CHAPTER FOUR

WAVE SHAPING CIRCUITS

 Introduction
 Wave generating circuits
 Multivibrators
 Schmitt trigger circuits
 Timer circuits (the 555 timer)
 modulator and demodulator circuits

1
 4.1 Introduction

What is wave shaping circuits?

• It is the one used to change to an input

waveform to yield a desired output waveform.

• used to change the shape of a waveform from

alternating current or direct current.

2
 Cont.
For example
 clipper circuit is used to
Removing undesired portion of a signal

3
Cont.

4
Cont’d

5
 Cont.
What is linear wave shaping circuits?

• It is called a linear circuit because the output of such a circuit is a linear

function of its input. if you give a sinusoidal input the output will be

sinusoidal.

What is nonlinear wave shaping?

• The process whereby the form of sinusoidal signals is going to be altered

by transmitting through a non-linear network is called non-linear wave

shaping. Non-linear elements (like diodes, transistors) in combination

with resistors can function as clipper circuit.

6
 Cont.
 Wave shaping circuits can be classifying:
based on the components constituting an electrical
waveform signal (frequency, amplitude, pulse width, period
and direction).
 Filters
 Multivibators
 Rectifiers
 Oscillators
 Schmitt trigger
 Timer circuit
 Modulator and demodulator
7
 4.2 Wave generating circuits
Filters:
Filters are circuits that are capable of passing signals
within a band of frequencies while rejecting or
blocking signals of frequencies outside this band. This
property of filters is also called “frequency
selectivity”.

Depending on component

 active filter

 passive filter 8
 cont,.
Depending on functionality both passive and active

filters can be:

 Low pass filter

 High pass filter

 Band pass filter

 Band reject filter

 all pass filter 9

 Cont.

Fig.4.1 ideal filter response 10

Cont.
Passive filters: The circuits built using RC, RL, or RLC circuits.

Active filters : The circuits that employ one or more op-amps in the
design an addition to resistors, capacitors and inductors.

11
 Active versus Passive Filters
Both active and passive filters are used in electronic circuits. However, active
filters offer the following advantages over passive filters:

 Flexibility of gain and frequency adjustment: Since op-amps can provide

a voltage gain, the input signal in active filters is not attenuated as it is in
passive filters. It is easy to adjust or tune active filters.

 No loading effect: Because of the high input resistance and low output
resistance of op-amps, active filters do not cause loading of the input
source or the load.

 Cost and size: Active filters are less expensive than passive filters because
of the availability of low-cost op-amps and the absence of inductors.
12
Cont.

 Parasitic: Parasitic are reduced in active filters because of their smaller

size.

 Digital integration: Analog filters and digital circuitry can be

implemented on the same IC chip.

 Filtering functions: Active filters can realize a wider range of filtering

functions than passive filters.

 Gain: An active filter can provide gain, whereas a passive filter often

exhibits a significant loss

13
Cont.

Active filters also have some disadvantages:

 Bandwidth: Active components have a finite bandwidth, which limits the

applications of active filters to the audio-frequency range. Passive filters

do not have such an upper-frequency limitation and can be used up to

approximately 500 MHz.

 Drifts: Active filters are sensitive to component drifts due to

manufacturing tolerances or environmental changes; in contrast, passive

filters are less affected by such factors.

14
 Cont.

 Power supplies: Active filters require power supplies,

whereas passive filters do not.

 Distortion: Active filters can handle only a limited range of

signal magnitudes; beyond this range, they introduce

unacceptable distortion.

 Noise: Active filters use resistors and active elements, which

produce electrical noise.

15
 4.2.1. RC low pass& high pass circuits
1. RC low -pass filter
A low-pass filter is a filter that passes frequencies from 0Hz to
critical frequency, fc and significantly attenuates all other frequencies.

Fig.4.2.(a) passive RC low pass

filter circuit Fig.4.2.(b) active RC low pass
filter circuit

16
 cont.

