B.E./ B. Tech.
DEGREE EXAMINATION NOVEMBER/DECEMBER 2016
Third Semester
Electrical andElectronicsEngineering
EE 6303 – LINEAR INTEGRATED CIRCUITS AND APPLICATIONS
(Regulations 2013)
Time: Three hours Maximum: 100 marks
Answer ALL questions
PART −A (10×2=20 marks)
1. What are the ideal characteristics of an OP – AMP? [ND –
2015]
(i ) Open loop gain infinite
(ii) Input impedance infinite
(iii) Output impedance low
(iv) Bandwidth infinite
(v) Zero offset, ie, Vo=0 when V1=V2=0
2. Mention the frequency compensation methods.
1. Dominant-pole compensation
2. Pole-zero compensation.
3. What do you mean by input offset current and offset voltage?(A/M 15)
Input offset current is defined as the algebraic difference between the currents into the (-) input and (+)
input. For 741 IC the maximum value is 200 nA
It is the voltage that must be applied between the input terminals of an op-amp to nullify the
output. Since this voltage could be positive or negative its absolute value is listed on the data
sheet.
4. Define CMRR.(A/M 15)
It is defined as the ratio of the differential voltage gain to common mode voltage gain.
CMRR= ρ = Ad/Ac
5. Why IC 741is not used for high frequency applications.(M/J 16)
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.
6. Draw the circuit diagram of an integrator and give its output equation.(M/J 16)
7. Draw an adder circuit using an op-amp to get the output expression as V0 = - (0.1 V1 + V2 +10 V3 where V1, V2
and V3 are the inputs.(N/D 14)
8. A 100 pF capacitor has a maxi1um charging current of 150 microamps. What is the slew rate?(N/D 14)
It is defined as the maximum rate of change of output voltage with time. The slew rate is specified in
V/µsec
Slew rate = S = dVo / dt |max
S = Imax / C
9. Write some applications of operational amplifier.
Differential amplifier
Non-inverting amplifier
Voltage follower (unity buffer amplifier)
Instrumentation amplifier, Precession rectifier
10. What is integrator?
The operational amplifier integrator is an electronic integration circuit. Based on
the operational amplifier (op-amp), it performs the mathematical operation of integration with
respect to time; that is, its output voltage is proportional to the input voltage integrated over
time.
11. What are the D.C Characteristics of Op-amp?
1. Input Bias current.
2. Input offset current.
3. Input offset voltage.
4. Thermal Drift.
12. Define slew rate and state its significance
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.
PART – B (80 marks)
11. (a) (i) What is Slew rate? List the causes of the Slew rate and explain its significance in applications.
(10) [ND – 2015]
It is defined as the maximum rate of change of output voltage with time. The slew rate is
specified in V/µsec
Slew rate = S = dVo / dt |max
It is specified by the op-amp in unity gain condition.
The slew rate is caused due to limited charging rate of the compensation capacitor and current
limiting and saturation of the internal stages of op-amp, when a high frequency large
amplitude signal is appliedIt is given by
dVc /dt = I/C
For large charging rate, the capacitor should be small or the current should be large.
S = Imax / C
For 741 IC the charging current is 15 µA and the internal capacitor is 30 pF. S= 0.5V/ µsec
S = 2 π f Vm V / sec
(ii) Briefly explain the methods used for frequency compensation. (6) [ND – 2015]
Because operational amplifiers are so ubiquitous and are designed to be used with
feedback, the following discussion will be limited to frequency compensation of these
devices.
It should be expected that the outputs of even the simplest operational amplifiers will
have at least two poles. An unfortunate consequence of this is that at some critical
frequency, the phase of the amplifier's output = −180° compared to the phase of its input
signal. The amplifier will oscillate if it has a gain of one or more at this critical frequency.
This is because (a) the feedback is implemented through the use of an inverting input
that adds an additional −180° to the output phase making the total phase shift −360° and
(b) the gain is sufficient to induce oscillation.
A more precise statement of this is the following: An operational amplifier will oscillate at
the frequency at which its open loop gain equals its closed loop gain if, at that frequency,
1. The open loop gain of the amplifier is ≥ 1 and
2. The difference between the phase of the open loop signal and phase
response of the network creating the closed loop output = −180°.
Mathematically,
ΦOL – ΦCLnet = −180°
Practice
Frequency compensation is implemented by modifying the gain and phase
characteristics of the amplifier's open loop output or of its feedback network, or
both, in such a way as to avoid the conditions leading to oscillation. This is
usually done by the internal or external use of resistance-capacitance networks.
