1. ELEC207 Linear Integrated Circuits1 1

2. “Op-Amps and Linear Integrated Circuits”
TEXTBOOKS:
Ramakant A, Gayakward
“Op-Amps and Linear Integrated Circuits”
Prentice Hall of India, New Delhi, 4th Edition.
REFERENCES:
1. Behzad Razavi
“Design of Analog CMOS Integrated Circuits”
McGraw Hill, 2001.
2. D. Roy Choudhry, Shail Jain
“Linear Integrated Circuits”
New Age International Pvt. Ltd., 2000. 2 2

3. INTRODUCTION: Analog and Digital Signals: Analog Signal Digital Signal
Analog signal exhibits a continuous variation over its range of activity. The vast majority of signals in the world around us are analog. The electronic circuits that process such signals are known as analog circuits.
Analog Signal
An alternative form of signals is digital signal which is a sequence of numbers and each number represents the signal magnitude at an instant of time.
The analog signal can be converted to digital form by a process called sampling in which the signal is allowed only at regular interval of time. The sampled signal is no longer continuous, rather it is said to be quantized or discretized or digitized. The digital signals can be processed using electronic circuits known as the digital circuits.
Digital Signal 3 3

4. Linear Amplifier:
When the output of the amplifier is a proportional change of the input, the amplifier is referred as Linear amplifier.
For linear amplifier the input and output relationship can be given as
Vout(t) = A Vin(t)
where, A is a constant known as the amplifier gain.
Input
Output
Linear Amplifier
When the output of the amplifier is not a proportional change of the input, the amplifier is referred as non-linear amplifier.
Input
Non-linear Amplifier
Output
A single silicon chip with a large number of semiconductor devices fabricated on it, is referred as Integrated Circuit (IC). The Integrated circuits with linear input and output relationship are referred as Linear Integrated Circuits. 4 4

5. Operational Amplifier (Op-Amp)-μA741
Op-amp is a Linear Integrated Circuit used to amplify dc as well as ac signals and also in signal conditioning, filtering or to perform mathematical operations such as addition, subtraction, integration and differentiation.
The input-output relationship of the op-amp is Vo = A (Vn – Vi)
where, A is the open loop gain of the op-amp.
741 Op-Amp Pin Diagram 5 5

6. Op-Amp Schematic Symbol
Op-Amp (contd…) A
Op-Amp Schematic Symbol
An ac signal (or dc voltage) applied to the non-inverting terminal produces an in-phase (or same polarity) signal at the output.
An ac signal (or dc voltage) applied to the inverting terminal produces an 180o out-of-phase (or opposite polarity) signal at the output. 6 6

7. CHARACTERISTICS OF AN OP-AMP
Input Resistance (Ri): equivalent resistance that can be measured at either the inverting or non-inverting input terminal with the other terminal connected to the ground.
Output Resistance (Ro): equivalent resistance that can be measured between the output terminal of the op-amp and the ground.
Voltage Gain: ratio of the output voltage and the differential input voltage.
Common-mode rejection ratio (CMRR): ratio of differential gain (Ad) and the common mode gain (Acm).Ad is very large so CMRR is also large.
Slew Rate (SR): maximum rate of change of output voltage per unit of time. It indicates how rapidly the output of an op-amp can change in response to changes in the input frequency.
Gain-Bandwidth Product (GBP)/Closed-loop bandwidth/Unity gain bandwidth: bandwidth of the op-amp when the voltage gain is 1.

8. CHARACTERISTICS OF AN IDEAL OP-AMP
Infinite voltage gain A
Infinite input resistance, so that any signal source can drive it and there is no loading of the preceding stage.
Zero output resistance, so that the output can drive an infinite number of other devices and so that it can supply as much current as necessary to the load.
Zero output voltage when input voltage is zero.
Infinite bandwidth so that any frequency signal from 0 to ∞ Hz can be amplified without attenuation. This ensures that the gain of the op-amp will be constant over the frequency range from d.c (zero frequency) to infinite frequency. So op-amp can amplify d.c as well as a.c signals.
Infinite common-mode rejection ratio (CMRR) so that the output common-mode noise voltage is zero.
Infinite slew rate so that output voltage changes occur simultaneously with input voltage changes.

