Download It is defined as the percentage change in the output voltage from a

UNIT-V: WAVEFORM GENERATORS AND SPECIAL FUNCTION ICs
PARTA (2 Marks)
1. Define line regulation.[AUC April 2004]
It is defined as the percentage change in the output voltage from a
change in the input voltage.
2. Name a timer IC and a voltage regulator IC?.[ AUC April2005,May 2010]
Timer IC : IC 555 , Voltage regulator : IC 7805 and IC 7905
3. Define ripple rejection with respect to voltage regulators.[ AUC April 2005]
Voltage regulators stabilize the output voltage against changes in input voltage.
Ripple is equivalent to a periodic change in the input voltage.Therefore, a voltage
regulator attenuates the ripple that comes in with the unregulated input voltage. This
process is called as ripple rejection.
4. In a monostable multivibrator using 555 timer ,R=100 KΩ and time delay is 100ms find the
value of c. [AUC June 2006 ,2011]
5. Draw the internal block diagram of an IC voltage regulator [AUC June 2006]
6. State the conditions required for designing a video amplifier [AUC June2006]
All the frequencies must be amplified equally to maintain the same relative
amplitudes.
The relative phases of all the frequency components in the output must be same as
the input.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 1
7. What is a switched capacitor filter? Mention any two advantages[AUC Nov 2006]
Switched capacitor filters are MOS switches that are used to simulate high value
resisters.
Advantages:
Very high value resistors can be easily simulated using small value
capacitors.
It does not requires any external reactive components like inductors
and capacitors.
Low system cost.
High accuracy
Excellent temperature stabilllity.
8. What is the basic principle of operation of a F to V converter.[ AUC Nov2007]
The operation of F to V converter depends on the ON time and OFF time of the
monostable multivibrator. The ON time of the monostable multivibrator will charge
the capacitor more frequently. At the same time the OFF time of the monostable
multivibrator will decrease the charging rate of the capacitor and hence the capacitor
will discharge to a higher value and output voltage V0 is raised to a higher level.
Thus the output voltage is proportional to input frequency.
9. What is a staged tuned amplifiers[AUC Nov 2007]
Stage tuned amplifiers are designed to amplify a signal over a selective narrow band
of frequencies centered at f0.
10. What is the basic principle of switching voltage regulator[AUC April 2008]
In a switching regulator the series pass transistor is used as a switch. It is operated
in either cut –off or saturation region. In cut-off region the current is very small while
in saturation region the voltage across transistor is very small. In any case the power
dissipation across the transistor is very small and maximum power is delivered to the
load. But due to cut-off region the power is transmitted to the load in the form of
discrete pulses. The pulse width modulation is the basic technique used in such
regulators. By varying the duty cycle of the pulse waveform used to control on / off of
the transistor, the average value of the voltage given to the load can be controlled
proportionally.
11. List the characteristics of optocoupler[AUC April 2008,May2010]




Current intensity ratio
Isolation impedance
Response time
CMRR
12. What are the advantages of a switched capacitor filter.?[ AUC Nov2008]
Advantages:
 Very high value resistors can be easily simulated using small value
capacitors.
 It does not requires any external reactive components like inductors
and capacitors.
 Low system cost.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 2
13. Give the working principle of optocouplers. [AUC June 2009]
The basic opto-coupler circuit consists of a LED and a photo diode. When the LED is
forward biased,the electron in the LED is excited and result in the emission of
light. This emitted light is sensed by the sensing device. The current flowing through
the sensing device is directly proportional to the intensity of light emitted the by
emitting device.
14. Mention the application of 555 timers in monostable mode of operation.[AUC May2010]




missing pulse detector
Linear ramp generator
Frequency divider
Pulse width modulation.
15. What is monostable multivibrator. [AUC Nov09]
Monostable Multivibrator is one, in which one of the states is stable, but the other state
is unstable (transient). A trigger pulse causes the circuit to enter the unstable state. After
entering the unstable state, the circuit will return to the stable state after a set time. Such
a circuit is useful for creating a timing period of fixed duration in response to some
external event. This circuit is also known as a one shot.
