Analog & Digital Integrated Circuits - Material (Short Answers)

Analog & Digital IC’s S.MATHANKUMAR, AP/EEE
TWO MARK QUESTIONS WITH ANSWERS
DEPARTMENT OF BIOMEDICAL ENGINEERING 1
Subject Name ANALOG AND DIGITAL IC’s
Objective SHORT QUESTIONS WITH ANSWERS
UNIT - I PART – A
1. Define virtual short.
The differential input voltage Vd between the noninverting and inverting input
terminals is essentially zero and the input current of the op-amp is always zero.
2. What is differential amplifier?
The differential amplifier amplifies the difference between two input voltage
signals.
3. Define slew rate.
The maximum rate of change of output voltage with respect to time is called slew
rate of op-amp. The slew rate equation is, S=2πfVm in V/Sec
(Or)
The slew rate is defined as the maximum rate of change of output voltage caused
by a step input voltage and is usually specified in V/µs. Slew rate of 741 IC is
0.5V/µS.

4. List the important characteristics of an ideal op-amp.
Characteristics of an ideal operational amplifier:
S.NO Characteristics Symbol Values
1. Open loop voltage gain AOL ∞ (infinity)
2. Input impedance Ri ∞ (infinity)
3. Output impedance Ro 0 (zero)
4. Zero offset voltage Vo 0 (zero)
5. Band width BW ∞ (infinity)
6. CMMR ρ ∞ (infinity)
7. Slew rate S ∞ (infinity)
8. Power Supply Rejection Ratio PSRR 0 (zero)
5. Define CMRR of an Op – Amp.
Definition of CMRR:
The relative sensitivity of an op-amp to a difference signal as compared to a
common –mode signal is called the common –mode rejection ratio. It is expressed
in decibels.
CMRR= Ad/Ac
(Or)
Common Mode Rejection Ratio (CMRR) is defined as the ratio of difference
mode gain to common mode gain. It‟s ideal value is infinity, and it is given by,
CMRR = | Ad| / |Ac|
Where, Ad is difference mode gain and
Ac is common mode gain.
6. Differentiate Average & peak detector.
It is sometimes necessary to measure the maximum positive excursion (peak
value) or negative excursion (valley value) of a waveform over a given time
period. There may also be a requirement to capture and hold some maximum value
of a positive or negative pulse. A circuit that performs this function is a peak
detector.

7. Mention some of the linear and non linear applications of Op - Amp.
Linear applications of op – amps :
Adder, subtractor, voltage –to- current converter, current –to- voltage converters,
instrumentation amplifier, analog computation, power amplifier, etc are some of
the linear op-amp circuits.
Non – linear applications of op-amps:-
Rectifier, peak detector, clipper, clamper, sample and hold circuit, log amplifier,
anti –log amplifier, multiplier are some of the non – linear op-amp circuits.
8. Draw the integrator circuit using Op - Amp.
An op – amp acts as an Integrator
9. Define precision diode.
Precision diode is the diode which is placed in the feedback loop of an op-
amp which is capable of rectifying input signals of the order of millivolts.
The major limitation of ordinary diode is that it cannot rectify voltages below
the cut – in voltage of the diode. A circuit designed by placing a diode in the
feedback loop of an op – amp is called the precision diode and it is capable of
rectifying input signals of the order of millivolt.

10.What is hysteresis?
The graph indicates that once the output changes its state, it remains there
indefinitely until the input voltage crosses any of the threshold voltage levels.
This is called hysteresis of Schmitt trigger. The hysteresis is also called dead band
or dead zone.
11. Draw the circuit diagram of Astable vibrator.
Astable Multivibrator Circuit Diagram Using the 555 Timer
12. What is meant by filters? What are the types?
Filter is a frequency selective circuit that passes signal of specified band of
frequencies and attenuates the signals of frequencies outside the band.
 Based on functions: Low pass filter, High pass filter, Band pass filter, Band
reject filter
 Based on order of transfer function: first, second, third higher order filters.
 Based on configuration: Bessel, Chebychev, Butterworth filters.

13. Define PSRR of an Op - Amp.
Definition of PSRR:
Power Supply Rejection Ratio (PSRR) is defined as the change in op-amp‟s input
offset voltage due to variations in supply voltage. It is expressed in microvolt per
volt or in db.
(Or)
PSSR is Power Supply Rejection Ratio. It is defined as the change in the input
offset voltage due to the change in one of the two supply voltages when other
voltage is maintained constant. Its ideal value should be zero.
14.Write down the applications of precision diode.
 Essentially a precision diode can be used in all the application circuits of
conventional p-n diodes (except power applications) to allow for low –
level processing.
 Rectifiers
 Limiters
 Clamping circuits
15.What is Peak detector?
A peak detector is a circuit which notes and remembers the peak positive or
negative value of an input signal for an infinite period of time until it is rest.
16. Draw the circuit diagram of triangular generator.

