Sub Code: C
Analog Electronic and Digital Electronic L T P C
B18EE3140 H
Circuit Design
Duration :14 Wks 2 1 0 3 4
1. To provide an insight into the modeling of bipolar junction
transistors, biasing techniques.
2. To illustrate the application and its design of BJTs as
amplifiers and oscillators.
3. Illustrate Boolean laws and minimization techniques for
Course Objectives: simplification of expressions like minterm, maxterm using K-
Map and QMT
4. Introduce various application oriented circuits which can be
implemented in real world examples for making the learners
attuned to Logic concepts.
5. Introduce and differentiate between the Combinational and
Sequential Circuits.
At the end of this course, student will be able to:
1. Describe the operation, applications and characteristics of
devices BJT.
2. Analyze and design circuits such as amplifiers and oscillators
using BJT.
Course outcomes 3. Define a Boolean term, expression, SOP, POS, and construct
the K-map/QMT Table for real time application
implementation
4. Design arithmetic and combinational logic circuits using gates,
encoders, decoders, multiplexers and de-multiplexers.
5. Design specified synchronous or asynchronous sequential
logic circuits using appropriate flip flops.
COURSE CONTENTS
Unit 1: Transistors [11 Hrs]
DC load line, Q point effect on signal swing, biasing techniques, discussion on bias stability,
BJT transistor modeling (re and h models) for various CE configurations (fixed bias, voltage
divider bias and emitter bias), Small signal BJT amplifiers: analysis of CE configuration
using re-model, h- parameter model; emitter follower.
Unit 2: Amplifiers and Oscillators [11 Hrs]
Darlington connections, Feedback Amplifiers: Characteristics of feedback, feedback
topologies, Power amplifiers: classification and application, series fed class A amplifier,
Transformer coupled Class A amplifiers, Class B Push-Pull amplifiers, Complementary Push-
Pull and Transformer–coupled load Push-Pull, Amplifier distortions.
Oscillators: Principle of operation (Barkhausen’s Criteria, positive feedback concept),
Introduction to Audio frequency Oscillators, Radio frequency Oscillators, Crystal Oscillators.
(BJT Version Only)
Unit 3: Minimization Techniques Analysis and Design of combinational Circuits
[10 Hrs]
Introduction to combinational logic circuits, generation of switching equation from truth
table. Minimization Techniques: Boolean algebra expression minimization. Minterm,
Maxterm, Sum of Products (SOP), Product of Sums (POS), Karnaugh map (3, 4, 5 Variable)
and Quine - McCluskey method of minimization Design procedure of Half adder, Full Adder,
Half subtractor, Full subtractor, Carry Look Ahead adder, BCD adder, Comparator – 1bit
and 2 bit , Principle of Encoder and Decoder with cascading of decoders. Principle of
Multiplexers and Demultiplexer with cascading of Mux and Boolean function
implementation using Mux and decoders.
Unit 4: Sequential circuits Design and Logic Families [10 Hrs]
Basic bistable element, S R Latch , application of SR latch as a switch debouncer, Edge
triggering – Level Triggering, Flip-flops - SR, JK, D, T, and Master-Slave – Characteristic
table and equation. Registers, Shift Register, Universal shift register, Counters: Binary Ripple
Up/Down Counter, Design of synchronous Mod- n counter using flip-flop. Logic families:
Diode-Transistor Logic, Transistor-Transistor Logic, Emitter-Coupled Logic, NMOS and
PMOS Logic, CMOS Logic.
Text Books:
1. Robert L. Boylestad and Louis Nashelsky, “Electronic Devices and Circuit Theory”,
PHI/Pearson Education. 9th Edition.
2. John M Yarbrough, “Digital Logic Applications and Design”, Thomson Learning,
1st Edition, 2001.
3. Donald D Givone, “Digital Principles and Design”, Tata McGraw-Hill 1st Edition,
2002.
Reference books:
1. Jacob Millman & Christos C. Halkias , “Integrated Electronics”, Tata - McGraw
Hill, 2nd Edition, 2010.
2. David A. Bell , “Electronic Devices and Circuits” , PHI, 5th Edition, 2009.
3. Muhammad H. Rashid, “Electronic Circuits and Applications”, Cengage learning,
1st Edition
4. Muhammad H. Rashid, “Electronic Devices and Circuits”, Cengage Learning, 1 st
Edition.
5. D P Leach, A P Malvino, & Goutham Saha, “ Digital Principles and applications”,
Tata McGraw-Hill, 7th Edition, 2010.
6. Moshe Morris Mano, “Digital Design” Prentice Hall, 3rd Edition, 2008.
UNIT 1- TRANSISTORS
The transistors have replaced the vacuum tubes for the following advantages
1. Low operating voltage
2. Higher efficiency
3. Small size and ruggedness
4. Does not require any filament power
Uses of a transistor
A transistor acts as an Amplifier, where the signal strength has to be
increased.