16. Define ripple factor.[ AUC Nov 09]
Ripple factor (γ) may be defined as the ratio of the root mean square (rms) value of the
ripple voltage to the absolute value of the dc component of the output voltage, usually
expressed as a percentage. However, ripple voltage is also commonly expressed as the
peak-to-peak value.
17. What are the limitations of linear voltage regulators? [AUC MAY 2011,2012]



The input step down transformer is bulky and expensive because of low line
frequency.
Because of low line frequency, large values of filter capacitors are required to
decrease the ripple.
Efficiency is reduced due to the continuous power dissipation by the transistor as
it operates in the linear region.
18. Mention any two applications of timer IC 555 in monostable mode. [AUC MAY 2011]


missing pulse detector
Linear ramp generator
19. What are the limitations of three terminal voltage regulators? [AUC MAY 2011]



The input step down transformer is bulky and expensive because of low line
frequency.
Because of low line frequency, large values of filter capacitors are required to
decrease the ripple.
Efficiency is reduced due to the continuous power dissipation by the transistor as
it operates in the linear region.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 3
20. What is a switched capacitor filter? [AUC MAY 2012]
A switched capacitor is an electronic circuit element used for discrete time signal
processing. It works by moving charges into and out of capacitors when switches are
opened and closed.
Part -B ( 16 Marks)
1. Explain the working of a timer circuit.[AUC April 2004]
The 555 timer as an Astable Multivibrator:
An Astable multivibrator, often called a free running multivibrator, is a rectangular wave
generating circuit. Unlike the monostable multivibrator, this circuit does not require an
external trigger to change the state of the output, hence the name free running.
However, the time during which the output is either high or low is determined by 2
resistors and capacitors, which are externally connected to the 555 timer.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 4
The above figures show the 555 timer connected as an astable multivibrator and its model graph
Initially, when the output is high :
Capacitor C starts charging toward Vcc through RA & RB. However, as soon as voltage across
the capacitor equals 2/3 Vcc. Upper comparator triggers the FF & output switches low.
When the output becomes Low:
Capacitor C starts discharging through RB and transistor Q1, when the voltage across C equals
1/3 Vcc, lower comparator output triggers the FF & the output goes High. Then cycle repeats.
The capacitor is periodically charged & discharged between 2/3 Vcc & 1/3 Vcc respectively. The
time during which the capacitor charges from 1/3 Vcc to 2/3 Vcc equal to the time the output is
high & is given by
Where RA & RB are in ohms. And C is in farads.
Similarly, the time during which the capacitors discharges from 2/3 Vcc to 1/3 Vcc is equal to the
time, the output is low and is given by,
where RB is in ohms and C is in farads.
Thus the total period of the output waveform is
Equation 4 indicates that the frequency f 0 is independent of the supply voltage Vcc. Often the
term duty cycle is used in conjunction with the astable multivibrator. The duty cycle is the ratio of
the time tc during which the output is high to the total time period T. It is generally expressed as
a percentage.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 5
2. Explain the working of a video amplifier circuit.
733 Video amplifier
Features
Wide badwidth
Offers an input resistance of 250Ω.
Gain of 10,100 and 400 are selectable.
External frequency compensation is not required.
Provides high common mode rejection ratio.
Operation
It consist of cascaded BJT differential amplifiers and a balanced emitter follower stage.
The wide bandwidth is achieved by the use of low value resistances for the two
differential amplifier stages and the use of internal feedback loops.
It has a differential input and output.. Single ended and balanced input signals may be
connected for amplification.
The input stage comprises of transistors Q1 , Q2 and the load resistors R1 and R2.
The transistor Q7 provides current sink biasing for the first differential stage. Resistors
R3 and R6 provides negative feedback path for the first stage.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 6
The second stage formed by Q3 and Q4 is driven by the balanced output available from
the first stage.