17. Write short notes on Schmitt trigger and List out the applications.
Schmitt trigger is a generic name of threshold circuits with positive feedback having
a loop gain > 1.
Schmitt trigger is a regenerative comparator. It converts sinusoidal input into a square
wave output. The output of Schmitt trigger swings between upper and lower threshold
voltages, which are the reference voltages of the input waveform.
(Or)
Schmitt trigger is an inverting comparator with positive feedback. It converts an
irregular-shaped waveform to a square wave or pulse, and is called as squaring circuit.
Schmitt trigger devices are typically used in open loop configurations for noise
immunity and closed loop negative feedback configurations to implement bistable
regulators, triangle/square wave generators, etc.
Applications:
 Squaring circuit
 Sine-to-Square comparator
 Amplitude comparator
 As flip flops
18.What are the limitations of integrated circuits?
The integrated circuits have few limitations also, as listed below:
 In an IC the various components are part of a small semi-conductor chip
and the individual component or components cannot be removed or
replaced, therefore, if any component in an IC fails, the whole IC has to be
replaced by the new one.
 Limited power rating as it is not possible to manufacture high power (say
greater than 10 Watt) ICs.
 Need of connecting inductors and transformers exterior to the semi-
conductor chip as it is not possible to fabricate inductors and transformers
on the semi-conductor chip surface.
 Operations at low voltage as ICs function at fairly low voltage.

 Quite delicate in handling as these cannot withstand rough handling or
excessive heat.
 Need of connecting capacitor exterior to the semi-conductor chip as it is
neither convenient nor economical to fabricate capacitances exceeding
30pF. Therefore, for higher values of capacitance, discrete components
exterior to IC chip are connected.
 High grade P-N-P assembly is not possible.
 Low temperature coefficient is difficult to be achieved.
 Difficult to fabricate an IC with low noise.
 Large value of saturation resistance of transistors.
 Voltage dependence of resistors and capacitors.
 The diffusion processes and other related procedures used in the fabrication
process are not good enough to permit a precise control of the parameter
values for the circuit elements. However, control of the ratios is at a
sufficiently acceptable level.
19.Draw the Op – Amp symbol and state its important terminals.

20.State the advantages of voltage follower.
 Very large input resistance, of the order of MΩ.
 Low output impedance, almost zero. Hence it can be used to connect high
impedance source to a low impedance load, as a buffer.
 It has a large bandwidth.
 The output follows the input exactly without a phase shift.
21.When inverting amplifier is called phase inverter?
When the gain of the inverting amplifier is unity and it is used to change the phase
of the input produce the output then it is called phase inverter.
22.Draw the circuit diagram of subtractor.
Basic differential amplifier used as a Subtractor
23.List the applications of differentiator circuit.
 In the wave shaping circuits to detect the high frequency components in the
input signal.
 As a rate – of – change detector in the FM demodulators.
 It is used in wave shaping circuits to detect the frequency in an input signal.
 It is also used as rate of change detector in modulations.

24.State the applications of V to I converter and I to V converter.
Applications of V to I converter
 Low voltage d.c. voltmeter
 Low voltage a.c. voltmeter
 Diode tester and match finder
 Zener diode tester
Applications of I to V converter
 Photodiode detector
 PhotoFET detector
25.Draw the circuit diagram of full wave rectifier using Op - Amp.
Full wave rectifier using op-amp
26.List out the applications of an instrumentation amplifier and what are the
requirements of instrumentation amplifier?
Applications of an instrumentation amplifier
☼Temperature controller ☼Data acquisition systems
☼Temperature indicator ☼Light intensity meter
☼Analog weight scale
Requirements of instrumentation amplifier
☼It should be finite, accurate and stable ☼Easy gain adjustment
☼High input impedance ☼Low output impedance
☼High CMRR ☼Low power consumption