9. COMPARISON BETWEEN CHARACTERISTICS OF AN IDEAL AND REAL OP-AMP (μA741)
Parameters
Ideal
Real
Open Loop Gain A ∞
105
Input Impedance Ri
2 MΩ
Output Impedance Ro
75 Ω
Gain-Bandwidth Product
1 MHz
CMRR
90 dB
Slew rate
0.5V/µs 9

10. Equivalent Circuit of an Op-Amp
Vo = A(v+ - v-)
The output voltage is directly proportional to the algebraic difference between the two input voltages.
The op-amp amplifies the difference between the two input voltages and not the input voltage itself.
Hence the polarity of the output voltage depends on the polarity of the difference voltage. vo
Voltage Gain (A) of the Amplifier
where Vin=V+ – V-
and in Decibels or (dB) is given as: 10 10

11. Logarithm Decibels or (dB) Example: A = 20dB This means 20 log A = 20
or log A = 1
or log10A =1
or A = 101 = 10
A = 60dB
20 log A = 60
or log A = 3
or log10A = 3
or A = 103 = 1000
A in dB is defined as
A dB = 20 log10 A
or A dB = 20 log A 11 11

12. Distortion
The output voltage never excess the DC voltage supply of the Op-Amp

13. Open-loop Frequency Response13

14. Therefore, GBP = Gain x Bandwidth or A x BW.
From this frequency response curve we can see that the product of the gain against frequency is constant at any point along the curve.
Also that the unity gain (0dB) frequency also determines the gain of the amplifier at any point along the curve. This constant is generally known as the Gain Bandwidth Product or GBP.
Therefore, GBP = Gain x Bandwidth or A x BW.
For example, from the graph above the gain of the amplifier at 100KHz,
A = 20dB or 10, then the
GBP = 100,000Hz x 10 = 1,000,000Hz.
Similarly, the gain at 1KHz, A = 60dB or 1000, therefore the GBP = 1,000 x 1,000 = 1,000,000Hz. The same!. 14

15. Virtual Short
As V1 and V2 are not shorted but are at equal potential, therefore called virtual short. 15

16. Two ideal Op-Amp Properties
The voltage between V+ and V is zero V+ = V
The current into both V+ and V terminals is zero.
I+ = 0 ; I- = 0

17. Non-Inverting Amplifier
If RF = Rin, Vout = - Vin
Non-Inverting Amplifier
Due to virtual short Vin = V1 17

18. Solution: Solution: Example
Find the closed loop gain of the following inverting amplifier circuit.
Solution:
Example
If Rin is 10kΩ, what value of Rf is required to produce a non-inverting amplifier with voltage gain of 25?
Solution: 18

19. Voltage Follower (Unity Gain Buffer)
In non-inverting amplifier if we make the feedback resistor, Rf = 0 then the circuit will have a fixed gain of "1" and is called a Voltage Follower.
As the input signal is connected directly to the non-inverting input of the amplifier the output signal is not inverted resulting in the output voltage being equal to the input voltage
Vout = Vin
The input impedance of the voltage follower circuit is very high, typically above 1MΩ as it is equal to that of the operational amplifiers input impedance. 19

20. Integrator V2 = 0 Integrator Circuit +1V -1VIF
Integrator Circuit
The output voltage Vout is inversly proportinal to the negative of RinC and proportional to the integral of the input voltage Vin with respect to time. The minus sign (-) indicates a 1800 phase shift between input and output.
+1V
-1V 20

21. Practical Integrator
Design Steps:
Practical Integrator Circuit 21

22. Differentiator V2 = 0 Differentiator Circuit
The output voltage Vout is a constant (-RfC) times the derivative of the input voltage Vin with respect to time. The minus sign indicates a 1800 phase shift between input and output.
If we apply a constantly changing signal such as a Square-wave or Triangular type signal to the input of a differentiator circuit, the resultant output signal will be changed and whose final shape is dependant upon the RC time constant. 22