Fig.4.2.(c) General diagram of an active filter.

17
 Cont.

roll-off rate

Actual response Ideal response

Fig.4.3 actual and ideal response of low pass filter

18
 Cont.
Pass-band of a filter is the range of frequencies that are allowed
to pass through the filter with minimum attenuation (usually
defined as less than -3 dB of attenuation).
Transition region shows the area where the fall-off occurs.

19
 Cont.
Stop-band is the range of frequencies that have the most

attenuation.

Critical frequency, fc, (also called the cutoff frequency) defines

the end of the passband and normally specified at the point where

the response drops – 3 dB (70.7%) from the passband response.

20
 Cont.

 At low frequencies, XC is very high and the capacitor circuit

can be considered as open circuit. Under this condition, Vo = Vin

or AV = 1 (unity).

 At very high frequencies, XC is very low and the Vo is small as

compared with Vin. Hence the gain falls and drops off gradually
as the frequency is increased.

21
 Cont.

 The bandwidth of an ideal low-pass filter is equal to fc:

BW  f c
The critical frequency of a low-pass RC filter occurs when
XC = R and can be calculated using the formula below:

1
fc 
2 RC
22
 Cont.
2. RC high-pass filter
A high-pass filter is a filter that significantly attenuates or rejects all
frequencies below fc and passes all frequencies above fc.
 The passband of a high-pass filter is all frequencies above the critical
frequency.

Fig.4.4.(a) passive RC high pass Fig.4.4.(b) active RC high pass

filter circuit filter circuit
23
 Cont.

Actual response Ideal response

Fig.4.4.(c) actual and ideal response of high pass filter

24
 Cont.

At zero frequency the reactance of the capacitor is infinity and

so it blocks the input and hence the output is zero. This capacitor
called blocking capacitor.

The critical frequency of a high-pass RC filter occurs when

XC = R and can be calculated using the formula below:

1
fc 
2 RC

25
 4.2.2 RL low pass & high pass circuits

1. RL low pass filter

Fig.4.5.(a) passive RL low pass

filter circuit
26
 Cont.
• Vo can be found from the voltage divider formula:

27
Cont.

28
Cont.

29
 Cont.
2. RL high pass filter
A high pass RL filter is a filter composed of a resistor and
inductor which passes through high-frequency signals. To build
a high pass RL filter, the inductor is placed in parallel to the
power source signals entering the circuit.

30
 Cont.

Fig.4.6.(a) passive RL high pass

Fig.4.6.(b) response of RL high
filter circuit
pass filter circuit

31
 Cont.

• In such a filter resistance R offers fixed opposition.

the reactance of the inductor L increases with the
increase in frequency, so high frequencies signal
develops across L but signal frequency below cut off
frequency develops negligible voltage across
inductor L.
• High frequency output voltage developed across
inductor is given by the equation

32
 Cont.

lower cut-off frequency for RL high pass filter becomes

33
 4.2.3 RLC series& parallel circuits
1. RLC series circuit (as simple filter)

Fig. 4.7 (a), low-pass, (b), band-pass, and (c) high-pass filters.
34
 Cont.

Fig. 4.7. (d) Series RLC circuit

35
 Cont.

• In the above circuit, the three components are

all in series with the voltage source. The
governing differential equation can be found by
substituting into Kirchhoff's voltage law (KVL)
the constitutive equation for each of the three
elements.

36
 Cont.
Using KVL

37
 Cont.

 α and ω0 are both in units of angular frequency. α is called

the neper frequency, or attenuation, and is a measure of how fast
the transient response of the circuit will die away after the
stimulus has been removed. ω0 is the angular resonance
frequency.

38
 Cont.
• For the case of the series RLC circuit these two
parameters are given by

• A useful parameter is the damping factor, ζ,

which is defined as the ratio of these two;

39
 Cont.
• Transient response
The differential equation for the circuit solves in three different ways
depending on the value of ζ. These are underdamped (ζ < 1),
overdamped (ζ > 1) and critically damped (ζ = 1). The differential
equation has the characteristic equation

The roots of the equation in s are

40
 Cont.
2. RLC parallel circuit

Fig. 4.8.(a) Parallel RLC band stop or Notch Filter.