☼Low thermal drift ☼High slew rate
27.State the applications of monostable multivibrator.
☼Frequency divider ☼Missing pulse detector
☼Pulse width modulation ☼Pulse position modulation
☼Linear ramp generator
28.What is clamper circuit? How clamper circuits are classified?
A clamper is an electronic circuit that prevents a signal from exceeding a
certain defined magnitude by shifting its DC value. The clamper does not
restrict the peak-to-peak excursion of the signal, but moves it up or down by
a fixed value.
Based on which point of the wave D.C. is inserted the clamper circuits are
classified into.
 Negative clamper.
 Positive clamper.
29. List the applications of Log amplifiers.
 Analog computation may require functions such as ln x, log x, sin hx
etc. These functions can be performed by log amplifiers
 Log amplifier can perform direct dB display on digital voltmeter and
spectrum analyzer
 Log amplifier can be used to compress the dynamic range of a signal

UNIT – II PART – A
1. Define phase comparator.
A phase detector or phase comparator is a frequency mixer, analog multiplier or
logic circuit that generates a voltage signal which represents the difference in
phase between two signal inputs. It is an essential element of the phase-locked
loop (PLL).
2. Define lock in range.
The range of frequencies over which the PLL can maintain locked with the
incoming signal is called the lock in range.
(Or)
The range of frequencies over which the PLL can maintain lock with the incoming
signal is called the lock-in range or tracking range. It is expressed as a percentage
of the VCO free running frequency.
3. Define capture range.
The range of frequency over which the PLL can acquire lock with an input signal
is called capture range.
(Or)
The range of frequencies over which the PLL can acquire lock with an input signal
is called the capture range. It is expressed as a percentage of the VCO free running
frequency.
4. List out the typical applications of Phase Locked Loop.
 Frequency multiplication
 Frequency division
 Frequency translation
 AM detection
 FM detection.

5. Define voltage controlled oscillator.
The voltage controlled oscillator is defined as the one in which the frequency of
the output signal (Vo) is controlled by the control voltage (Vc).
6. What is monotonicity?
The converter is said to have good monotonicity if it does not miss any step
backward when stepped through its entire range by a counter.
7. Draw the blocks of PLL.
8. Write the operation of Voltage to Frequency ADC.
Voltage-to-frequency analog-to-digital converters use an input voltage to an
output pulse train with a frequency proportional to the input voltage. Output
frequency is determined by counting pulses over a fixed time interval and the
voltage is inferred from the known relationship.
Voltage-to-frequency analog-to-digital conversion has a high degree of noise
rejection, because the input signal is effectively integrated over the counting
interval. Voltage-to-frequency conversion is commonly used to convert slow and
often noisy signals. It is also useful for remote sensing applications in noisy
environments. The input voltage is converted to a frequency at the remote location
and the digital pulse train is transmitted over a pair of wires to the counter. This
eliminates the noise that can be introduced in the transmission of an analog signal
over a long distance.

9. Define counter ramp ADC.
A delta-encoded ADC or counter-ramp has an up-down counter that
feeds a digital to analog converter (DAC). The input signal and the DAC both go
to a comparator. The comparator controls the counter.
The circuit uses negative feedback from the comparator to adjust the
counter until the DAC's output is close enough to the input signal. The number is
read from the counter. Delta converters have very wide ranges and high resolution,
but the conversion time is dependent on the input signal level, though it will
always have a guaranteed worst-case.
10.What is pull- in time?
The total time taken by the PLL to establish lock is called pull-in time. It depends
on the initial phase and frequency difference between the two signals as well as on
the overall loop gain and loop filter characteristics.
11.Define resolution of DAC.
It is the smaller change in voltage which may be produced at the output or input of
the converter.
(Or)
This is defined as the smallest possible change in the analog output voltage. Its
value depends on the number of bits in the digital input applied to DAC.
12.State advantage of R- 2R ladder type DAC.
 Easier to build, as only two values of resistors are needed.
 It is possible to increase the no. of bits just by adding more sections of same
R/2R values.

13.Which is the fastest ADC and why?
A pipeline ADC generally several flash ADCs in series. Because of this you have
a latency due to each flash. Plus you have to have op-amps in a pipeline ADC and
compensating the op-amps limits the speed of the ADC. A flash ADC will be the
fastest ADC you can have, but for it to have the same resolution as a 10-12 bit
pipeline, it will consume a lot more area, input capacitance, and possibly more
power. But the benefit is that the output data will be available usually 1/2-1 clock
cycle later instead of several clock cycles later in a pipeline.
A Flash ADC is the fastest ADC in terms of speed but it has added disadvantage
that we have to compromise on power verse resolution. Pipelined ADC comes
second in terms of power.
14.List out the advantages of DAC.
 Simple to construct
 Economic
15.Define amplitude modulation.
The amplitude of high frequency carrier wave is varied in accordance with the
message signal.
16.List out the disadvantages of ADC.
 Accuracy & stability of this ADC depends on the resistors
 A wide range of resistors required
 Finite resistance of the switches may disturb the currents.
17. Give the specification of DAC.
☼Resolution ☼Linearity ☼Accuracy
☼Monotonic ☼Setting time ☼Stability
☼Temperature ☼Speed