Resistor R9 and R10 act as load resistors for the second differential stage.
The transistor Q9 provides the current sink bias.
Q10 and Q11 act as current sink bias for the emitter followers.
The resistors R11 and R12 provide the negative feedback from the output terminals to
the balanced input terminals of the second stage.
The diode connected transistor Q8 along with resistors R8 and R15 provides the overall
biasing for the circuit.
The external gain adjustment for the amplifier can be done by modifying the maount
of series feedback connected in the first stage.
This is accomplished by externally interconnecting resistor taps .
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 7
The frequency response for the video amplifier are as shown.
3. Write short notes on i) optocoupler ii) switched capacitor filter[April 2004,June2006]
i) OPTOCOUPLERS/OPTOISOLATORS:
o Optocouplers or Optoisolators is a combination of light source & light detector in
the same package.
o They are used to couple signal from one point to other optically, by providing a
complete electric isolation between them. This kind of isolation is provided
between a low power control circuit & high power output circuit, to protect the
control circuit.
o Depending on the type of light source & detector used we can get a variety of
optocouplers.
They are as follows,
(i) LED – LDR optocoupler
(ii) LED – Photodiode optocoupler
(iii) LED – Phototransistor optocoupler
Characteristics of optocoupler:
(i) Current Transfer Ratio (CTR)
(ii) Isolation Voltage
(iii) Response Time
(iv) Common Mode Rejection
(i) Current Transfer Ratio:
It is defined as the ratio of output collector current (Ic) to the input forward current (If)
CTR = Ic/If * 100%
Its value depends on the devices used as source & detector.
(ii) Isolation voltage between input & output:
It is the maximum voltage which can exist differentially between the input & output
without affecting the electrical isolation voltage is specified in K Vrms with a relative
humidity of 40 to 60%.
(iii)Response Time:
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 8
Response time indicates how fast an optocoupler can change its output state. Response time
largely depends on the detector transistor, input current & load resistance.
(iv)Common mode Rejection:
Eventhough the optocouplers are electrically isolated for dc & low frequency signals, an
impulsive input signal (the signal which changes suddenly) can give rise to a
displacement
current Ic= Cf*dv/dt. This current can flow between input & output due to the capacitance Cf
existing between input & output. This allow the noise to appear in the
output.
Types of optocoupler:
(i) LED – Photodiode optocoupler:
LED photodiode shown in figure, here the infrared LED acts as a light source & photodiode is
used as a detector.
The advantage of using the photodiode is its high linearity. When the pulse at the input
goes high, the LED turns ON. It emits light. This light is focused on the photodiode.
In response to this light the photocurrent will start flowing though the photodiode. As
soon as the input pulse reduces to zero, the LED turns OFF & the photocurrent through
the photodiode reduces to zero. Thus the pulse at the input is coupled to the output side.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 9
(ii) LED – Phototransistor Optocoupler:
The LED phototransistor optocoupler shown in figure. An infrared LED acts as a light
source and the phototransistor acts as a photo detector.
This is the most popularly used optocoupler, because it does not need any additional
amplification.
When the pulse at the input goes high, the LED turns ON. The light emitted by the LED
is focused on the CB junction of the phototransistor.
In response to this light photocurrent starts flowing which acts as a base current for the
phototransistor.
The collector current of phototransistor starts flowing. As soon as the input pulse reduces
to zero, the LED turns OFF & the collector current of phototransistor reduces to zero.
Thus the pulse at the input is optically coupled to the output side.
Advantages of Optocoupler:
Control circuits are well protected due to electrical isolation.
Wideband signal transmission is possible.
Due to unidirectional signal transfer, noise from the output side does not get coupled to
the input side.
Interfacing with logic circuits is easily possible.
It is small size & light weight device.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 10
Disadvantages:
Slow speed.
Possibility of signal coupling for high power signals.