41
 Cont.

Fig.4.8(b)Parallel RLC band pass filter

42
 Band-pass filter

• A band pass filter allows signals with a range of frequencies

(pass band) to pass through and attenuates signals with
frequencies outside this range.

Actual response Ideal response

Fig.4.9(a) actual and ideal response of band-pass filter 43
 Cont’d
• Band pass filters are often a cascade of an LPF and
an HPF

Fig.4.9(b) A simple active band pass filter

44
 Cont.
The bandwidth (BW) is defined as the difference between the
upper critical frequency (fc2) and the lower critical frequency
(fc1).
BW  f c 2  f c1

The frequency about which the pass band is centered is called the
center frequency, fo and defined as the geometric mean of the critical
frequencies.

f o  f c1 f c 2

45
 Cont.
The quality factor (Q) of a band-pass filter is the ratio
of the center frequency to the bandwidth.

fo
Q
BW

The higher value of Q, the narrower the bandwidth

and the better the selectivity for a given value of fo.
 (Q>10) as a narrow-band or (Q<10) as a wide-band
46
 Band-stop filter
It rejects frequencies within the bandwidth, while
passing all others.

Actual response Ideal response

Fig.4.10 actual and ideal response of band-stop filter

47
 4.3 Multivibrators
what is multivibrator?
 It is an electronic circuit that generates square, rectangular,
pulse waveforms, also called nonlinear oscillators or function
generators.
 It is an electronic circuit used to implement a variety of simple
two-state devices such as relaxation oscillators, timers and
flip-flops.
 It consists of two amplifying devices (transistors, vacuum
tubes or other devices) cross-coupled by resistors or
capacitors.

48
 Cont.
 Multivibrator is basically a two amplifier circuits
arranged with regenerative feedback.
What is regenerative(posetive) feedback?
 Feedback in which the portion of the output signal
that is returned to the input has a component that is in
phase with the input signal.

49
 Cont.
Multivibrators types
 Bistable (flip-flops) multivibrator
 Monostable multivibrator
 Astable multivibrator

50
 Cont.
 Circuits have two, well defined states, which can be either
stable or unstable

 A stable state is a state, in which the circuit, in absence of a

driving signal, can remain for an unlimited period of time
 The circuit can remain in an unstable state only for a limited
period of time, after which, in the absence of any exterior
command signals, it switches into the other state.

51
Cont.

52
 Cont.
1. Astable Multivibrator:
 A free-running multivibrator that has NO stable states but
switches continuously between two states this action produces
a train of square wave pulses at a fixed frequency.
 Astable multivibrator circuit consist of two cross coupled RC
amplifiers.
 Consists of two amplifying devices cross-coupled by resistors
and capacitors.
 Typically, R2 = R3, R1 = R4, C1 = C2 and R2 >> R1.
 With no external signal applied, the transistors alternately
switch from cutoff to saturation at a frequency determined by
the RC time constants of the coupling circuits.

53
 Basic mode of operation for astable
multivibrator
The circuit has two states
 State 1: VC1 LOW, VC2 HIGH, Q1 ON (saturation) and Q2 OFF.
 State 2: VC1 HIGH, VC2 LOW, Q1 OFF and Q2 ON (saturation).
 it continuously oscillates from one state to the other.
(Application in Oscillators)

Fig.4.11 circuit of astable multivibrator using BJT 54

 Initial Power-Up
 When the circuit is first powered up, neither transistor is ON.
 Both VB1 and VB2 rise via base resistor R3 and R2
respectively.
 Any one of the transistor will conduct faster than other due to
some circuit imbalance.