18.What are the different types of ADC?
 Flash type converter
 Counter type converter
 Tracking type converter
 Successive approximation type converter.
19.What is linear error?
The linear error is defined as the amount by which the actual output differs from
the ideal straight line output characteristics of DAC.
20.Define settling time.
It represents the time it takes for the output to settle within a specified band
±½LSB of its final value following a code change at the input (usually a full scale
change). It depends upon the switching time of the logic circuitry due to internal
parasitic capacitance & inductances. Settling time ranges from 100ns. 10µ s
depending on word length & type circuit used.
(Or)
It is a comparison of actual output voltage with expected output. It is expressed in
percentage. Ideally, the accuracy of DAC should be, at worst, ±½ of its LSB.
21.What are the stages through which PLL operates?
 Free running
 Capture
 Phase lock
22.Where is the successive approximation type ADC used?
The Successive approximation ADCs are used in applications such as data
loggers & instrumentation where conversion speed is important.

23.Listout the advantage and draw backs of dual slop ADC.
Advantages:
 For accurate measurement of slowly varying signals, such as thermocouples
and weighing scales.
 It provides excellent noise rejection of ac signals whose periods are integral
multiples of the integration time T.
Disadvantages:
 Long conversion time.
 The dual slope ADC has long conversion time. This is the main drawback
of dual slope ADC.
24. Write the two problems associated with DM.
Problems associated with DM:
 Slope overload
 Granular noise
25.Define Voltage to Frequency conversion factor.
Voltage to Frequency conversion factor is defined as,
Ky = Δf0 / ΔVC = 8f0 / VCC
Where, ΔVC is the modulation voltage required to produce the frequency shift Δfo.

UNIT – III PART – A
1. Define radix.
Total numbers available in a number system is called base or radix.
(Or)
The number of values that a digital (one character) can assume is equal to the base
of the system. It is also called as the radix of the system.
2. What are the number systems?
 Decimal number system
 Binary number system
 Octal number system
 Hexadecimal number system
3. Define the following: minterm and maxterm.
Minterm
The „n‟ variables forming an AND term, with each variable being primed or
unprimed, provide in 2n
possible combinations, called Minterm or Standard
Products. Each individual term in standard SOP form is called minterm.
Maxterm
The „n‟ variables forming an OR term, with each variable being primed or
unprimed, provide in 2
n
possible combinations, called Maxterm or Standard
Sums. Each individual term in standard POS form is called maxterms.
4. What is meant by prime implicant?
A prime implicant is a group of minterms which cannot be combined with any
other minterm or groups.

5. Convert the given binary (1101010110)10 into gray code.
 The most significant bit (MSB) in the Gray code is the same as the
corresponding MSB in the binary number.
 Going from left to right, perform an EX-OR operation between the adjacent
pair of binary code bits to get the next code bit.
Binary : 1 1  0  1  0  1  0  1  1  0
         
Gray : 1 0 1 1 1 1 1 1 0 1
Gray code : 1011111101
6. Distinguish between 1’s and 2’s complements.
In 1's complement, a negative number is represented by taking all the bits in the
positive number and inverting them: 0's become 1's, 1's become 0's.
The 2‟s complement of a binary number is obtained by adding 1 to the 1‟s
complement of that number.
Therefore , 2’s complement = 1’s complement + 1
7. Define the terms disjunction and conjunction.
Disjunction
A coordinate construction that uses a disjunctive conjunction (usually or or either .
. . or) to indicate a contrast. The items on either side of the disjunctive conjunction
are called disjunctions.
Logical disjunction is an operation on two logical values, typically the values of
two propositions, that produces a value of false if and only if both of its operands
are false. More generally a disjunction is a logical formula that can have one or
more literals separated only by ORs. A single literal is often considered to be a
degenerate disjunction.

Conjunction
In logic and mathematics, a two-place logical operator and, also known as logical
conjunction, results in true if both of its operands are true, otherwise a value of
false.
The analogue of conjunction for a (possibly infinite) family of statements is
universal quantification, which is part of predicate logic.
8. What are the logic gates?
Logic gates are the basic elements that make up a digital system. The electronic
gate is a circuit that is able to operate on a number of binary inputs in order to
perform a particular logical function.
There are three basic logic gates each of which performs a basic logic function,
they are called NOT, AND and OR.
The three basic logic gates can be combined to provide more complex logical
functions. Four important logical functions are NAND, NOR, XOR and XNOR.
9. Define SOP and POS.
Sum Of Products
The logical sum of those fundamental products that produce output 1‟s in the truth
table. The corresponding logic circuit is an AND – OR circuit or equivalent
NAND – NAND circuit.
Product Of Sum
The logical product of those fundamental sums that produce output 0‟s in the truth
table. The corresponding logic circuit is an OR - AND circuit or equivalent NOR –
NOR circuit.

