Chapter Four
Chapter Four
Chapter Four
1
4.1 Introduction
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?
function of its input. if you give a sinusoidal input the output will be
sinusoidal.
Depending on component
active filter
passive filter 8
cont,.
Depending on functionality both passive and active
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:
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.
size.
Gain: An active filter can provide gain, whereas a passive filter often
14
Cont.
unacceptable distortion.
16
cont.
17
Cont.
roll-off rate
19
Cont.
Stop-band is the range of frequencies that have the most
attenuation.
the end of the passband and normally specified at the point where
20
Cont.
or AV = 1 (unity).
compared with Vin. Hence the gain falls and drops off gradually
as the frequency is increased.
21
Cont.
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.
1
fc
2 RC
25
4.2.2 RL low pass & high pass circuits
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.
31
Cont.
32
Cont.
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.
35
Cont.
36
Cont.
Using KVL
37
Cont.
38
Cont.
• For the case of the series RLC circuit these two
parameters are given by
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
40
Cont.
2. RLC parallel circuit
41
Cont.
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
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
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)
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.
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State 1(cont’d)
• As C1’s voltage cannot change instantaneously, VB2
drops by VCC.
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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.
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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
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
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)
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APPLICTION OF MULTIVIBRATOR
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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)
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
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.
83
example
• For the figure above having R1=10KΩ,
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
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
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