23. Practical Differentiator
Design Steps:
Select fa equal to the highest frequency to be differentiated.
Assume C1<1μF and then calculate RF.
Choose fb = 20 fa, calculate R1.
Calculate CF such that
R1C1 = RFCF
Practical Differentiator Circuit 23

24. Example
Example
Design a practical integrator circuit with a dc gain of 10, to integrate a square wave
of 10kHz.
Design a practical differentiator circuit to differentiate an input signal that varies in frequency from 10Hz to about 1kHz. 24

25. Summing Amplifier
Summing Amplifier
However, if all the input impedances, (Rin) are equal in value the final equation for the output voltage is given as: 25

26. A Scaling Summing Amplifier can be made if the individual input resistors are "NOT" equal. Then the equation would have to be modified to:
Example: Find the output voltage of the following Summing Amplifier circuit. 26

27. Difference Amplifier Due to virtual short Va = Vb Va Vb
Difference Amplifier Circuit
Difference Amplifier
Due to virtual short Va = Vb
When R1 = R3 and R2 = R4 the transfer function becomes:
If R1 = R2, then
Thus the amplifier becomes a Unity Gain Differential Amplifier. 27

28. Instrumentation Amplifier
In a number of industrial and consumer applications, the measurement of physical quantities is usually done with the help of transducers. The output of transducer has to be amplified in order to drive the indicator or display system. This function is performed by an instrumentation amplifier which are high gain differential amplifiers with high input impedance and a single ended output. 28

29. Instrumentation Amplifier
The output stage is a standard basic difference amplifier 29

30. Input current of op-amp A1 & A2 both are zero
Input current of op-amp A1 & A2 both are zero. Hence I remains same through Rf1,Rf2 and RG. Applying Ohm’s law between nodes E & F we get. 30

31. Comparators
Comparator is a circuit which compares the amplitude of one voltage input with another and produce either a high or a low output voltage depending on which input is higher.
Vin R1
Vout + - R2
+VS VT
An example of a comparator circuit is shown. The input is compared with a reference set by the voltage-divider. Notice that there is no feedback; the op-amp is operated in open-loop, so the output will be in saturation i.e.
Vin > VT , VOUT is Positive
Vin < VT , VOUT is Negative 31

32. Example:
Sketch the output of the comparator in relationship to the input; assume the maximum output is ±13 V.
Solution:
The threshold is +4.2 V. The output is in positive saturation when Vin > +4.2 V
Vout
Vin
+10 V
-10 V
0 V
+4.2 V
+13 V
-13 V
VT= VT
Vin R1 + - R2
VS = +15 V
10 kW
3.9 kW
Vout
Vin < VT (=4.2V), VOUT is Negative
Vin > VT (=4.2V), VOUT is Positive 32

33. Example:
Show the output of the comparator for the last example if the inputs to the op-amp are reversed.
Solution:
The threshold is still +4.2 V but now the output is in negative saturation when Vin > +4.2 V.
+10 V
Vin R1 + - R2
VS = +15 V
10 kW
3.9 kW VT
+4.2 V
Vin
0 V
-10 V
+13 V
Vout
0 V
Vin < VT (=4.2V), VOUT is Positive
Vin > VT (=4.2V), VOUT is Negative
-13 V 33

34. Schmitt trigger
Schmitt trigger is a regenerative comparator. It is a comparator circuit that incorporates positive feedback.
It converts sinusoidal input into a square wave output. The output of Schmitt trigger swings between upper and lower threshold voltages, which are the reference voltages of the input waveform. Depending on the side at which we apply the external input, a Schmitt trigger can be of the inverting or the non-inverting type. 34

35. Non-Inverting Schmitt trigger
According to Superposition Theorem:
For the circuit to switch its output state from Vsat+ to Vsat-VP=0. VP
Non-Inverting Schmitt Trigger
When Vout = Vsat-, value of Vin to switch output state from Vsat- to Vsat+ is
When Vout = Vsat+, value of Vin to switch output state from Vsat+ to Vsat- is
The hysteresis width VH is 35
Hysteresis Loop