10.What do you meant by LSB and MSB?
The binary digits (0 and 1) are also called as bits. Thus binary system is a two bit
system.
The leftmost bit in a given binary number with the highest weight is called as
Most Significant Bit ( MSB ) whereas the rightmost bit in a given number with the
lowest weight is called as Least Significant Bit ( LSB ).
11.How will you find 2’s complement of a binary number?
i) Find 1‟s complement by replacing 0 by 1 and 1 by 0.
ii) Then add 1 to the 1‟s complement.
12.What are the alphanumeric codes?
 ASCII ( American Standard Code for Information Interchange )
 EBCDIC ( Extended Binary – Coded – Decimal Interchange Code )
 Five bit Baudot Code ( Hollerith Code )
13.Convert the given gray code (1011011101) into binary code.
The most significant bit (MSB) in the binary code is the same as the corresponding
bit in the gray code.
To obtain the next binary digit, perform an EX-OR operation between the bit just
written down and the next gray code bit.
Gray : 1 0 1 1 0 1 1 1 0 1
        
Binary : 1 1 0 1 1 0 1 0 0 1
Binary code : 1101101001

14.Define karnaugh maps.
The Karnaugh map is a graphical representation of a truth table that can be used
to reduce a logic circuit to its simpliest terms.
A Karnaugh map or k map is a pictorial form of truth table, in which the map diagram is
made up of squares, with each squares representing one minterm of the function.
15.Write the truth table of AND, OR and NAND gates.
Truth table of AND Truth table of OR Truth table of NAND
16.What is meant by excess 3 decimal numbers?
The excess – 3 code is a modified form of binary number. It is obtained by simply
adding „3‟ to a decimal number.
For example, to encoded the decimal number „4‟ into an excess 3 code, first add 3.
i.e. 4+3=7.
17.Define the law of Boolean algebra.
De Morgan suggested two theorems that form important part of Boolean algebra.
They are,
1) The complement of a product is equal to the sum of the complements.
(A.B)' = A'+B'
2) The complement of a sum term is equal to the product of the
complements.
(A+B)' = A'.B'
A B AB AB
0 0 0 1
0 1 0 1
1 0 0 1
1 1 1 0
A B AB
0 0 0
0 1 0
1 0 0
1 1 1
A B A+B
0 0 0
0 1 1
1 0 1
1 1 1

18.What is meant by universal gates?
A universal gate is a gate which can implement any Boolean function without
need to use any other gate type.
The NAND and NOR gates are universal gates.
In practice, this is advantageous since NAND and NOR gates are economical and
easier to fabricate and are the basic gates used in all IC digital logic families.
NAND and NOR both are called as universal gate. You can built any circuit or
design using either NAND or NOR, this will be so flexible to fabricate any circuit
in a single chip
19.What is Multivariable Theorem?
In the multivariable theorem, the multiple inputs (x,y and z) can be either a logic 0
or a logic 1.
Example:
x+ (y+z) = (x+y) +z = x+y+z (associative law)
x (yz) = (xy) z = xyz (associative law)
20.Convert (25)10 to binary.
25
14 – 1
7 – 0
3 – 1
1 – 1
2510 = 111012

21.What are the types of karnaugh map?
A Karnaugh map provides a pictorial method of grouping together expressions
with common factors and therefore eliminating unwanted variables. The Karnaugh
map can also be described as a special arrangement of a truth table.
 Minimization Technique
 2-Variable K-Map
 Grouping/Circling K-maps
 Inverse Function
22.Write the truth table of X-OR and X-NOR.
Truth table of X-OR Truth table of X-NOR
23.Distinguish between Boolean addition and Binary addition.
Boolean addition
Let us begin our exploration of Boolean algebra by adding numbers together:
The first three sums make perfect sense to anyone familiar with elementary
addition. The last sum, though, is quite possibly responsible for more confusion
A B AB
0 0 0
0 1 1
1 0 1
1 1 0
A B AB
0 0 1
0 1 0
1 0 0
1 1 1

