Ex06 - Answer Key-lic Te1 210114

KONGUNADU COLLEGE OF ENGINEERING AND TECHNOLOGY

NAMAKKAL- TRICHY MAIN ROAD, THOTTIAM

DEPARTMENT OF ECE

TERMINALEXAMINATION-I

EC 2254 LINEAR INTEGRATED CIRCUITS

IV-SEMESTER

PART A

1. What is an integrated circuit?An integrated circuit(IC) is a miniature, low cost electronic circuit consisting of activeand 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. Name the different methods used in fabrication of integrated resistors.Diffused ResistorEpitaxial ResistorPinched ResistorThin Film Resistor

3. What is current mirror?A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading

4. Define CMRR of an OP-AMP. 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.

5. State the limitations of discrete circuits.As IC is small in size, it is unable to dissipate large amount of power. Increase in current may produce enough heat which may destroy the device.At present, coils, inductors and transformers cannot be produced in IC form.

6. Mention two important features of an instrumentation amplifier.High gain accuracy, high CMRR, high gain stability with low temperature co-

efficient, low dc offset, low output impedance

7. How does precision rectifier differ from the conventional rectifier?In ordinary rectifier the cut in voltage is 0.6 V of the diode. The circuit acts like ideal diode and can be placed in the feedback of an op-amp. Here the cut in voltage is divided in to open loop gain(Vr).The output vo follows as both positive half cycle and negative half cycle and capable of rectifying input signal order of milli volt.

8. Compare the performance of inverting and non inverting operational amplifier configurations.INVERTING AMPLIFIER

GAIN (AV) = -R2 / R1

NON-INVERTING AMPLIFIER

GAIN (AV) = 1+(R2 / R1)

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ANSWER KEY

9. Why is frequency compensation required in operational amplifier?The compensating network is connected externally to the op amp for modifying the response suiting the requirement. The compensating network alters the response so that -20 Db/decade of roll-off rate is achieved over a broad range of frequency.

10. Draw and write equation of an integrator using an op-amp.

PART-B11. a. Describe in detail the processing steps involved in the fabrication of monolithic IC.

MANUFACTURING PROCESSES OF MONOLITHIC ICsSilicon wafer preparationEpitaxial growthOxidationPhotolithographyDiffusionMetallizationCircuit probingScribing and separating into chipsMounting and packagingEncapsulation

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EPITAXIAL GROWTH

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11.(b) Draw the circuit diagram of the output stage of the IC 741 OP AMP and explain its

operation with clearly indicating the protection mechanisms indicated.

12.(a) With a neat circuit diagram and with necessary equations, explain the concept of

Widlar Current source and Wilson current source used in op-amp circuit.

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12.(b)(i) Draw the circuit of an Op-amp differential amplifier and give the expression for

CMRR.

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(ii) Define Slew Rate. Explain the cause of slew rate and derive an expression for Slew rate

for an op-amp voltage follower.

For 741 IC, the maximum internal capacitor charging current is limited to about 15μA. So the

slew rate of 741 IC is

SR = dVc/dt |max = Imax/C .

For a sine wave input, the effect of slew rate can be calculated as consider volt follower ->

The input is large amp, high frequency sine wave .

If Vs = Vm Sinwt then output V0 = Vm sinwt . The rate of change of output is given by

dV0/dt = Vm w coswt.

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The max rate of change of output across when coswt =1

(i.e) SR = dV0/dt |max = wVm.

SR = 2∏fVm V/s = 2∏fVm v/ms.

13.(a) With the help of circuits and necessary equations, explain how log and antilog Computations are performed using IC 741.

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ANTILOG AMPLIFIER

13.(b) With neat circuit diagrams and mathematical expressions, explain the operation of the following op-amp applications:

(i) Scale changer (ii) Voltage follower (iii) Non-Inverting adder (iv) Integrator.

Non-Inverting adder

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SCALE CHANGER/INVERTER

INTEGRATOR

14.(a) With relevant circuits, explain the circuit of a Voltage to current converters if the load is (i)Floating (ii) Grounded

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14.(b) (i) Explain the steps involved in the design of a band pass filter using OPAMP.

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14.b(ii) Write a note on Schmitt trigger.

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SCHMITT TRIGGER

15.(a)(i)Explain the positive clipper circuit using an op-amp and diode with neat diagrams.

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15.a.(ii)Explain the operation of Instrumentation amplifier.The important features of an instrumentation amplifier are1. high gain accuracy2. high CMRR3. high gain stability with low temperature coefficient4. low output impedance5.low dc offset

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15. (b) Discuss the DC and AC characteristics of op-amp.AC Characteristics:

For small signal sinusoidal (AC) application one has to know the ac

characteristics such as frequency response and slew-rate.

Frequency Response:

Slew Rate:

Another important frequency related parameter of an op-amp is the slew rate. (Slew rate is

the maximum rate of change of output voltage with respect to time. Specified in V/μs).

Reason for Slew rate:

There is usually a capacitor within 0, outside an op-amp oscillation. It is this capacitor which

prevents the o/p voltage from fast changing input. The rate at which the volt across the

capacitor

I -> Maximum amount furnished by the op-amp to capacitor C. Op-amp should have the

either a higher current or small compensating capacitors.

For 741 IC, the maximum internal capacitor charging current is limited to about 15μA. So the

slew rate of 741 IC is

SR = dVc/dt |max = Imax/C .

For a sine wave input, the effect of slew rate can be calculated as consider volt follower ->

The input is large amp, high frequency sine wave .

If Vs = Vm Sinwt then output V0 = Vm sinwt . The rate of change of output is given by dV0/dt = Vm w coswt.

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increases is given by

dVc/dt = I/C --------(1)

The max rate of change of output across when coswt =1(i.e) SR = dV0/dt |max = wVm.

SR = 2∏fVm V/s = 2∏fVm v/ms.

Thus the maximum frequency fmax at which we can obtain an undistorted output volt of peak

value Vm is given by

fmax (Hz) = Slew rate/6.28 * Vm .

called the full power response. It is maximum frequency of a large amplitude sine wave with

which op-amp can have without distortion.

DC Characteristics of op-amp:Current is taken from the source into the op-amp inputs respond differently to current

and voltage due to mismatch in transistor.

DC output voltages are,

1. Input bias current

2. Input offset current

3. Input offset voltage

4. Thermal drift

Input bias current:

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Input offset current 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 currents 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 Input bias current Input bias current IB as the average value of the base currents entering into terminal of an op-amp IB=IB

+ + IB-

Thermal drift Bias current, offset current and offset voltage change with temperature. A circuit carefully nulled at 25oc may not remain so when the temperature rises to 35oc. This is called drift.

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