36. Example:
For the Schmitt trigger shown below calculate the upper threshold voltage, lower threshold voltage and the hysteresis width if the saturation voltages are ±12V.
Solution: 3k 1k 36

37. Output of a Schmitt Trigger
Output of a Comparator
Output of a Schmitt Trigger 37

38. Precision Half-Wave Rectifier
The precision rectifier/super diode, is a configuration obtained with an op-amp in order to have a circuit behaving like an ideal diode and rectifier
The rectifier which rectifies the AC voltage below 0.7V is referred as precision rectifier.
useful for high-precision signal processing
When the input vi is negative, there is a negative voltage on the diode, so it works like an open circuit, there is no current in the load and the output voltage is zero.
When the input vi starts increasing in the positive direction, the output voltage at the op-amp output pin also starts increasing until the diode is forward biased. When the diode is forward biased it closes a feedback loop and the op-amp works as a voltage follower. Hence the output voltage is equal to the input. 38

39. Precision Half-Wave Rectifier-(Continued)IL
Transfer Characteristics 39

40. Precision Full Wave Rectifier with Transfer Characteristics:
(a) Precision Full Wave Rectifier
(b) Transfer Characteristics 40

41. Precision Full-Wave Rectifier/Absolute Value RectifierIL 41

42. Astable Multivibrator/Square Wave Generator:
No external signal is required to produce the changes in state
Initially the voltage across the capacitor is 0V at the instant the dc supply voltage is applied, i.e voltage at the inverting terminal is 0.
With Vo = Vsat+, VP=VUT and the capacitor C starts charging towards Vsat+ through the feed back path provided by the resistor Rf.
As long as the capacitor voltage VC is less than VUT, the output voltage remains at Vsat+.
As soon as VC charges to a value slightly greater than VUT , the voltage at the inverting terminal becomes greater than that at the non-inverting terminal.
This switches the output voltage from Vsat+ to Vsat- and thus VP = VLT 42

43. Astable Multivibrator Contd:
As Vo switches to Vsat-, the capacitor starts to discharge through Rf to VLT
When VC becomes slightly lesser than VLT, output voltage Vo switches back to Vsat+.
Then the capacitor voltage will become 0V from VLT and then recharge to VUT and the process is repeating and the waveform becomes periodic. 43

44. Frequency of Oscillation of Astable Multivibrator:
The frequency of oscillation is determined by the time the capacitor takes to charge from VUT to VLT and vice versa.
The voltage across the capacitor is
where VC(t) is the instantaneous voltage across the capacitor, Vinitial is the initial voltage and Vmax is the voltage towards which the capacitor is charging
Consider the charging of the capacitor from VLT(initial voltage) to VUT(instantaneous voltage), Vsat+ is the maximum voltage and T is the time constant(=RfC)
At t=T1, voltage across the capacitor reaches VUT 44

45. Therefore the total time is
The time taken by capacitor to charge from VUT to VLT is same as time required for charging capacitor from VLT to VUT.
Therefore the total time is 45

46. Example: For the astable multivibrator circuit discussed above if R2=100kΩ, R1=86kΩ, Vsat+=+15V, Vsat-=-15V, Rf=100kΩ and C=0.1µF, find (a) VUT, (b) VLT and (c) frequency of oscillation.
Solution: 46

47. Triangular Wave Generator:
If a square wave from a square wave generator is integrated we get a triangular wave at the output of the integrator. The frequency of the square wave and the triangle wave is the same.
The time period
T = T1 + T2
During T1
Similarly during T2 47

48. the time period T can be expressed as T=T1+T2=2T1
If the magnitude of the saturation levels L+ and L- are equal then T1 = T2 and
the time period T can be expressed as T=T1+T2=2T1
For a Schmitt trigger as VH=(VTH – VTL) = 2(R1 / R2) VS = 2(R1 / R2)L+, therefore
Therefore frequency of the triangle wave can be expressed as
The complete circuit for triangular wave generator is shown in Fig. (d)
Note:
Fig. (d) Triangular Wave Generator 48