55
 Cont.
State-1
 we assume Q1 conducts first and Q2 off (C1 is fully charged).
 Since Q1 conducts and Q2 off hence Vc1 = 0V and Vc2 = VCC.
 VB1 charges up through R3 from below ground towards VCC.
 When VB1 reaches VON (of VBE, ≈1V), Q1 turns on and pulls VC1 from
VCC to VCESat ≈ 0V.
 Due to forward-bias of the BE junction of Q1, VB1 remains at 1V.

56
 State 1(cont’d)
• As C1’s voltage cannot change instantaneously, VB2
drops by VCC.

57
 State 1(cont’d)
 Q2 turns off and VC2 charges up through R4 to VCC (speed set by
the time constant R4C2).
 VB2 charges up through R2 towards VCC (speed set by R2C1,
which is slower than the charging up speed of VC2).

58
 cont’d
 State 2
 When VB2 reaches VON, Q2 turns on and pulls VC2 from VCC
to 0V.
 VB2 remains at VON.

59
 State 2 (cont’d)
 As C2’s voltage cannot change instantaneously, VB1 drops by
VCC.

60
 State 2 Cont’d
 Q1 turns off and VC1 charges up through R1 to VCC, at a rate
set by R1C1.
 VB2 charges up through R3 towards VCC, at a rate set by

R3C2, which is slower.

61
 Cont.

 Back to state 1
 When VB1 reaches Von, the circuit enters state 1 again,
and the process repeats.

62
Switching time & Frequency for Astable Multivibrators

63
Cont.

64
 Cont.
2. monostable multivibrator
 A one-shot multivibrator that has only ONE stable state
and is triggered externally with it returning back to its first
stable state.
• capacitive path between VC2 and VB1 removed.
• In stable state any one transistor conducts
and other is off.

65
 Cont’d
Application of external trigger change the state.
 Stable for one state (state 2 here)
• – Q1 OFF and Q2 ON
• – VC1 High, VC2 Low
 When VB2 is momentarily pulled to ground by an external signal
• VC2 rises to VCC
• Q1 turns on
• VC1 pulled down to 0V
• – Enter state 1 temporarily
 When the external signal goes high
• VB2 charges up to VCC through R2
• After a certain time T, VB2=VON, Q2 turns on
• VC2 pulled to 0V, Q1 turns off
• Enters state 2 and remains there
• Can be used as a timer
66
 Cont’d
3. Bistable Multivibrator:
 has two stable states.
 Both capacitors removed
 Can be forced to either state by Set or Reset signals
 Moves to the other stable state only when triggered.
 The circuit can be flipped from one state to the other by an
external event or trigger. (Application in Flip flop)
 Bistability can be obtained by connecting an amplifier in a
positive feedback loop having loop gain greater than unity.

67
 Cont’d

Fig. 4.13 circuit of bistable multivibrator using BJT

68
 Cont’d

 If Set is low,
• Q1 turns off
• VC1 (Vout) and VB2 rises towards VCC
• Q2 turns on
• VC2 pulled to 0V
• VB1 is latched to 0V
• Circuit remains in state 2 until Reset is low
 If Reset is low
• Similar operation
• Circuit remains in state 1 until Set is low
• Behave as an RS flip-flop (memory element)

69
 APPLICTION OF MULTIVIBRATOR

Some of the uses of multivibrators are

•They are used as a Frequency dividers
•Used as a sawtooth generators
•They are used as wave and pulse generators
•They are used as standard frequency source
•They are used in radar and tv circuits
•They are also used as a memory elements in computer

70
 4.4.schmit trigger circuit
What is Schmitt trigger circuit?
In electronics, a Schmitt trigger is a comparator circuit
with hysteresis implemented by applying positive
feedback to the noninverting input of a comparator or
differential amplifier.
 It is an active circuit which converts an analog input
signal to a digital output signal.
 compares a regular or irregular waveform with a
reference signal and converts the waveform to a square
or pulse wave.
71
 Cont’d
• It is often known as a squaring circuit. It is
also known as a bistable multivibrator
because it has two stable states, low and high.
It can remain in one state indefinitely;
• it moves to the other stable state only when a
triggering signal is applied.