than any other single statement in digital electronics, because it seems to run
contrary to the basic principles of mathematics. Well, it does contradict principles
of addition for real numbers, but not for Boolean numbers. Remember that in the
world of Boolean algebra, there are only two possible values for any quantity and
for any arithmetic operation: 1 or 0. There is no such thing as "2" within the scope
of Boolean values. Since the sum "1 + 1" certainly isn't 0, it must be 1 by process
of elimination.
Binary addition
These are computed without regard to the word size, hence there can be no sense
of "overflow." Work through the columns right to left, add up the ones and express
the answer in binary. The low bit goes in the sum, and the high bit carries to the
next column left.
Adding binary numbers is a very simple task, and very similar to the
longhand addition of decimal numbers. Unlike decimal addition, there is little
to memorize in the way of rules for the addition of binary bits:
0 + 0 = 0
1 + 0 = 1 & 0 + 1 = 1
1 + 1 = 10 & 1 + 1 + 1 = 11
24.What is meant by multilevel gates networks?
 Maximum number of gates cascaded in series between an input and output is
the number of levels of gates.
 Example: sum of products form is a two level gate network.
 We do not count inverter gates.
 The number of levels can be increased by factoring the sum of products
expression for AND-OR network, or by multiplying out some terms in the
product of sum expression for OR-AND network.

25.What are the drawbacks of K –map method?
 It is a manual technique. We cannot use computers for K – map solution.
 Minimization is extremely complicated as the number of variables exceeds 6.
 The simplification process depends heavily on human user‟s abilities.
26.State De Morgan’s law.
De Morgan's laws are rules relating the logical operators "and" and "or" in terms
of each other via negation. With two operands A and B:
27.What is the number of bits in ASCII code? What is the need for ASCII code?
 ASCII is abbreviated from American Standard Code for Information
Interchange. It is a seven bit code.
 This code allows the manufacturer to standardize I/O hardware such as
keyboards, video displays and so on.
28.Simplify : A AB A B  

UNIT – IV PART – A
1. Define half adder.
A half-adder is a combinational circuit that can be used to add two bits. It has two
inputs that represent the two bits to be added and two outputs, with one producing
the SUM output and the other producing the CARRY.
2. What do you meant by Encoder?
An encoder is a combinational circuit that converts binary information from 2n
input lines to a maximum of „n‟ unique output lines. The general structure of
encoder circuit is
3. List out the basic types of PLD.
 PROM - Programmable Read Only Memory
 GAL - Generic Array Logic
 PLA - Programmable logic arrays
 PAL - Programming Array Logic
 Structured ASIC
 CPLD - Complex Programmable Logic Devices
 FPSC - Field Programmable System Chip
 EPLD - Erasable Programmable Device
 FPOA - Field Programmable Object Array
 Mask-Programmable Gate Array (MPGA)
 FPGA - Field Programmable Gate Array

4. Define full adder.
A full adder is a combinational circuit that forms the arithmetic sum of three
Input bits. It consists of 3 inputs and 2 outputs.
Two of the input variables, represent the significant bits to be added. The third
input represents the carry from previous lower significant position.
The block diagram of full adder is given by,
5. What is PLA? How its differ from ROM?
The most flexible PLD is the programmable logic array (PLA), where both the
AND and OR arrays can be programmed. The product term in the AND array may
be shared by any OR gate to provide the required sum of product implementation.
A PLA or PAL (programmable array logic) device is like a baby FPGA which can
be programmed to perform basic logic functions. Tens to hundreds of gates on a
PAL can be connected to perform simple logic functions. A PAL is often read
only, in that after programming you have to perform a complete erase to update it.
A ROM does not perform logic functions, but stores data. A type of ROM might
be EPROM, erasable programmable read only memory.
You can use a ROM as a logic device, by implementing a simple logic table
lookup. Like a truth table. However, it is somewhat wasteful and expensive to do
this compared to actually using a PAL or even a CPLD/FPGA.

6. Define half subtractor.
A half-subtractor is a combinational circuit that can be used to subtract one binary
digit from another to produce a DIFFERENCE output and a BORROW output.
The BORROW output here specifies whether a „1‟ has been borrowed to perform
the subtraction.
7. What is meant by decoder?
A decoder is a combinational circuit that decodes the binary information on „n‟
input lines to a maximum of 2n
unique output lines.
The general structure of decoder circuit is
8. Differentiate sum and carry.
The sum output is 1 when any of inputs (A and B) is 1 and the carry output is 1
when both the inputs are 1.
9. What is meant by a full subtractor? Draw a full subtractor circuit.
A full subtractor performs subtraction operation on two bits, a minuend and a
subtrahend, and also takes into consideration whether a „1‟ has already been
borrowed by the previous adjacent lower minuend bit or not.

