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Linear Integrated Circuits : Basics of Operational Amplifiers : Book Bank, Important Questions and Answers: Linear Integrated Circuits : Basics of Operational Amplifiers

**BASICS OF OPERATIONAL AMPLIFIERS**

**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.**

*Miniaturization
and hence increased equipment density.

*Cost
reduction due to batch processing.

*Increased
system reliability due to the elimination of soldered joints.

*Improved
functional performance.

*Matched
devices. *Increased operating speeds.

*Reduction
in power consumption.

**3. Define sheet resistance.**

Sheet
resistance is defined as the resistance in ohms /square offered by the diffused

**4. What is the use of buried n+
layer in monolithic IC transistor? **

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 is active load? Where it is
used and why? **

The
active load realized using current source in place of the passive load in the
collector arm of differential amplifier makes it possible to achieve high
voltage gain without requiring large power supply voltage.

**7. Why open loop OP-AMP
configurations are not used in linear Applications? **

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 applications.

**8. Define virtual ground of a
OP-Amp?**

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.

**9. 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.

**10. 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 currents.

**11. 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

**12. 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.

**13. What is the advantage of widlar
current source over constant current source? **

Using
constant current source output current of small magnitude(micro amp 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.

**14. Mention the advantages of Wilson
current source. **

(i)
Provides high output resistance.

(ii)
Offers low sensitivity to transistor base current

**15. Define sensitivity **

Sensitivity
is defined as the percentage or fractional change in output current per
percentage or fractional change in power-supply voltage.

**16. What are the limitations in a
temperature compensated Zener-reference source? **

A
power supply voltage of at least 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

**17. 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 to instability.

**18. 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.

**19. Define slew rate. **

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.

**20. 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.

**21. What causes slew rate? **

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.

**22. What happens when the common
terminal of V+ and V- sources is not grounded?
**

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.

** 23. What is an integrated circuit?**

IC
is a miniature low cost electronic circuit consisting of active and passive
components that are irreparably joined together on a single crystal chip of
silicon.

**24. What is current mirror? **

A
constant current source (current mirror) uses a transistor in the active mode
of operation where the collector current is relatively independent of the
collector voltage.

**25. Define slew rate and CMRR.**

Slew
rate is the maximum rate of change of output voltage caused by a step input
voltage.

CMRR
is defined as the ratio of differential mode gain to common mode gain.

**26. Why are active loads preferred
than passive loads in the input stage of an operational amplifier?**

A
large value of resistance requires large chip area.

**27. Name the different methods used
in fabrication of integrated resistors. **

Diffused
resistor, Epitaxial resistor, pinched resistor & Thin film resistor.

**28. What are the two requirements to
be met for a good current source? **

Transistors
should be matched in order to have same Vbe.

**29. State the limitations of
discrete circuits. **

Operating
speed is low due to parasitic capacitance effect.

Power
consumption is more.

**30. What is meant by monolithic IC?**

An
integrated circuit or monolithic integrated circuit (also referred to as an IC,
a chip, or a microchip) is a set of electronic circuits on one small plate
("chip") of semiconductor material, normally silicon.

**31. What are the characteristics of
an ideal op-amp? **

•
Infinite open-loop gain G = Vout / 'v

•
Infinite input impedance Rin, and so zero input current.

•
Zero input offset voltage.

•
Zero output impedance.

•
Infinite bandwidth with zero phase shift and infinite slew rate.

**32. A differential amplifier has a
differential voltage gain of 2000 and common mode gain of 0.2. Determine CMRR
in db.**

CMRR=
20 LOG [2000/0.2] =80 dB

**33. Mention two advantages of active
load over passive load in an operational amplifier.**

**34. Define input bias current and
input offset current of an operational amplifier.**

*Input
Bias Current:*

Ideally,
no current flows into the input terminals of an op amp. In practice, there are
always two input bias currents, IB+ and IB-

Input
bias current IB=IB+ (-) IB- / 2

*Input
Offset Current:*

One
of the practical op –amp limitations that the input bias current for the two
inputs may be slightly different. Even though the inputs are designed to be
symmetrical, slight differences which occur in the manufacturing process may
give slightly different bias currents. This offset current is typically on the
order of a tenth of the input bias current, with 10nA being a representative
offset current for a 741.

