Anna University, Chennai
(IC FABRICATION AND CIRCUIT CONFIGURATION FOR LINEAR ICs)
(TWO MARKS QUESTION AND ANSWERS)
1. Define an Integrated circuit.
An integrated circuit(IC) is a miniature ,low cost electronic circuit consisting of active
and passive components fabricated together on a single crystal of silicon.The active components
are transistors and diodes and passive components are resistors and capacitors.
2. Mention the advantages of integrated circuits over discrete
components. (May 2010)
*Miniaturisation and hence increased equipment density.
*Cost reduction due to batch processing.
*Increased system reliability due to the elimination of soldered
*Improved functional performance.
*Matched devices. *Increased operating
*Reduction in power consumption.
3. Define sheet resistance.(may 2010)
Sheet resistance is defined as the resistance in ohms /square offered by
the diffused area.
4. What is the use of buried n+ layer in monolithic IC transistor?(MAY
The buried n+ layer provides a low resistance path in the active collector
region for the flow of current
5. What are the two common methods for obtaining integrated capacitors?
• Monolithic junction capacitor
• Thin-flim capacitor
6.What are the basic processes involved in fabricating ICs using planar
1.Silicon wafer (substrate) preparation
9.Assembly processing & packaging
7. What is active load? Where it is used and why? (MAY/JUNE 2010 )
The active load realized using current source in place of the passive load in the collector
arm of differencial amplifier makes it possible to achieve high voltage gain without requiring large power supply voltage.
8. Why open loop OP-AMP configurations are not used in linear
The open loop gain of the op-amp is not a constant and it varies with changing the
temperature and variations in power supply.Also the bandwidth of the open loop op-amp is
negligibly small.For this reasons open loop OP-AMP configurations are not used in linear
9. Define virtual ground of a OP-Amp? (may/june 2010)
A virtual ground is a ground which acts like a ground. It is a point that is at the fixed ground potential (0v),though it is not practically connected to the actual ground or common
terminal of the circuit.
10.List out the steps used in the preparation of Si - wafers.
1.Crystal growth &doping
2.Ingot trimming & grinding
4.Wafer policing & etching
11. Write the basic chemical reaction in the epitaxial growth process of pure
The basic chemical reaction in the epitaxial growth process of pure silicon is the
hydrogen reduction of silicon tetrachloride.
1200C SiCl4 + 2H2 <> Si + 4 HCl
12. What are the two important properties of SiO2?
1.SiO2 is an extremely hard protective coatng & is unaffected by almost all reagents except by hydrochloric acid. Thus it stands against any contamination.
2.By selective etching of SiO2 , diffusion of impurities through carefully defined
windows in the SiO2 can be accomplished to fabricate various components.
13. Explain the process of oxidation.
The silicon wafers are stacked up in a quartz boat & then inserted into quartz furnace
tube. The Si wafers are raised to a high temperature in the range of 950 to 1150oC & at the same
time, exposed to a gas containing O2 or H2O or both.The chemical action is
Si + 2H2O > Si O2+ 2H2
14. What is meant by molecular beam epitaxy(MBE)?
In the molecular beam epitaxy, ilicon along with dopants is evaporated.The evaporated species are transported at a relatively high velocity in a vacuum to the substrate.The relatively
low vapour pressure of silicon & the dopants ensures condensation on a low temperature
substrate.Usually, silicon MBE is performed under ultra high vacuum (UHV) condition of 10-8
to 10-10 Torr
15. What are the advantages and limitations implantation of ion
• Accurate control over doping
• Very good reproducibility
• Precise resistance value
• A room temperature process
• Annealing at higher temperature is required for avoiding the crystal
• The possibility of dopant implanting through various layers wafer.
16.Define input offset voltage.
A small voltage applied to the input terminals to make the output voltage as zero when
the two input terminals are grounded is called input offset voltage.
17. Define input offset current. State the reasons for the offset currents at the input of the op-amp.
The difference between the bias currents at the input terminals of the op-amp is called as
input offset current. The input terminals conduct a small value of dc current to bias the input
transistors.Since the input transistors cannot be made identical,there exists a difference in bias
18. Define CMRR of an op-amp.(DEC 09)
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.
19.What are the applications of current sources?
Transistor current sources are widely used in analog ICs both as biasing elements and as
load devices for amplifier stages.
20. Justify the reasons for using current sources in integrated circuits.
(i) Superior insensitivity of circuit performance to power supply variations and
(ii) More economical than resistors in terms of die area required to provide bias
currents of small value.
(iii) When used as load element, the high incremental resistance of current source
results in high voltage gain at low supply voltages.
21. What is the advantage of widlar current source over constant current
Using constant current source output current of small magnitude(microamp range) is not
attainable due to the limitations in chip area. Widlar current source is useful for obtaining small
output currents.Sensitivity of widlar current source is less compared to constant current source.
22.Mention the advantages of Wilson current source.
(i) Provides high output resistance.
(ii) Offers low sensitivity to transistor base currents
Sensitivity is defined as the percentage or fractional change in output current per percentage or fractional change in power-supply voltage.