49. Fig. (1) Triangular Wave Generator
Example: In Fig.(1) the op-amp saturation voltage Vsat = ±10V. The capacitor C = 0.01µF and R1 =10kΩ
are used. If the frequency of the generator is 1kHz and the triangular wave has 10V peak to peak amplitude design the values of R and R2 .
Fig. (1) Triangular Wave Generator
Solution: Given f = 1kHz, C = 0.01µF, R1 =10kΩ and VS = ±10V
The triangular wave peak to peak amplitude = (VTH – VTL) = 2(R1 / R2) Vsat = 10V
Therefore
2(R1 / R2) x 10V = 10V
or 2(R1 / R2) = 1
or R2 = 2R1 = 2 x 10k
R2 = 20kΩ
Since
Therefore 49

50. Saw-tooth Wave Generator
VCONT R4
In triangular wave the rise and fall time are always equal. The saw-tooth wave has unequal rise and fall time. The triangular wave generator can be converted in to saw-tooth wave generator by injecting a variable DC voltage VCONT at the non-inverting input terminal of the integrator as shown in Fig. (a).
If the VCONT is more towards –VC the rise time of saw-tooth wave becomes longer than the fall time as shown in Fig. (b). On the other hand if the VCONT is moved towards +VC, the fall time becomes longer than the rise time. Also the frequency of the saw tooth wave decreases as R4 is adjusted towards +VC or –VC. However, the amplitude of saw-tooth wave remain constant.
Fig. (a) Saw-tooth wave generator V2 t
Time
Period
Rise
Fall Time t
Fig.(b) Saw-tooth waveform 50

51. Signals with mixed Frequencies
FILTERS
A filter is a device that passes electric signals of certain frequencies or
frequency ranges while preventing the passage of others.
Signals with mixed Frequencies
FILTER
INPUT
OUTPUT
Filtered Output
Filter circuits are used in a wide variety of applications:
In the field of telecommunication, Data acquisition systems, System power supplies etc.
The electronic filters may be Passive Filters or Active Filters. 51

52. Passive Filters:
The passive filters are the electronic filters that are made only from passive elements -- resistors, capacitors, inductors, and transformers. A passive filter has guaranteed stability.
Examples of Passive Filters: R C L
VIN
VOUT R C
VIN
VOUT
VIN R C
VOUT
First-order Low Pass Filter
First-order High Pass Filter
Second-order Band Pass Filter
All of these filters usually consist of passive components such as inductors (L), resistors (R), and capacitors (C). They are then called LRC filters. In the frequency range (1 Hz to 1 MHz), however, the inductor value becomes very large and the inductor itself gets quite bulky, making economical production difficult. In these cases, active filters become important which do not use the passive inductor. 52

53. Active Filters:
The active filters are the type of electronic filters, that use of one or more active components i.e. voltage amplifiers or buffer amplifiers (transistors, operational amplifiers etc.) along with R, C elements. Active filters use no passive inductance and provide gain.
Example of Active Filter:
Second-Order Active Low-Pass
As the order of the filter increases the slope of the frequency response becomes more steep which is desirable but hardware complexity increases. 53

54. Types of Filters-Ideal Frequency Responses:
1. Low pass Filter (LPF) High pass (HPF) Band pass (BPF) f G
Pass Band
Stop
Band fH
Pass
Band
Stop f G fL f G
Pass Band
Stop
Band fL fH f0
4. Band Reject (BRF) All pass (APF)
Pass Band
Stop
Band fL fH f0 f G G f
All pass
Real Low-pass Responses 54

55. Frequency Response of Practical FiltersfL fH f
A(dB)
BPF fH f
A(dB)
LPF
Pass-
band fL f
A(dB)
HPF fL fH f
A(dB)
BRF
Active filters provide low output impedance which avoids the need of buffer at the output and also provide the filter gain. 55