Input
Offset Current Ios= |IB+ (-) IB-|

**35. The output of an operational
amplifier is 5V peak sine wave whose slew rate is 0.5V/μs. Find the maximum
allowable frequency of the signal. **

f_{m}
=S/2πV_{m}

Given
S=0.5V/µs V _{m}=5V

fm=0.5*10^{-6}/2π*5=1.59*10^{-08}

**36. Find the maximum frequency for
sine wave output voltage 10V peak to Peak with an op-amp whose slew rate is
1V/µs.**

f_{m}
=S/2πV_{m}

Given
S=1V/µs V _{m}=10V

fm=1*10^{-6}/2π*10=1.59*10^{-08}

**37. Differentiate the ideal and
practical characteristics of an op-amp.**

*Ideal
operational amplifier are characterized by*

•
Infinite gain

•
Infinite input resistance

•
Zero output resistance (order of 10’s of ohms)

•
Infinite bandwidth (practically restricted by slew rate)

•
Linear irrespective of entire analog signal range No offsets and, so on.

*Practical
operational amplifier are characterized by*

•
Input resistance is of the of mega ohm Order due to differential stage at the
front end

•
Output resistance order of tens of ohms.

•
Practically bandwidth of Opamp restricted by slew rate.

**38. What
do you mean by a band-gap referenced biasing circuit?**

The
biasing sources referenced to VBE has a negative temperature coefficient and VT
has a positive temperature co-efficient. Band gap reference circuit is one in
which the output current is referenced to a composite voltage that is a
weighted sum of VBE and VT so that by proper weighting, zero temperature
coefficient can be achieved.

**39. Define thermal drift.**

The
bias current, offset current & offset voltage change with temperature. A
circuit carefully nulled at 25oC may not remain so when the temperature raises
to 35oC.This is called thermal drift. Often, offset current drift is expressed
in nA/ oC and offset voltage drift in mV/ 0C.

**40. Define supply voltage rejection
ratio (SVRR)**

The
change in OPAMP’s input off et voltage due to variations in supply voltage is
called the supply voltage rejection ratio. It is also called Power Supply
Rejection Ratio (PSRR) or Power Supply Sensitivity (PSS)

.

**41. Define an operational amplifier.**

An
operational amplifier is a direct-coupled, hi h ga amplifier consisting of one
or more differential amplifier. By properly selecting the external components,
it can be used to perform a variety of mathematical operations.

**42. Mention the characteristics of
an ideal op-amp. **

• Open loop voltage gain is infinity.

• Input impedance is infinity.

• Output impedance is zero.

• Bandwidth is infinity.

• Zero offset.

**43. 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.

**44. 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

**45. What is frequency response of
Op-amp?**

The
plot showing the variations in magnitude and phase angle of the gain due to
change in frequency is called frequency response of Op-amp. The plot is used to
find the bandwidth and cut-off frequencies of Op-amp.

**46. Define Unity Gain Bandwidth of
Op-amp.**

For
a certain frequency of the input signal, the gain of the Op-amp reduces to 0
dB. This means 20 log |AOL (f) | is 0dB i.e. |AOL (f) | = 1. Such a frequency
is called gain cross over frequency or unity gain bandwidth (UGB).

**47. Define slew rate.**

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 opamp’s
output voltage should change instantaneously in response to input step voltage.

**PART-B **

1.
Explain the method of improving CMRR.

2.
(a) Write a note on Widlar biasing circuit.

(b) Explain the various circuits used to
provide constant current bias in a differential amplifier.