Applications:
Optocouplers are used basically to isolate low power circuits from high power circuits.
At the same time the control signals are coupled from the control circuits to the high
power circuits.
Some of such applications are,
AC to DC converters used for DC motor speed control
High power choppers
High power inverters
One of the most important applications of an optocoupler is to couple the base driving
signals to a power transistor connected in a DC-DC chopper.
Note that the input & output waveforms are 180º out of phase as the output is taken at
the collector of the phototransistor.
Switched capacitor filter
R3
v1
1k
Iave = (v1-v2)/R
a
b
C2
v1
v2
1n
b'
v2
a'
Operation may be explained in two modes
Mode 1 : switch position is at aa’
Charge through capacitor is given by
Q = C(V1-V2) ------ 1
Mode 2 : switch position is at bb’
Charge through capacitor is given by
Q’ = -c (v1 – v2)
Current through capacitor is given by
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 11
Iave = Q – Q’ / tclk
=c( v1 –v2) –( - c( v1-v2)) / tclk
=2c (v1 –v2) / tclk
Iave = (v1- v2) / tclk /2c ----2
Comparing 1 and 2
V1-v2 / r = v1 – v2 / tclk /2c
R= tclk / 2c
R= 1/ 2c .fclk
By adjusting clock frequency of the external capacitor “ R” can be simulated.
4. Design an astable multivibrator using 555 timer to produce a 1Khz square waveform for
duty cycle D=0.50[AUC Nov 2004, May 2010]
5. Draw and explain the functional block diagram of a 723 regulator.[ AUC Nov
2004,May2010]
IC 723 – GENERAL PURPOSE REGULATOR
Disadvantages of fixed voltage regulator:
1. Do not have the shot circuit protection
2. Output voltage is not adjustable
These limitations can be overcomes in IC723.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 12
Features of IC723:
1. Unregulated dc supply voltage at the input between 9.5V & 40V
2. Adjustable regulated output voltage between 2 to 3V.
3. Maximum load current of 150 mA (ILmax = 150mA).
4. With the additional transistor used, ILmax upto 10A is obtainable.
5. Positive or Negative supply operation
6. Internal Power dissipation of 800mW.
7. Built in short circuit protection.
8. Very low temperature drift.
9. High ripple rejection.
The simplified functional block diagram can be divided in to 4 blocks.
1. Reference generating block
2. Error Amplifier
3. Series Pass transistor
4. Circuitry to limit the current
1. Reference Generating block:
The temperature compensated Zener diode, constant current source & voltage reference
amplifier together from the reference generating block. The Zener diode is used to generate a
fixed reference voltage internally. Constant current source will make the Zener
diode
to
operate at affixed point & it is applied to the Non – inverting terminal of error amplifier. The
Unregulated input voltage ±Vcc is applied to the voltage reference amplifier as well as error
amplifier.
2. Error Amplifier:
Error amplifier is a high gain differential amplifier with 2 input (inverting & Non-inverting). The
Non-inverting terminal is connected to the internally generated reference voltage.
The
Inverting terminal is connected to the full regulated output voltage.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 13
3. Series Pass Transistor:
Q1 is the internal series pass transistor which is driven by the error amplifier. This transistor
actually acts as a variable resistor & regulates the output voltage. The collector of transistor Q1
is connected to the Un-regulated power supply. The maximum collector voltage of Q1 is limited
to 36Volts. The maximum current which can be supplied by Q1 is 150mA.
4. Circuitry to limit the current:
The internal transistor Q2 is used for current sensing & limiting. Q2 is normally OFF transistor. It
turns ON when the IL exceeds a predetermined limit.
Low voltage , Low current is capable of supplying load voltage which is equal to or between 2 to
7Volts.
Vload = 2 to 7V
Iload = 150mA
6.
Explain the working of i) isolation amplifier ii) Voltage regulator[AUC Apr 2005,NOV
2007,2009 ,APR 2010 ,NOV 2012]
Isolation amplifiers :
Isolation amplifiers are hybrid IC ‘s and contain an input amplifier , LED , a photodiode and an
output amplifier.