24.What are the limitations in a temperature compensated zener-reference source?
A power supply voltage of atleast 7 to 10 V is required to place the diode in the
breakdown region and that substantial noise is introduced in the circuit by the avalanching diode.
25.In practical op-amps, what is the effect of high frequency on its performance?
The open-loop gain of op-amp decreases at higher frequencies due to the presence of
parasitic capacitance. The closed-loop gain increases at higher frequencies and leads toinstability.
26. What is the need for frequency compensation in practical op-amps?
Frequency compensation is needed when large bandwidth and lower closed loop gain is
desired. Compensating networks are used to control the phase shift and hence to improve the stability.
27.Define slew rate.(MAY 2010)
The slew rate is defined as the maximum rate of change of output Voltage caused by a
step input voltage.An ideal slew rate is infinite which means that op-amp's output voltage should
change instantaneously in response to input step voltage.
28.Why IC 741 is not used for high frequency applications?
IC741 has a low slew rate because of the predominance of capacitance present in the
circuit at higher frequencies. As frequency increases the output gets distorted due to limited slew rate.
29. What causes slew rate?(DEC 09)
There is a capacitor with-in or outside of an op-amp to prevent oscillation. The capacitor
which prevents the output voltage from responding immediately to a fast changing input.
30. What happens when the common terminal of V+ and V- sources is not
grounded? (DEC 09)
If the common point of the two supplies is not grounded, twice the supply voltage will
get applied and it may damage the op-amp.
APPLICATION OF OPERATIONAL AMPLIFIERS
1. Mention some of the linear applications of op - amps. (DEC 09)
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.
2. Mention some of the 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.
3.What are the areas of application of non-linear op- amp circuits?
1. Industrial instrumentation
3. Signal processing
4. What is voltage follower?(MAY 2010)
A circuit in which output folloes the input is called voltage follower.
5. What is the need for an instrumentation amplifier?
In a number of industrial and consumer applications, the measurement of physical quantities is usually done with the help of transducers. The output of transducer has to be
amplified So that it can drive the indicator or display system. This function is performed by an
6. List the features of instrumentation amplifier:
1.High gain accuracy
3.High gain stability with low temperature co-efficient
4Low dc offset
5.Low output impedance
7. What are the applications of V-I converter?
₃ 1.Low voltage dc and ac voltmeter
2.L E D
3.Zener diode tester
8. Define Bandpass filter. (MAY 2010)
The bandpass filter is the combination of high and low pass filters, and this allows a specified range of frequencies to pass through.
9. Write transfer function of op amp as an integer. (MAY 2010)
The transfer function of the integer is
10. What do you mean by a precision diode?
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.
11. Write down the applications of precision diode.
1.Half - wave rectifier
2.Full - Wave rectifier
3.Peak - value detector
12. Define Logarithmic and antilogarithmic amplifier. (MAY 2010)
When a logarithmic PN junction is used in the feedback network of op-amp, the circuit
exhibits log or antilog response.
The logarithmic amplifier is a current to voltage converter with the
transfer characteristics v0=vi In(If/Ii)
Antilog amplifier is a decoding circuit which converts the logarithmically encoded
signal back to the original signal levels as given by vl=vR10-kvi
13. Differentiate Schmitt trigger and comparator. (MAY 2010)
1. It compares the input signal with
references voltage then yields the
2. It need not consist of feedback
3. comparator output need not to be
14. List the applications of Log amplifiers:
1. It operates between two reference
points namely UTP<P.
2. It employs positive feedback
3. Its output is square wave.
1.Analog computation may require functions such as lnx, log x, sin hx etc. These
functions can be performed by log amplifiers
2.Log amplifier can perform direct dB display on digital voltmeter and spectrum analyzer
3.Log amplifier can be used to compress the dynamic range of a signal
15. What are the limitations of the basic differentiator circuit?
1.At high frequency, a differentiator may become unstable and break into oscillations
2,The input impedance decreases with increase in frequency , thereby making the
circuit sensitive to high frequency noise.
16. Write down the condition for good differentiation .
1.For good differentiation, the time period of the input signal must be greater than or
equal to Rf C1
2.T > R f C1 Where, Rf is the feedback resistance
3.Cf is the input capacitance
17. What is a comparator?(MAY 2010)
A comparator is a circuit which compares a signal voltage applied at one input of an op-
amp with a known reference voltage at the other input. It is an open loop op - amp with output +
18. What are the applications of comparator?
1.Zero crossing detector
3.Time marker generator
19. What is a Schmitt trigger?(DEC 09,MAY 10)
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.
20. What is a multivibrator?
Multivibrators are a group of regenerative circuits that are used extensively in timing
applications. It is a wave shaping circuit which gives symmetric or asymmetric square output. It
has two states either stable or quasi- stable depending on the type of multivibrator.
21. What do you mean by monostable multivibrator?
Monostable multivibrator is one which generates a single pulse of specified
duration in response to each external trigger signal. It has only one stable state.
Application of a trigger causes a change to the quasi-stable state.An external trigger signal
generated due to charging and discharging of the capacitor produces the transition to the original stable state
22. What is an astable multivibrator?
Astable multivibrator is a free running oscillator having two quasi-stable states. Thus,
there is oscillations between these two states and no external signal are required to produce the change in state.