3.
Explain the method of improving the CMRR using active load.

4.
Explain the operation of differential amplifier and give its differential gain,
common mode gain and CMRR.

5.
Draw and explain the basic band gap reference circuit.

6.
Derive the slew rate equation for an op-amp.

7.
Explain in detail about the frequency compensation applied to operational
amplifiers.

8.
(a) Explain the method of improving the slew rate of an op-amp.

(b)
Draw and explain briefly the equivalent circuit of an op-amp.

9.
Draw and explain the internal block diagram of an op-amp.

10.
Define and explain slew rate. What is full-power bandwidth? Also explain the
methods adopted to improve slew rate [10]

11.
Define output off-set voltage. Explain methods of nullify offset voltage

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

(ii)
Define Slew Rate. Explain the cause of slew rate and derive an expression for
Slew rate for an op-amp voltage follower.

13.
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.

14.
With neat circuit diagram explain the operation of

(i)
Voltage reference circuit using temperature compensation

(ii)
Voltage reference circuit using avalanche diode reference 16. (i) List and explain the non-ideal DC
characteristics of an operational amplifier.

(ii)
Explain the AC characteristics of an operational amplifier

17.
Compare different configurations of Differential Amplifier.

18.
For a dual input, balanced output differential amplifier, Rc = 2.2kΩ, Re
=4.7kΩ, Rs1 = Rs2 = 50 Ω. The supply voltages are ± 10V. The hfe for the
transistor is 50. Assume silicon transistors and hie = 1.4kΩ. Determine the
operating point values, differential gain common mode gain and CMRR.

19.
State the advantages of Integrated circuits over discrete components.

20.
Explain the working of BJT Differential Amplifier with Active Load.

21.
Write down the characteristics and respective values of an ideal operational
amplifier.

22.
Explain the internal circuit diagrams of IC 741.Discuss its AC and DC
performance Characteristics.

23.
With simple schematic of differential amplifier explain the function of
operational amplifier?

24.
With the neat diagram, explain the input side of the internal circuit diagram
of IC741. .

25.
What is the need for the frequency compensation in an OPAMP? With a suitable
illustration, explain the pole-zero frequency compensation technique.

26.
Draw the circuit diagram of a basic current mirror and explain its operation

27.
With a schematic diagram, explain the effect of RE on CMRR in differential
amplifier.

28.
Discuss about the methods of improve the CMRR..

29.
For the DC level shifter shown in Fig 1, Determine the level shift between
input and output voltages.

30.
Obtain the level shift Vo for the circuit shown in Fig. 2.

31.
A differential Amplifier has i) CMRR=1000 and ii)CMRR = 10,000.The first set of
inputs is V_{1}=100µV and V_{2}=-100µV.The
second set of input is V_{1}=1100µV
and V_{2}= 900µV.Calculate the percentage difference in output voltage
obtained for the two sets of input voltages and also comment on this. (PO2)

32.
Design a i) constant current source ii) Widlar current source for generating a
constant current of Io=10µA.Assume Vcc=10V, Vbe=0.7 V, β=125, VT=25Mv.Find
R1.Justify which circuit can be used for generating small current?

33.
Design a differential Amplifier for a differential gain of 5000 and CMRR 100.If
the inputs are 290µV and 250 µV, Find the output voltage.

34.
An operational amplifier has a slew rate of 2V/µs. If the peak output is 15 V,
What is the power bandwidth?

35,
An operational amplifier has a slew rate of 35V/µs. How long will it take for
the output to change from 0 to 15V?

36.The
output of an opamp voltage follower is a triangular wave for a square wave
input of frequency 2MHz and 8Vpp amplitude. What is the slew rate of
opamp?

37.
Design a simple current source to provide an output current of 150µA.Assume Vcc=5V,Vbe=0.6
V, β=125.

38.A
peak to peak input signal of 400mV has to produce a peak to peak undistorted
output voltage of 3V with a rise time of 4µs.Can IC741 be used for such
application.? Justify.

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