The input signal modulates the light output of the LED. The light emitted by the LED is detected
by a photodiode and converted into an electrical signal.
i) OPTOCOUPLERS/OPTOISOLATORS:
Optocouplers or Optoisolators is a combination of light source & light detector in the same
package.They are used to couple signal from one point to other optically, by providing a
complete electric isolation between them. This kind of isolation is provided between a low power
control circuit & high power output circuit, to protect the control circuit.Depending on the type of
light source & detector used we can get a variety of optocouplers.
They are as follows,
(i) LED – LDR optocoupler
(ii) LED – Photodiode optocoupler
(iii) LED – Phototransistor optocoupler
Characteristics of optocoupler:
(i) Current Transfer Ratio (CTR)
(ii) Isolation Voltage
(iii) Response Time
(iv) Common Mode Rejection
(i) Current Transfer Ratio:
It is defined as the ratio of output collector current (Ic) to the input forward current (If)
CTR = Ic/If * 100%
Its value depends on the devices used as source & detector.
(ii) Isolation voltage between input & output:
It is the maximum voltage which can exist differentially between the input & output without
affecting the electrical isolation voltage is specified in K Vrms with a relative humidity of 40 to
60%.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 14
(iii)Response Time:
Response time indicates how fast an optocoupler can change its output state. Response time
largely depends on the detector transistor, input current & load resistance.
(iv)Common mode Rejection:
Eventhough the optocouplers are electrically isolated for dc & low frequency signals, an
impulsive input signal (the signal which changes suddenly) can give rise to a displacement
current Ic= Cf*dv/dt. This current can flow between input & output due to the capacitance Cf
existing between input & output. This allow the noise to appear in the output.
Types of optocoupler:
(i) LED – Photodiode optocoupler:
LED photodiode shown in figure, here the infrared LED acts as a light source & photodiode is
used as a detector.
The advantage of using the photodiode is its high linearity. When the pulse at the input
goes high, the LED turns ON. It emits light. This light is focused on the photodiode.
In response to this light the photocurrent will start flowing though the photodiode. As
soon as the input pulse reduces to zero, the LED turns OFF & the photocurrent through
the photodiode reduces to zero. Thus the pulse at the input is coupled to the output side.
(ii) LED – Phototransistor Optocoupler:
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 15
The LED phototransistor optocoupler shown in figure. An infrared LED acts as a light
source and the phototransistor acts as a photo detector.
This is the most popularly used optocoupler, because it does not need any additional
amplification.
When the pulse at the input goes high, the LED turns ON. The light emitted by the LED
is focused on the CB junction of the phototransistor.
In response to this light photocurrent starts flowing which acts as a base current for the
phototransistor.
The collector current of phototransistor starts flowing. As soon as the input pulse reduces
to zero, the LED turns OFF & the collector current of phototransistor reduces to zero.
Thus the pulse at the input is optically coupled to the output side.
Advantages of Optocoupler:
Control circuits are well protected due to electrical isolation.
Wideband signal transmission is possible.
Due to unidirectional signal transfer, noise from the output side does not get coupled to
the input side.
Interfacing with logic circuits is easily possible.
It is small size & light weight device.
Disadvantages:
Slow speed.
Possibility of signal coupling for high power signals.
Applications:
Optocouplers are used basically to isolate low power circuits from high power circuits.
At the same time the control signals are coupled from the control circuits to the high
power circuits.
Some of such applications are,
AC to DC converters used for DC motor speed control
High power choppers
High power inverters
One of the most important applications of an optocoupler is to couple the base driving
signals to a power transistor connected in a DC-DC chopper.
Note that the input & output waveforms are 180º out of phase as the output is taken at
the collector of the phototransistor.
Voltage Regulator
IC 723 – GENERAL PURPOSE REGULATOR
Disadvantages of fixed voltage regulator:
1. Do not have the shot circuit protection
2. Output voltage is not adjustable
These limitations can be overcomes in IC723.