72
 Cont’d
• Schmitt triggers can be classified into two
types depending on the type of op-amp
configuration used: inverting or noninverting.

73
 Cont’d

(a) (b)

Fig 4.14(a) inverting schmitt trigger,

(b) noninverting schmitt trigger

74
 Cont’d
What is difference between Schmitt trigger and comparator?
 A comparator will give either +vsat or -vsat according to the given
input.
 but in a schmitt trigger, the output voltage depends upon the
voltage divider that is attached to the non inverting side of op-amp.
 It is also called a regenerative comparator because it remembers its
old state.

75
 Cont’d

fig.4.15 inverting schmitt trigger

76
 Cont’d

Fig. 4.16 circuit of schmitt trigger using BIT

77
 Response of schmitt trigger for different wave form
• Example
1. for complex waveform

78
 Cont’d
• Different switching thresholds for positive and negative-going
inputs
• Hysteresis voltage = VT+- VT-
2. Transform of waveform

79
 Cont’d
3.

4.

80
 Cont’d
• As long as Vin less than Vut. Vo is +Vsat and using voltage
divider rule.

• As long as Vin greater than Vlt. Vo is -Vsat and using voltage

divider rule.

• Where Vut = upper threshold voltage

• Vlt= lower threshold voltage
• Vsat= saturated voltage
• Vhy= hysteresis voltage
81
 Schmitt Trigger with Reference Voltage
• some applications require shifting the crossover
voltage in either the positive or the negative direction
along the vS-axis. This can be accomplished by adding
a reference voltage Vref to the circuit in below.

fig. 4.17 schmitt trigger with reference voltage 82

 Cont’d
• Assuming that VLt and VHt are symmetric
about the zero-axis, the switching voltage is
given by

83
 example
• For the figure above having R1=10KΩ,

• determine the value of RF and Vref

solution

84
 4.5. Timer circuit (555 timer)
• It is one of the most popular and versatile integrated
circuits.
• It is a combination of digital and analog circuits.
 It is known as the “time machine” as it performs a wide
variety of timing tasks.
 555-Timers, like op-amps can be configured in different
ways to create different circuits.

85
 Cont’d
• Each pin has a function

8
R

VCC
7
DIS

3
Q
6
2 THR
TR

GND
5
CV
NE555

1
Fig. 4.18 pin diagram of 555 timer
86
 Cont’d

Fig .4.19 functional block diagram of 555 timer

87
 Operation of 555 timer
 the timer consists of two comparators CM1 and CM2, an RS flip-
flop, a discharge transistor Q1, and a resistive voltage divider
string.
 The voltage divider sets the voltage at the inverting terminal of
CM1 to 2VCC ⁄3 and the voltage at the noninverting terminal of
CM2 to VCC ⁄3.

88
 Cont’d
• The reset input has the highest priority in setting the state of the
flip-flop.
• Thus, Q is low if the reset input is low, regardless of the inputs to
the comparators. If the reset is not in use, then it is connected to
the positive DC supply VCC so that it does not affect the state of
the flip-flop.
• If the trigger input becomes lower than the voltage at the
noninverting input of CM2 (i.e., <VCC ⁄3), the output of CM2
(i.e., the S input to the flip-flop) will be high. As a result, the Q
output of the flip-flop will be set to high.

89
 Cont’d
• If the threshold input becomes higher than the voltage at the
inverting input of CM1 (i.e., >2VCC ⁄3), the output of CM1
will be high.
• As a result, the Q output of the flip-flop will be reset to low.
• Thus, will be high, and the discharge transistor Q1 will be on
(in saturation), providing a discharge path.

90
 Cont’d
 Applications for the 555 Timer include:
• Bounce-free switches and Cascaded timers
• Frequency dividers
• Voltage-controlled oscillators
• Pulse generators and LED flashers

91