56. First-Order Non-Inverting Active Low-Pass Filter:
VIN
VOUT V1
The first-order active low pass filter is shown in Fig.1. Its transfer gain can be analyzed as follows.
Fig. 1.
Frequency
Gainn f0
0.7
Pass-Band
and
Therefore the transfer gain of the active low pass filter of Fig. 1 can be expressed as
Frequency response characteristics of LPF
Where, the pole frequency or
filter gain, A=
The f0 is also known as the low-pass cut-off frequency or 3-dB frequency. 56

57. First-Order Non-inverting Active High-Pass Filter:
The first-order active high pass filter is shown in Fig. 1. Its transfer gain can be analyzed as follows.
Fig. 1.
and
Frequency f0
0.7
Pass-Band
Therefore the transfer gain of the active high pass filter of Fig. 2 can be expressed as
Frequency response characteristics of HPF
Where the filter gain A=
The f0 is also known as the high-pass cut-off frequency or 3-dB frequency.
And the pole frequency 57

58. Design Steps for a First-Order Active Low-Pass and High-Pass Filter:
Step 1: Choose a value of cutoff frequency fo
Step 2: Select a value of C ≤ 1μF
Step 3: Calculate R using
Step 4: Finally, select value of R1 and R2 dependent on the desired pass band gain
Example: Design an active low pass non-inverting filter for low pass band gain of 2 and the cutoff frequency f0 = 10kHz.
Solution:
The transfer gain of the filter can be expressed as: C R1 R2 R
VIN
VOUT V1
where
and
For gain A = 2, Let R1 = R2 = R = 10k. 58

59. Band-Pass Filter: frequency selector
Allows one particular band of frequencies to pass.
The pass band is between the two cut off frequencies fH and fL such that fH >fL
Any frequency outside this band gets attenuated.
The pass band which is between fH & fL is called band width (BW)
BW= fH - fL
The frequency at the center of the pass band is called center frequency (fC)
There are two types of band pass filters based on the figure of merit or quality factor Q
Wide band pass filter: for Q<10, pass band is wide & large BW.
Narrow band pass filter: for Q>10, pass band is very narrow & the BW is very small.
Higher the value of Q, narrower is the pass band
For wide band pass filter
Wide Band Pass Filter is realized by cascading high pass and low pass filter
Atotal = AH x AL 59

60. Solution: Design of the low pass filter
Example: Design a wide band pass filter having fL = 400 Hz & fH = 2 kHz and pass band gain of 4. Calculate the Q factor of the filter.
Solution: Design of the low pass filter 4 1 2 3
Design of the high pass filter
Atotal = AH x AL = 4
Let AH = AL = 2 60

61. All-pass filter:
The filters that do not filter any frequencies of a complex input signal, but just add a linear phase shift to each frequency component, are called all-pass filters. The first order all pass filter using op-amp is shown in Fig. 1. Its transfer function can be analyzed as follows.
Fig.1: First-Order All-Pass
VIN
VOUT R C V1 V2
Using super position theorem. Assume input to the non-inverting terminal as zero. Now the circuit acts as an inverting amplifier.
Assume input to the inverting terminal as zero. Now the circuit acts as non-inverting amplifier. 61

62. Magnitude of the transfer function is62

63. Solution: The phase angle between input and output Φ =-2tan-1(2πfCR)
Example: For the all pass filter of Fig. 1, if R = 15.9K, C = 0.01μF, find the phase angle Φ if the frequency of the input VIN is 1 KHz.
Fig.1: First-Order All-Pass
VIN
VOUT R C V1 V2
Solution: The phase angle between input and output
Φ =-2tan-1(2πfCR)
=-2tan-1(2πx1x103 x0.01x10-6 x15.9x103 )
= -900
This means that the output VOUT has the same frequency and amplitude as VIN but lags VIN by 900. 63

64. Second-Order Active Low-Pass Filter:
The active second order LPF is shown in Fig. (a). Its transfer function can be analyzed by applying the KVL at node v1 and node v2. v2=v0. For simplicity, use conductance values rather than resistances.
Fig. a. 64