As a result, there are three bits to be handled at the input of a full subtractor,
namely the two bits to be subtracted and a borrow bit designated as Bin. There are
two outputs, namely the DIFFERENCE output D and the BORROW output Bo.
The BORROW output bit tells whether the minuend bit needs to borrow a „1‟ from
the next possible higher minuend bit.
10.Draw the logic diagram of half – adder.
11.Write a design procedure of combinational circuits.
The Design of the combinational circuits starts from the verbal outline of the
problem and ends in a logic circuit diagram or a set of Boolean functions from
which the logic diagram can be easily obtained.
Steps:
 Determine Number of inputs and number of outputs to the system and assign
names to the input and output variables.
 Setup a truth table which specifies the output(s) as a function of the input
variables.
 Drive the Algebraic Expression for the outputs.
 Drive simplified algebraic expressions for the output functions using K-maps,
the Tabulation method, or other similar procedures.
 Draw the circuit diagram.

12.Differentiate decoder and encoder.
13.What is the similar between a half adder and a half subtractor?
In this both half adder and subtractor produce a same output for sum and difference.
14.What do you meant by Comparator?
A comparator is a circuit which compares a signal voltage applied to one input of
an op-amp with known reference voltage at other input. It is basically an op-amp
with output + (Vsat).

15.Define Multiplier.
A multiplexer is a digital switch which allows digital information from several
sources to be routed into a single output line.
The basic multiplexer has several data-input lines and a single output line. The
selection of a particular input line is controlled by a set of selection lines.
Normally there are 2n
input lines and n selection lines.
16.What is meant by multiple bit adders?
A multi-bit adder adder does not require much more circuitry. The only
improvement that it must have over the 1-bit adder is that it has to account for the
carry bit coming from the previous bit addition. The change is that after the first
adder, there needs to be another one (since we are adding the carry bit in addition
to the regular two bits). Since there can be only one carry bit for the operation, and
both of the adders can yield a carry bit, both carry bits need to be OR'ed together
to be taken into account.
17.Write short notes on ROM.
A Read-only memory (ROM) is essentially a memory device in which permanent
binary information is stored. The binary information must be specified by the
designer and is then embedded in the unit to form the required interconnection
pattern. Once the pattern is established, it stays within the unit even when power is
turned OFF and ON again.
There are four types.
 Masked ROM,
 Programmable ROM (PROM)
 Erasable ROM (EPROM)
 Electrically Erasable ROM (EEROM)

18.Define propagation delay.
Speed of operation expressed interms of propagation delay. It is defined as the
time taken for the output of a gate to change after the inputs have charged. It is
measured in „ns‟.
19.What is PAL? How it differs from ROM and PLA?
The PAL consists of a programmable array of AND gates that connects to a fixed
array of gates. This structure allows any sum - of – products (SOP) logic
expression with a defined number of variables to be implemented.
20.What is meant by Combinational circuits?
A combinational circuit consists of logic gates whose outputs at anytime are
determined directly from the present combination of inputs, without regard to
previous inputs.
21.Draw the logic diagram of BCD to Excess 3 code converter.

22.Write the truth table of Full – adder.
23.What is meant by binary decoder?
A binary decoder has „n‟ bit binary input and a one activated output out of 2n
outputs. A binary decoder is used when it is necessary to activate exactly one of 2n
outputs based on an n – bit input value.
24.What are the difference between decoder and demultiplexer?
Decoder:
It is a digital device that converts coded information into another code or
non – coded form. It is a multi – input multi – output logic circuit. The number of
outputs is greater than the number of inputs (n:2n
).
Demultiplexer:
Demultiplex means “one into many”. A demultiplexer is a combinational
logic circuits with one input many outputs. But applying control signal, we can
steer the input signal to one of the output lines.

25.Write short notes on priority encoder.
A priority encoder is an encoder that includes the priority function. The operation
of the priority encoder is such that if two or more inputs are equal to 1 at the same
time, the input having the highest priority will take precedence.