Features of IC723:
1. Unregulated dc supply voltage at the input between 9.5V & 40V
2. Adjustable regulated output voltage between 2 to 3V.
3. Maximum load current of 150 mA (ILmax = 150mA).
4. With the additional transistor used, ILmax upto 10A is obtainable.
5. Positive or Negative supply operation
6. Internal Power dissipation of 800mW.
7. Built in short circuit protection.
8. Very low temperature drift.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 16
9. High ripple rejection.
The simplified functional block diagram can be divided in to 4 blocks.
1. Reference generating block
2. Error Amplifier
3. Series Pass transistor
4. Circuitry to limit the current
1. Reference Generating block:
The temperature compensated Zener diode, constant current source & voltage reference
amplifier together from the reference generating block. The Zener diode is used to
generate a fixed reference voltage internally. Constant current source will make the
Zener diode to operate at affixed point & it is applied to the Non – inverting terminal of
error amplifier. The Unregulated input voltage ±Vcc is applied to the voltage reference
amplifier as well as error amplifier.
2. Error Amplifier:
Error amplifier is a high gain differential amplifier with 2 input (inverting & Non-inverting). The
Non-inverting terminal is connected to the internally generated reference voltage.
The
Inverting terminal is connected to the full regulated output voltage.
3. Series Pass Transistor:
Q1 is the internal series pass transistor which is driven by the error amplifier. This transistor
actually acts as a variable resistor & regulates the output voltage. The collector of transistor Q1
is connected to the Un-regulated power supply. The maximum collector voltage of Q1 is limited
to 36Volts. The maximum current which can be supplied by Q1 is 150mA.
4. Circuitry to limit the current:
The internal transistor Q2 is used for current sensing & limiting. Q2 is normally OFF transistor. It
turns ON when the IL exceeds a predetermined limit.
Low voltage , Low current is capable of supplying load voltage which is equal to or between 2 to
7Volts.
Vload = 2 to 7V
Iload = 150mA
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
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7. Explain the internal details of 555 timer.[ AUC Nov 2006]
PIN CONFIGURATION OF 555 TIMER:
Pin description:
Pin description:
Pin 1: Ground:
All voltages are measured with respect to this terminal.
Pin 2: Trigger:
The o/p of the timer depends on the amplitude of the external trigger pulse applied to this pin.
Pin 3: Output:
There are 2 ways a load can be connected to the o/p terminal either between pin3 & ground
or between pin 3 & supply voltage
(i) When the input is low:
The load current flows through the load connected between Pin 3 & +Vcc in to the output
terminal & is called the sink current.
(ii) When the output is high:
The current through the load connected between Pin 3 & +Vcc (i.e. ON load) is zero. However
the output terminal supplies current to the normally OFF load. This current is called the source
current.
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Pin 4: Reset:
The 555 timer can be reset (disabled) by applying a negative pulse to this pin. When the reset
function is not in use, the reset terminal should be connected to +Vcc to avoid any false
triggering.
Pin 5: Control voltage:
An external voltage applied to this terminal changes the threshold as well as trigger voltage. In
other words by connecting a potentiometer between this pin & GND, the pulse width of the
output waveform can be varied. When not used, the control pin should be bypassed to ground
with 0.01 capacitor to prevent any noise problems.
Pin 6: Threshold:
This is the non inverting input terminal of upper comparator which monitors the voltage across
the external capacitor.
Pin 7: Discharge:
This pin is connected internally to the collector of transistor Q1. When the output is high Q1 is
OFF. When the output is low Q is (saturated) ON.
Pin 8: +Vcc:
The supply voltage of +5V to +18V is applied to this pin with respect to ground.
Block Diagram of 555 Timer IC:
From the above figure, three 5k internal resistors act as voltage divider providing bias voltage of
2/3 Vcc to the upper comparator & 1/3 Vcc to the lower comparator. It is possible to vary time
electronically by applying a modulation voltage to the control voltage input terminal (5).