65. 65

66. Second-Order Active High-Pass Filter:
The active second order HPF is shown in Fig. (a). 66

67. 67

68. Second-Order Active Band-Pass Filter:
The active second order BPF is shown in Fig. (a).
Fig. a. 68

69. 69

70. Cascading Filter Stages for Higher-Order Filters:
Ist Order
2nd Order
3rd Order
4th Order
5th Order
6th Order 70

71. Fourth-Order Active RC Low-Pass Filter:
Second
Order
VIN
VOUT
Fourth-Order
The two second order low pass sections are cascaded through as shown in the Figure to construct a 4th order low filter.
Fourth-Order Active RC Low-Pass Filter 71

72. 555 Timer - An Introduction
The 8-pin 555 timer is one of the most useful ICs ever made. With just a few external components it can be used to build many useful circuits such as monostable and astable multivibrator, linear ramp generators, burglar and toxic gas alarms, voltage regulators, etc.
Features of IC 555 Timer
adjustable duty cycle i.e. time delays ranging from few microseconds to several hours
two operating modes: monostable (one-shoot) multivibrator and astable (free-running) multivibrator
available in 3 packages: 8-pin metal can, 8-pin mini DIP or a 14-pin DIP (IC556 consists of two 555 timers)
operates with a supply voltage in the range of +5V to +18V
Its output is compatible with TTL, CMOS and op-amp circuits. 72

73. 555 Timer SR-Flip-Flop: Transistor Switch:
The 555 timer consists of two comparators, an SR flip-flop, a transistor Q that operates as a switch and a voltage divider with three equal resistors.
SR-Flip-Flop:
The flip-flop is a two-state device whose output can be either a high voltage level (set) or a low voltage level (reset). The state of the output depends on the input signals as shown in the Table.
Inputs
Outputs S R
Remark NC 1
RESET
SET ? NA S R Q Q’
Transistor Switch:
VQ’ S
For
VQ’=5V, Switch S is closed
VQ’=0V, Switch S is open 73

74. Internal Diagram of the 555 Timer IC
Functions of 555 Timer Pins
OUTPUT R S Q
FLIP
FLOP
VCC
100Ω
COMPARATOR1
COMPARATOR2
THRESHOLD
TRIGGER
DISCHARGE
GROUND
VTH
VTL
555 Timer IC
The 555 timer’s IC Pin Diagram
Pin 1: Ground
Pin 8: Supply +Vcc (+5V to +18V)
Pin 3: Output
Internal Diagram of the 555 Timer IC 74

75. Functions of 555 Timer Pins
Pin 2 and 6: Trigger and Threshold input - A string of three resistors (R) are of equal value (5kΩ) therefore the comparator 1 has a reference of 2/3 Vcc and the comparator 2 has a reference of 1/3 Vcc. The comparators output controls the state of the flip-flop.
For Threshold > 2/3 Vcc, Flip Flop is reset, output is low
For Trigger < 1/3 Vcc, Flip Flop is set, output is high
Pin 4: Reset Input - Used to interrupt a timing interval and discharge the capacitor before it reaches 2/3 Vcc making output low.
Pin 5: Control Voltage Input – Used to change this reference levels by applying external voltage at the control voltage input.
Pin 7: Discharge - Connected internally to the collector of the NPN transistor. When the output is high , the transistor is off and acts as an open circuit. When output is low, the transistor is saturated and acts as a short circuit. 75

76. Monostable (one shot) Multivibrator using 555 Timer:VC R
VOUT Tr 1
The 555 timer can be operated as a monostable multivibrator by connecting an external resistor and capacitor.
When trigger is applied, a single positive pulse is produced at the output.
The duration of this output pulse is set by the values of external R and C. C
Initially when the output is low, the transistor is ON and capacitor is shorted to ground and Vc =0. As soon as VTr goes below VTL, S=1 then Q = 1, Q1 is OFF and capacitor starts charging towards Vcc through R. When Vc equals 2/3 Vcc, comparator 1's output switches from 0 to 1, i.e. R=1 so Q=0, Q1 on and hence capacitor rapidly discharges through the transistor. The output of the monostable remains low until a trigger pulse is again applied. Then the cycle repeats. The RC time constant sets the width of the output pulse.
VCC
0 = LOW, 1 = HIGH 76

