UNIT-V: WAVEFORM GENERATORS AND SPECIAL FUNCTION ICs...

EC2254 –LINEAR INTEGRATED CIRCUITS – II/IV SEM ECE - L.M.I.LEO JOSEPH ASST.PROF/ECE PAGE 1

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.


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.


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.

http://en.wikipedia.org/wiki/Root_mean_square

http://en.wikipedia.org/wiki/Voltage

http://en.wikipedia.org/wiki/Absolute_value

http://en.wikipedia.org/wiki/Peak-to-peak


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.

http://en.wikipedia.org/wiki/Electronic_circuit

http://en.wikipedia.org/wiki/Discrete_signal

http://en.wikipedia.org/wiki/Signal_processing

http://en.wikipedia.org/wiki/Signal_processing

http://en.wikipedia.org/wiki/Switch


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.


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.


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 .


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:


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:

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.


a

v2

v1

b

R3

1k

b'

v2

v1Iave = (v1-v2)/R

a'

C2

1n

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

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


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.


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.


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%.


(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:


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.


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


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.


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


(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.


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.


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.


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.


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.


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


13. State barkhausen principle for oscillation and explain a RC phase shift oscillator using op-amp 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:


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.

UNIT-V: WAVEFORM GENERATORS AND SPECIAL FUNCTION ICs...

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