Or
(b) (i) Draw and explain the operation of a current to voltage converter. (8) [ND – 2015]
1. Open – loop gain a of the op-amp is very large.
2. Input impedance of the op-amp is very high. (i.e) the currents entering into the 2 input
Terminals are very small. IB1=IB2= 0 ---(2)
(ii) What are the limitations of an ordinary op-amp differentiator? Draw the circuit of a practical
differentiator that will eliminate these limitations. (8) [ND – 2015]
12. (a) (i) Design an op-amp circuit to give an output voltage V0 = 4V1-3V2+5V3-V4 where V1 ,V2 ,V3 and V4 are
inputs. (8)(A/M 15)
(ii) Explain voltage to current converter using operational amplifier
Also explain the application of OP-Amp as integrator (8)(A/M 15)
Integrator:
A circuit in which the output voltage waveform is the integral of the input voltage
waveform is the integrator or Integration Amplifier. Such a circuit is obtained by using a
basic
inverting amplifier configuration if the feedback resistor RF
is replaced by a capacitor CF .
The expression for the output voltage V0can be obtained by KVL eqn at node V2.
or
(b) (i) Explain in detail about the methods of frequency compensation
used in operational amplifiers. (10)(A/M 15)
(ii) What is slew rate and how it can be improved? (6)(A/M 15)
It is defined as the maximum rate of change of output voltage with time. The slew rate is
specified in V/µsec
Slew rate = S = dVo / dt |max
It is specified by the op-amp in unity gain condition.
The slew rate is caused due to limited charging rate of the compensation capacitor and current
limiting and saturation of the internal stages of op-amp, when a high frequency large
amplitude signal is appliedIt is given by
dVc /dt = I/C
For large charging rate, the capacitor should be small or the current should be large.
S = Imax / C
For 741 IC the charging current is 15 µA and the internal capacitor is 30 pF. S= 0.5V/ µsec
S = 2 π f Vm V / sec
13. (a) (i) Discuss the frequency response characteristics and compensation of an
operational amplifier. (16) )(M/J 16)
or
(b) (i) Explain the application of Op-Amp as differentiator (8) (M/J 16)
Uses:
Its used in waveshaping circuits to detect high frequency components in an input signal and
also as
a rate of change and detector in FM modulators.
(ii) Find V0 for the given circuit. (8) (M/J 16)
14. (a) (i) Consider lossy integrator as shown in Figure 2. For the component values R1 = 10 k, Rf = 100 k, Cf= .
Determine the lower frequency limit of integration and study the response of the inputs
(1) step input
(2) square input
(3) sine input
(ii) Design an adder sub tractor circuit for V0 = 2V1 +5V2-10V3 (6)(N/D 14)
or
15. (a) (i) Discuss in detail about the DC and AC characteristics of op amp (13) (N/D 16)
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
1. Input bias current
Input offset current:
Bias current compensation will work if both bias currents IB+ and IB- are equal.
Since the input transistor cannot be made identical. There will always be some small
difference between IB+ and IB-.
This difference is called the offset current|Ios|=IB+ -IB-——>(10)
Offset current I
for BJT op-amp is 200nA and for FET op-amp is 10pA. Even with bias current
Input offset voltage:
Inspite of the use of the above compensating techniques, it is found that the output voltage
may still not be zero with zero input voltage [Vo ≠ 0 with Vi = 0]. This is due to unavoidable
imbalances inside the op-amp and one may have to apply a small voltage at the input terminal
tomake output (Vo) = 0.
This voltage is called input offset voltage Vos. This is the voltage required to be applied at
the input for making output voltage to zero (Vo = 0).
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.
Offset current drift is expressed in nA/ºC.
These indicate the change in offset for each degree Celsius change in temperature.
(b) (i) Explain the differe3ntial amplifier using op amp (13) (N/D 16)
Differential Amplifier with Active load:
Differential amplifier are designed with active loads to increase the differential mode
voltage gain.
The open circuit voltage gain of an op-amp is needed to be as large as possible. This is
achieved by
cascading the gain stages which increase the phase shift and the amplifier also becomes
vulnerable
to oscillations. The gain can be increased by using large values of collector resistance. For
such a circuit, the voltage gain is given by
product must be made very large. However, there are limitations in
IC fabrication such as,
1. a large value of resistance needs a large chip area.
2. for large RC,
the quiescent drop across the resistor increase and a large power supply will be
required to maintain a given operating current.
3. Large monolithic resistor introduces large parasitic capacitances which limits the frequency
response of the amplifier.