(i) In the Stable state:
The output of the control FF is high. This means that the output is low because of power
amplifier which is basically an inverter. Q = 1; Output = 0
(ii) At the Negative going trigger pulse:
The trigger passes through (Vcc/3) the output of the lower comparator goes high & sets the
FF. Q = 1; Q = 0
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 19
(iii) At the Positive going trigger pulse: It passes through 2/3Vcc, the output of the upper
comparator goes high and resets the FF. Q = 0; Q = 1
The reset input (pin 4) provides a mechanism to reset the FF in a manner which overrides the
effect of any instruction coming to FF from lower comparator.
8. Explain the usage as astable multivibrator.
The 555 timer as an Astable Multivibrator:
An Astable multivibrator, often called a free running multivibrator, is a rectangular wave
generating circuit. Unlike the monostable multivibrator, this circuit does not require an external
trigger to change the state of the output, hence the name free running. However, the time during
which the output is either high or low is determined by 2 resistors and capacitors, which are
externally connected to the 555 timer.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
PAGE 20
The above figures show the 555 timer connected as an astable multivibrator and its model graph
Initially, when the output is high :
Capacitor C starts charging toward Vcc through RA & RB. However, as soon as voltage across
the capacitor equals 2/3 Vcc. Upper comparator triggers the FF & output switches low.
When the output becomes Low:
Capacitor C starts discharging through RB and transistor Q1, when the voltage across C equals
1/3 Vcc, lower comparator output triggers the FF & the output goes High. Then cycle repeats.
The capacitor is periodically charged & discharged between 2/3 Vcc & 1/3 Vcc respectively. The
time during which the capacitor charges from 1/3 Vcc to 2/3 Vcc equal to the time the output is
high & is given by
Where RA & RB are in ohms. And C is in farads.
Similarly, the time during which the capacitors discharges from 2/3 Vcc to 1/3 Vcc is equal to the
time, the output is low and is given by,
where RB is in ohms and C is in farads.
Thus the total period of the output waveform is
Equation 4 indicates that the frequency f 0 is independent of the supply voltage Vcc. Often the
term duty cycle is used in conjunction with the astable multivibrator. The duty cycle is the ratio of
the time tc during which the output is high to the total time period T. It is generally expressed as
a percentage.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
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9. a)Explain the working of 555 timer in monostable mode[AUC June2006,May 2008,Nov
2008,09]
Monostable Operation:
Model Graph:
Initially when the output is low, i.e. the circuit is in a stable state, transistor Q1 is ON & capacitor
C is shorted to ground. The output remains low. During negative going trigger pulse, transistor
Q1 is OFF, which releases the short circuit across the external capacitor C & drives the output
high.
Now the capacitor C starts charging toward Vcc through RA. When the voltage across the
capacitor equals 2/3 Vcc, upper comparator switches from low to high. i.e. Q = 0, the transistor
Q1 = OFF ;the output is high.
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Since C is unclamped, voltage across it rises exponentially through R towards Vcc with a time
constant RC (fig b) as shown in below. After the time period, the upper comparator resets the
FF,
i.e. Q = 1, Q1 = ON; the output is low.[i.e discharging the capacitor C to ground potential (fig c)].
The voltage across the capacitor as in fig (b) is given by
If the reset is applied Q2 = OFF, Q1 = ON, timing capacitor C immediately discharged. The
output now will be as in figure (d & e). If the reset is released output will still remain low until a
negative going trigger pulse is again applied at pin 2.
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10. Explain the working of a basic series voltage regulator.
Switching Regulators
Diode D1 and Inductor L1 play a very specific role in this circuit and are found in almost
every switching regulator. First, diode D1 has to be a Schottky or other very fast switching
diode. A 1N4001 just won't switch fast enough in this circuit.
