positive more will be the width of N-inversion layer and
therefore more electrons can flow from source to drain. Since insulator is
connected in gate circuit gate current is almost zero. This indicates that MOSFET
has very high input impedance. MOSFET‟s are used in microprocessors as an
electronic switch. Because of very small gate current, power required for MOSFET
is very small it can be operated with small button cell. By connecting
complementary MOSFET (P-channel) with (N-channel) MOSFET a new type of
circuit is formed called as “CMOS” Complementary MOSFET.
Fig. shows the I-V Characteristics of MOSFET. Drain current depends on DDV and
GSV .
(A load line can be drawn similar to transistor between DD
D
V
R
instead of CC
L
V
R
and
DDV instead of CCV of bipolar transistor.
iii) Uni-Junction Transistor (UJT)
UJT is a special type of unipolar transistor usually used a s an electronic switch. It
has two doped regions and three terminals BASE1, BASE2 and EMITTER. Refer
the fig. (a), it shows its structure and symbol. The emitter is heavily doped and
base is lightly doped. Since the device has one P-N junction and three leads, it is
commonly known as Uni-junction transistor (Uni means single).
Fig. (b) shows the equivalent circuit of a UJT. The resistance of the silicon bar is
called as inter-base resistance BBR . Two resistors in series represent the inter –
base resistance viz.

TRANSISTORS 103
a) B2R is the resistance of silicon bar between 2B and the point at which the emitter
junction lies.
b) B1R is the resistance of the bar between 1B and emitter junction. This resistance
is shown variable because its value depends upon the bias voltage across the P-N
junction.
The P-N junction is represented in the emitter by a diode D.
The circuit action of a UJT can be easily explained from its equivalent circuit.
i) with no voltage applied to the UJT, the inter-base resistance is given by;
BB B1 B2R = R + R
The value of BBR generally lies between 4 K Ω and 10KΩ
ii) If a voltage BBV is applied between the bases with emitter open, the voltage will
divide up across B1R and B2R .
B1
B1, RB1 BB
B1 B2
B1
RB1 BB
B1 B2
R
Voltage across R V = V
R +R
R
Or V /V =
R +R
The ratio RB1 BBV /V is called intrinsic stand – off ratio and is represented by letter
η.
Characteristics of UJT

TRANSISTORS 104
Fig. shows E EI -V characteristic curve. Initially, when the UJT is in cut-off as we
increase from zero, very small current flows from terminal B2 to the emitter. This
is known as leakage current.
Above a certain value of EV forward emitter current starts increasing till EV
reaches to peak point. After the peak point current through UJT suddenly
increases and EV decreases to a very low point. This part of curve is known as
negative resistance characteristics of UJT. When EI increases to a very low point
known as valley point the device goes into saturation.
SOLVED PROBLEMS
1) In a transistor characteristics Base current is 100μA , Calculate EI , CI , if β = 100.
 
B C B
C
E B C
Solution : I = 100μA, I = β×I
= 100 × 100μA
I =100×0.1mA 100μA=0.1mA
=10mA
Now Emitter current =I =I +I
=0.1mA+10mA
=10.1mA
2) If α of a transistor is 0.95 find β
α 0.95 0.95
Solution: β 19
1 α 1 0.95 0.05
   
 

TRANSISTORS 105
3) In a transistor characteristics base current is 100μ A, if β of the transistor is 100
calculate CI and EI and CEV if CCV = 24V and CR = 1K
 
B
C B
C B
E C B
CE CC C C
Solution: Given I = 100μA = 0.1mA
a) Since β = I /I
I = β × I
=100×0.1mA = 10mA
b) Now I = I + I
= 10mA+0.1mA=10.1mA
c) V =V -I R =24- 10 10-3 1 103 24 10 14 .V

     
4) A UJT has supply of 12 volts find peak emitter voltage if it‟s B1R is 6 KΩ and B2R is
4KΩ
B1 B2BB
B1
B1 B2
EP BB D
Solution: V =12V, R = 6KΩ R =4KΩ
R 6K
η = = =0.6
R +R 10K
V =ηV +V =7.2+0.7=7.9 Volt

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