23. What is a bistable multivibrator?
Bistable multivibrator is one that maintains a given output voltage level unless an
external trigger is applied . Application of an external trigger signal causes a change of state,
and this output level is maintained indefinitely until an second trigger is applied . Thus, it
requires two external
triggers before it returns to its initial state
24. Mention any two audio frequency oscillators.
i. RC phase shift oscillator
ii. Wein bridge oscillator
25. What are the characteristics of a comparator?
1.Speed of operation
3.Compatibility of the output
26. What is a filter?
Filter is a frequency selective circuit that passes signal of specified band of frequencies
and attenuates the signals of frequencies outside the band
27. What are the demerits of passive filters?
Passive filters works well for high frequencies. But at audio frequencies, the inductors
become problematic, as they become large, heavy and expensive.For low frequency applications,
more number of turns of wire must be used which in turn adds to the series resistance degrading
inductor's performance ie, low Q, resulting in high power dissipation.
28. What are the advantages of active filters?
Active filters used op- amp as the active element and resistors and capacitors as passive
29. Define low pass filter.
A low pass filter allows only low frequency signals upto a certain break point fH to pass
30. Define High pass filter.
A low pass filter allows only high frequency signals upto a certain break point fH to pass
ANOLOG MULTIPLIER AND PLL
1. List the basic building blocks of PLL:
2.Low pass filter
4.Voltage controlled oscillator
2. Define FSK modulation.(MAY 2010)
FSK is a type of frequency modulation ,in which the binary data or code is
transmitted by means of a carrier frequency that is shifted between two fixed
frequency namely mark(logic1) and space frequency(logic 0).
3. What is analog multiplier?(MAY 2010)
A multiplier produces an output v0, which is proportional to the product of two inputs vx
4. List out the various methods available for performing for analog multiplier.
• Logarithmic summing technique
• Pulse height /width modulation technique
• Variable transconductance technique
• Multiplication using gilbert cell
• Multiplication technique using transconductance technique
5. Mention some areas where PLL is widely used. (DEC 2009)
2. Satellite communication systems
3. Air borne navigational systems
4. FM communication systems
6. What are the three stages through which PLL operates?
7. Define lock-in range of a PLL. (MAY 2010)
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 theVCO free running frequency.
8. Define capture range of PLL. (MAY 2010)
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
9.Define Pull-in time.
The total time taken by the PLL to establish lok 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
10. Write the expression for FSK modulation.(MAY 2010)
11. Define free running mode .(MAY 2010)
An interactive computer mode that allows more than one user to have
simultaneous use of a program.
12. For perfect lock, what should be the phase relation between the incoming signal
and VCO output signal?
The VCO output should be 90 degrees out of phase with respect to the input
13.Give the classification of phase detector:
1.Analog phase detector .
2.Digital phase detector
14.What is a switch type phase detector?
An electronic switch is opened and closed by signal coming from VCO and the input
signal is chopped at a repetition rate determined by the VCO frequency.This type of phase
detector is called a half wave detector since the phase information for only one half of the input
signal is detected and averaged.
15.What are the problems associated with switch type phase detector?
1.The output voltage Ve is proportional to the input signal amplitude.This is undesirable
because it makes phase detector gain and loop gain dependent on the input signal amplitude.
2.The output is proportional to cosf making it non linear.
16.What is a voltage controlled oscillator?
Voltage controlled oscillator is a free running multivibrator operating at a set frequency
called the free running frequency.This frequency can be shifted to either side by applying a dc
control voltage and the frequency deviation is proportional to the dc control voltage.
17. Define Voltage to Frequency conversion factor.
Voltage to Frequency conversion factor is defined as,
Kv = fo / Vc= 8fo /Vcc
Vc is the modulation voltage
fo frequency shift
18.What is the purpose of having a low pass filter in PLL?
*It removes the high frequency components and noise.
*Controls the dynamic characteristics of the PLL such as capture range,
lock-in range,band-width and transient response.
*The charge on the filter capacitor gives a short- time memory to the PLL
19.Discuss the effect of having large capture range.
The PLL cannot acquire a signal outside the capture range, but once captured, it
will hold on till the frequency goes beyond the lock-in range.Thus , to increase the ability
of lock range,large capture range is required.But, a large capture range will make the PLL more
susceptible to noise and undesirable signal.
20.Mention some typical applications of PLL:
• Frequency multiplication/division
• Frequency translation
• AM detection
• FM demodulation
• FSK demodulation.
21.What is a compander IC? Give some examples.(DEC 2009)
The term companding means compressing and expanding.In a communication system, the audio signal is compressed in the transmitter and expanded in the receiver. Examples : LM 2704- LM 2707 ; NE 570/571.
22.What are the merits of companding?
*The compression process reduces the dynamic range of the signal before it
*Companding preserves the signal to noise ratio of the original signal and avoids
non linear distortion of the signal when the input amplitude is large.
*It also reduces buzz,bias and low level audio tones caused by mild interference