A transistor also acts as a switch to choose between available options.
It also regulates the incoming current and voltage of the signals.
Bipolar Junction Transistor is a three terminal device namely: Base, Emitter and collector.
Input signal of very small amplitude is applied at the base to get the magnified output signal
at the collector. Thus provides amplification of the signal. The amplification in the transistor
is achieved by passing input current signal from a region of low resistance to a region of high
resistance. This concept of transfer of resistance has given the name TRANSfer-resISTER .
There are two types of transistors: Unipolar Junction transistor(UJT) and Bipolar
Junction Transistor(BJT).
In Unipolar Junction transistor current conduction is only due to majority carriers. In bipolar
Junction Transistor current conduction takes place due to both types of charge carriers, ie
holes and electronics.
The BJTs are of two types:
Whatever may be the use of a transistor either as an amplifier or as a switch it must have
one input circuit and one output circuit. To facilitate the input and output circuit a transistor
should have four terminals – two for input and two for output circuit. To resolve the needs of
four terminals from only three terminals of the transistor, one terminal in the transistor is used
as common for both input and output circuit. Which terminal would be chosen as common in
the transistor, determined by the application of the transistor. Depending on the possibilities
of circuit configurations transistor connections are of three types.
Common Base configuration
Input is applied between emitter and base and output is taken from the collector and base. Here base
is used as the common terminal for both input and output circuits, hence it is known as common base
configuration. This configuration provides voltage gain but no current gain. This
Configuration provides good stability against increase in temperature.The CB configuration
is used for high frequency applications.
Common collector configuration
When collector is used as the common terminal for two circuits then it is known as common collector
configuration. Here collector is used as the common terminal for both input and output circuits. This
configuration provides current gain but no voltage gain. The voltage gain provided by this
circuit is less than 1. This circuit is mostly used for impedance matching. That means, to
drive a low impedance load from a high impedance source.
Common emitter configuration
Input is applied between emitter and base and output is taken from the collector and emitter. Here
emitter is used as the common terminal for both input and output circuits, hence it is known as
common emitter configuration.
Characteristics of CE Configuration
This configuration provides good current gain and voltage gain.
Keeping VCE constant, with a small increase in V BE the base current IB increases
rapidly than in CB configurations.
For any value of VCE above knee voltage, IC is approximately equal to βIB.
The input resistance Ri is the ratio of change in base emitter voltage (ΔVBE) to the
change in base current (ΔIB) at constant collector emitter voltage VCE.
Ri=ΔVBE/ΔIB, at constant VCE
As the input resistance is of very low value, a small value of V BE is enough to
produce a large current flow of base current IB.
The output resistance Ro is the ratio of change in collector emitter voltage (ΔVCE) to
the change in collector current (ΔIC) at constant IB.
Ro=ΔVCEΔICRo=ΔVCEΔIC at constant IB
As the output resistance of CE circuit is less than that of CB circuit.
This configuration is usually used for bias stabilization methods and audio frequency
applications.
Why CE configuration is widely used in amplifier circuits.
The main reasons for the wide spread use of this circuit arrangements are:
The CE configuration is the only configuration which provides both voltage gain as
well as current gain greater than unity.
In case of common base current gain is less than 1
In case of common collector voltage gain is less than 1
Power gain of the CE amplifier is much greater than the power gain provided by
other two configurations.
Transistor Biasing
The supply of suitable external dc voltage is called as biasing. Either forward or reverse
biasing is done to the emitter and collector junctions of the transistor.
These biasing methods make the transistor circuit to work in four kinds of regions such
as Active region, Saturation region, Cutoff region and Inverse active region (seldom
used). In order to operate transistor in the desired region we have to apply external dc
voltages of correct polarity and magnitude to the two junctions of the transistor. This is
nothing but biasing. This is understood by having a look at the following table.
When we bias a transistor we establish a certain current and voltage conditions for the
transistor. These conditions are known as operating conditions or dc operating point or
Quiescent point.
Operating points must be stable for proper operation of transistor
Operating points shifts with changes in transistor parameters such as β, I CO and V BE
Operating point varies with change in temperature as transistor parameters are
temperature dependent.
Emitter Junction Collector Junction Region of Operation
Forward biased Forward biased Saturation region
Forward biased Reverse biased Active region
Reverse biased Forward biased Inverse active region
Reverse biased Reverse biased Cut off region
Among these regions, Inverse active region, which is just the inverse of active region, is not
suitable for any applications and hence not used.
Active Region
This is the region in which transistors have many applications. This is also called as linear
region. A transistor while in this region, acts better as an Amplifier.
The following circuit diagram shows a transistor working in active region.
This region lies between saturation and cut-off. The transistor operates in active region when
the emitter junction is forward biased and collector junction is reverse biased.
In the active state, collector current is β times the base current, i.e.
IC=βIBIC=βIB
Where IC = collector current, β = current amplification factor, and IB = base current.