Inductor L1 must be a type of core that does not saturate under high currents. Capacitor
C1 is
normally a low ESR (Equivalent Series Resistance) type. To understand the action of D1
and L1, lets look at what happens when S1 is closed as indicated below:
As we see above, L1, which tends to oppose the rising current, begins to generate an
electromagnetic field in its core. Notice that diode D1 is reversed biased and is essentially
an open circuit at this point. Now lets take a look at what happens when S1 opens below:
The electromagnetic field that was built up in L1 is now discharging and generating a current
in the reverse polarity. As a result, D1 is now conducting and will continue until the field in L1
is diminished. This action is similar to the charging and discharging of capacitor C1. The use
of this inductor/diode combination gives us even more efficiency and augments the filtering of
C1.
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
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11. With neat diagram explain the working of step down switching regulator[AUC June 2006]
Buck Regulator:
The most commonly used switching converter is the Buck, which is used to down-convert a DC
voltage to a lower DC voltage of the same polarity. This is essential in systems that use
distributed power rails (like 24V to 48V), which must be locally converted to 15V, 12V or 5V with
very little power loss. The Buck converter uses a transistor as a switch that alternately connects
and disconnects the input voltage to an inductor (see Figure).
The lower diagrams show the current flow paths (shown as the heavy lines) when the switch is
on and off. When the switch turns on, the input voltage is connected to the inductor. The
difference between the input and output voltages is then forced across the inductor, causing
current through the inductor to increase. During the on time, the inductor current flows into both
the load and the outputcapacitor (the capacitor charges during this time).
When the switch is turned off, the input voltage applied to the inductor is removed. However,
since the current in an inductor can not change instantly, the voltage across the inductor will
adjust to hold the current constant. The input end of the inductor is forced negative in voltage by
the decreasing current, eventually reaching the point where the diode is turned on. The inductor
current then flows through the load and back through the diode. The capacitor discharges into
the load during the off time, contributing to the total current being supplied to the load (the total
load current during the switch off time is the sum of the inductor and capacitor current).
12. a)With circuit diagram explain the working principle of IC 723 voltage regulator.[ AUC Nov
2006,May2010]
Refer Qno.6
EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE
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13. State barkhausen principle for oscillation and explain a RC phase shift oscillator using opamp and derive for frequency of oscillation and gain of the amplifier.[ AUC May 2010]
Oscillation results from an unstable state; i.e., the feedback system can‘t find a stable state
because its transfer function can‘t be satisfied. Equation 1 becomes unstable when (1+Aβ) = 0
because A/0 is an undefined state. Thus, the key to designing an oscillator is to insure that Aβ =
1 (called the Barkhausen criterion), or using complex math the equivalent expression is Aβ = 1 –
180°. The – 180° phase shift criterion applies to negative feedback systems, and 0° phase shift
applies topositive feedback systems.
RC phase shift oscillator using op-amp in inverting amplifier introduces the phase shift of 180°
between input and output. The feedback network consists of 3 RC sections each producing 60°
phase shift. Such a RC phase shift oscillator using op-amp is shown in the figure.
The output of amplifier is given to feedback network. The output of feedback network drives the
amplifier. The total phase shift around a loop is 180 ° of amplifier and 180°due to 3 RC section,
hus 3600. This satisfies the required condition for positive feedback and circuit works as an
oscillator.
Without the simplification of all the resistors and capacitors having the same value, the
calculations become more complex:
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Oscillation criterion:
14. Design a 555 based square wave generator to produce a 1 KHZ square waveform.
Square wave generator:
With out reducing RA = 0 ohm, the astable multivibrator can be used to produce square wave
output. Simply by connecting diode D across Resistor RB. The capacitor C charges through RA
& diode D to approximately 2/3 Vcc & discharges through RB & Q1 until the capacitor voltage
equals approximately 1/3 Vcc, then the cycle repeats.
To obtain a square wave output, RA must be a combination of a fixed resistor & potentiometer
so that the potentiometer can be adjusted for the exact square wave.
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