UNIT – V PART – A
1. What is sequential circuit?
The Circuits whose outputs depend not only on the present input value but
also the past input value are known as sequential logic circuits.
The mathematical model of a sequential circuit is usually referred to as a
sequential machine.
2. Define flip-flop.
Flip-Flops are synchronous bistable devices (has two outputs Q and Q‟). An
edge-triggered Flip-Flop changes state either at the positive edge (rising edge)
or at the negative edge (falling edge) of the clock pulse and is sensitive to its
inputs only at this transition of the clock.
3. What is shift registers?
A register capable of shifting its binary information in one or both
directions is called shift register. The logical configuration of a shift register
consists of a chain of flip-flops in cascade, with the output of one flip-flop
connected to the input of the next flip-flop. All flip-flops receive common
clock pulses, which activate the shift from one stage to the next.
4. List out the various types of flip-flop.
All flip-flops can be divided into four basic types.
SR flip - flops
JK flip - flops
D flip - flops
T flip - flops

5. Define metastability.
A state which exists between either "valid" digital logic states {an undefined
voltage}.
(Or)
Whenever there are setup and hold time violations in any flip-flop, it enters a
state where its output is unpredictable: this state is known as metastable state
(quasi stable state); at the end of metastable state, the flip-flop settles down to
either '1' or '0'. This whole process is known as metastability.
6. Write the specifications of shift register IC’s
Voltage at Any Pin : - 0.3V to VCC +0.3V
Operating Temperature Range : -55°C to +125°C
Storage Temperature Range : -65°C to +150°C
Absolute Maximum VCC : 18V
Power Dissipation
Dual-In-Line : 700 mW
Small Outline : 500 mW
Operating VCC Range : 3V to 15V
Lead Temperature (Soldering, 10 sec.): 260°C
7. List the function of asynchronous inputs.
The memory commonly used in asynchronous sequential circuits is
time-delayed devices, usually implemented by feedback among logic
gates.
Thus, asynchronous sequential circuits may be regarded as
combinational circuits with feedback.

8. What is meant by trigger?
The state of a flip-flop is switched by a momentary change in the input
signal. This momentary change is called a trigger and the transition it
causes is said to trigger the flip-flop.
9. Derive the characteristic equation of T flip flop.
Characteristic equation of T flips flop:
Characteristic equation: Qn+1= TQn’+ T’Qn
10.What is universal shift register?
The register operates in any of the modes, like SISO, SIPO, PISO, PIPO or
bidirectional. If a register can be operated in all the five possible ways, it is
known as universal shift register.
(Or)
A register which is capable of shifting data both to the right and left is called a
bi-directional shift register. A register that can shift only one direction is called
a uni-directional shift register. If the register has shift and parallel load
capabilities, then it is called a shift register with parallel load or universal shift
register.

11.Draw a block diagram of sequential circuit.
12.Derive the characteristic equation of SR flip flop.
13.Write the specifications of counter IC’s.
Specification
* Logic Base Number : 4516
* Logic IC Function : Binary Up/Down Counter
* Package/Case : 16-DIP
* Supply Voltage Min : 3V
* Supply Voltage Max : 20V
* Operating Temperature Range : -55°C to +125°C

14.Define a register and shift register.
Register
Memory elements capable of storing one binary word. It consists of a group of
Flip-flops, which store the binary information.
Shift Register
A register capable of shifting its binary information in one or both directions is
called shift register.
15.Write down the 74194 IC specifications.
Logic Base Number : DM54194
Logic IC Function : 4- bit bidirectional universal Shift Register
Supply Voltage : 7V
Input Voltage : 5.5V
Package/Case : 16-DIP
Operating Free Air Temperature Range: -55°C to +125°C
Storage Temperature Range: -65°C to +150°C
16.What are the applications of flip-flop?
 Counters
 Frequency dividers
 Shift registers
 Storage registers
17.What is meant by synchronous circuit?
Synchronous sequential circuits change their states and output values at
discrete instants of time, which are specified by the rising and falling
edge of a free-running clock signal.
18.Define clock period.
Clock period is the time between successive transitions in the same
direction, that is, between two rising and two falling edges.

19.What is meant by state table?
A table, which consists time sequence of inputs, outputs and flip-flop states, is
called state table. Generally it consists of three section present state, next state
and output.
20.Draw a logic diagram of counters.
Decade counter Logic diagram
Logic diagram of a 3-bit binary counter
21.What is meant by state diagram?
A graphical representation of a state table is called a state diagram.
In the state diagram, a state is represented by a circle and the transition
between states is indicated by directed line connecting the circles. There is no
difference between a state table and a state diagram except in the manner of
representation.

22.Draw the state diagram of JK flip flop.
23.What is meant by asynchronous circuit?
In asynchronous sequential circuits, the transition from one state to
another is initiated by the change in the primary inputs; there is no
external synchronization.
24.Derive the characteristic equation of JK flip flop.
Characteristic equation of JK flips flop:
Characteristic Equation: Qn+1= JQ’+ K’Q
25. What are the bistable elements?
It is a circuit having two stable conditions (states). It can be used to store
binary symbols.

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