Saturation Region
This is the region in which transistor tends to behave as a closed switch. The transistor has
the effect of its collector and emitter being shorted. The collector and emitter currents are
maximum in this mode of operation.
The following figure shows a transistor working in saturation region.
The transistor operates in saturation region when both the emitter and collector junctions are
forward biased.
In saturation mode,
β<ICIBβ<ICIB
As in the saturation region the transistor tends to behave as a closed switch,
IC=IEIC=IE
Where IC = collector current and IE = emitter current.
Cutoff Region
This is the region in which transistor tends to behave as an open switch. The transistor has
the effect of its collector and base being opened. The collector, emitter and base currents are
all zero in this mode of operation.
The figure below shows a transistor working in cut-off region.
DC load line
When the transistor is given the bias and no signal is applied at its input, the load line drawn
under such conditions, can be understood as DC condition. Here there will be no amplification as
the signal is absent. The circuit will be as shown below
The transistor operates in cutoff region when both the emitter and collector junctions are
reverse biased.
As in cutoff region, the collector current, emitter current and base currents are nil, we can
write as
IC = IE = IB = 0
Where IC = collector current, IE = emitter current, and IB = base current.
Operating point(Q point)
When a value for the maximum possible collector current is considered, that point
will be present on the Y-axis, which is nothing but the Saturation point. As well, when a
value for the maximum possible collector emitter voltage is considered, that point will be
present on the X-axis, which is the Cutoff point.
When a line is drawn joining these two points, such a line can be called as Load line. This is
called so as it symbolizes the output at the load. This line, when drawn over the output
characteristic curve, makes contact at a point called as Operating point or quiescent
point or simply Q-point.
Consider the collector circuit of a biased transistor shown along with its characteristics.
KVL equation for the base circuit is
V CC −I B R B −V BE = 0
V −V BE
I B= CC
RB
Intersection of curves of different I B with DC line gives different operating points.
KVL equation for the collector circuit is
V CC −I C RC −V CE =0
V CC =I C RC +V CE
To determine two points on the line we assume V CE =V CC and V CE =0
When collector emitter voltage VCE = 0, the collector current is maximum and is equal to
VCC/RC. This gives the maximum value of VCE. This is shown as
VCE=VCC−ICRC
0=VCC−ICRC
IC = VCC/RC
This gives the point A (OB = VCC/RC) on collector current axis, shown in the above figure.
When the collector current IC = 0, then collector emitter voltage is maximum and will be
equal to the VCC. This gives the maximum value of IC. This is shown as
VCE=VCC−ICRC
=VCC
(AS IC = 0)
This gives the point A, which means (OA = V CC) on the collector emitter voltage axis shown
in the figure.
Hence we got both the saturation and cutoff point determined and learnt that the load line is
a straight line. So, a DC load line can be drawn.
Consider the output characteristics of a common emitter configuration with points A, B and
the line drawn between them called dc load line.
The load line is drawn by joining the saturation and cut off points. The region that lies
between these two is the linear region. A transistor acts as a good amplifier in this linear
region.
If this 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.
Q point effect on signal swing
Operating point can be selected at different position on DC load line. The selection of
operating point depends on application.
When the transistor is used as amplifier, the operating point should be at the centre of
DC load line to prevent distortion in the amplified output (Q point should remain stable
to achieve faithful amplification). Hence the quiescent point or Q-point is the value
where the Faithful Amplification is achieved.
Faithful amplification is the process of amplifying to obtain complete portions of
input signal by increasing the signal strength. This is done when AC signal is
applied at its input.
Case 1: When the Q point is near saturation region:
If the operating point is considered near saturation point, then the amplification will
be as shown in fig below. The collector current I C is clipped at positive half cycle.
Even with increase in base current, I C is not useful since distortion is present. So
this operating point is not suitable for transistor used as amplifier.
Case 2: When the Q point is near cut-off region:
If the operating point is considered near cut off point, then the amplification will be
as shown in fig below. The collector current I C is clipped at negative half cycle. So
this operating point is not suitable for transistor used as amplifier.
Case 2: When the Q point is at centre of DC load line :
If the operating point is considered at the centre of DC load line, the amplification
will be as shown in fig below. The output signal is sinusoidal waveform without any
distortion. Q point should be centred in the active region to assure maximum swing
for the input signal. Thus this Q point is the best operating point.
In the above graph, The input signal applied is completely amplified and reproduced
without any losses. This can be understood as Faithful Amplification.
The operating point is so chosen such that it lies in the active region and it helps in
the reproduction of complete signal without any loss.
Hence the placement of operating point is an important factor to achieve faithful
amplification. But for the transistor to function properly as an amplifier, its input
circuit (i.e., the base-emitter junction) remains forward biased and its output circuit
(i.e., collector-base junction) remains reverse biased.
The amplified signal thus contains the same information as in the input signal
whereas the strength of the signal is increased.