77. Monostable-Continued
If at t = 0, the trigger pulse is applied and the capacitor starts charging, then
(1)
At t = T, , therefore equation (1) reduces to or
Therefore,
Where C is in farads (F), R is in Ω and T is in seconds
Example- If R = 100KΩ, C = 0.01µF, then the pulse width
T = 1.1 x 100 x 103 x 0.01 x 10-6 = s = 1.1 ms 77

78. Astable ( Free Running ) Multivibrator using 555:VC
Initially, when the dc power is turned on, the capacitor is discharged and holds the trigger voltage at less than 1/3 Vcc, so the output of comparator 2 i.e S=1 and the transistor turns OFF so the external capacitor C starts charging through RA and RB. When the capacitor voltage reaches 2/3 Vcc the output of comparator 1 i.e. R=1 and the Q=0 and the transistor turns ON. The capacitor now discharges through RB and the transistor. The cycle repeats itself and the device oscillates. The capacitor is periodically charged and discharged between 2/3 Vcc and 1/3 Vcc respectively. During charging time output is high and during discharge time output is low. VC
VTH
VTL t
VOUT
VCC TH TL 78

79. Astable- Continued
During charging VC rises from VTL to VTH , thus the capacitor charging voltage VC can be expressed as
(1)
The charging starts at t = 0 and ends at t = TTH at VC = VTH =(2/3)VCC, therefore equation (1) reduces to
At VC = VTL , t = TL,, thus
Duty Cycle = TH / T =
During TL the capacitor discharges from VTH to VTL,, thus 79

80. Example- In the astable multivibrator, RA=2. 2kΩ,RB=3. 9k Ω and C = 0
Example- In the astable multivibrator, RA=2.2kΩ,RB=3.9k Ω and C = 0.1µF. Determine its positive pulse width, negative pulse width and the free-running frequency. 80

81. Voltage Controlled Oscillator (VCO) using 555 Timer
A VCO can be made from an Astable Multivibrator with variable control voltage.
The control voltage is externally set by the potentiometer.
With change in the control voltage, the upper threshold voltage changes and thus the time required to charge and discharge the capacitor changes. As a result the frequency of the output voltage changes.
If control voltage is increased, the capacitor will take more time to charge and discharge and therefore frequency will decrease.
On the other hand, if control voltage is decreased, the capacitor will take less time to charge and discharge and therefore increasing the frequency of the output signal. Thus by varying control voltage we can change frequency. 81

82. Voltage Controlled Oscillator (VCO) – 566VCO
provides simultaneous square wave and triangular wave outputs as a function of input voltage.
The triangular wave is generated by alternately charging the external capacitor C1 by one current source and then linearly discharging it by another.
The charge-discharge levels are determined by the Schmitt trigger which provides the square wave output.
Both the output waveforms are buffered so that the output impedance of each is 50Ω.
The typical amplitude of the triangular wave is 2.4Vpp and that of the square wave is 5.4Vpp. 82

83. 566 VCO – Typical Connection
The frequency of oscillation is determined by an external resistor R1, capacitor C1 and the voltage VC applied to the control terminal 5.
The frequency of the output waveforms is approximately
The maximum output frequency is 1MHz.
where 2kΩ<R1<20kΩ
Used in converting low-frequency signals such as ECG, EEG into an audio frequency range to be transmitted over telephone lines or two way radio communication for diagnostic purposes or to record on magnetic tapes for further reference.
Fig 1 83

84. Example- In the circuit of the VCO shown in Fig 1,if +V=12V, R2=1
Example- In the circuit of the VCO shown in Fig 1,if +V=12V, R2=1.5kΩ, R1=R3=10kΩ and C1=0.001μF.
a. Determine the frequency of the output waveforms.
b. Compute the modulation in the output frequencies if VC is varied between V and 11.5V.
c. Draw the square wave output waveform if the modulating input is a sine wave.
Solution:
c. During the positive half-cycle of the sine wave input, the control voltage VC will increase. Therefore the frequency of the output waveform will decrease and the time period will increase, Exactly the opposite action will take place during the negative half-cycle of the input. 84