Upload
chemuel-mardie-g-obedencio
View
222
Download
0
Embed Size (px)
Citation preview
7/27/2019 ECE 334 Lecture 27 Ocilators
1/8
1
Chapter Fifteen
Applications and Design of
Integrated Circuits
2
An electronic oscillator may be define in any one ofthe following four ways:i) It is a circuit which convert dc energy into ac
energy at a very high frequency
ii) It is an electronic source of alternating current orvoltage having sine, square or sawtooth or pulseshapes
iii) It is circuit which generates an ac output signalswithout requiring any externally applied inputsignal.
iv) It is an unstable amplifier.
3
Classification of Oscillators
Electronic oscillators may be broadly dividedinto following two groups:
i) sinusoidal (or harmonic) oscillators-whichproduce an output having sine wave form
ii) non- sinusoidal (or relaxation) oscillators-they produce an output which has square,rectangular or sawtooth wave form, employcircuit building blocks known asmultivibrators.
4
7/27/2019 ECE 334 Lecture 27 Ocilators
2/8
5 6
oscillations
7
The oscillation frequency of the phase shift amplifier is given by
8
7/27/2019 ECE 334 Lecture 27 Ocilators
3/8
9
Above equation
By adding the Wien bridge, oscillator becomes sensitive to signal of only one
Particular frequency. Hence we get an oscillator of good frequency stability.10
11
Schmitt trigger
A Schmitt trigger circuit, which can be used for
generating a class of waveform called
multivibrators.There are three types ofmultivibrators are:
Bistable
Monostable
Astable
The basic circuit to build a bistable multivibrators
is a comparator with positive feed back.
12
USES of multivibrators
As saw tooth generator
As square wave and pulse generator
As standers frequency source
For many specialized uses in radar and
TV circuits
As memory element in computers
7/27/2019 ECE 334 Lecture 27 Ocilators
4/8
13
Comparator
14
15
Comparators in which crossover voltage is a
function of resistor ratios
16
7/27/2019 ECE 334 Lecture 27 Ocilators
5/8
17
18
Bistable Multivibrator (BMV)
It is also called fip-flop multivibrators.It has two absolutely stable states. Itcan remain in either of these two statesunless an external trigger pulseswitches it from one state to other.Obviously, it does not oscillate.
19
20
As input voltage increases
7/27/2019 ECE 334 Lecture 27 Ocilators
6/8
21 22
23 24
Illustrating the use of hysteresis in the comparator characteristics as ameans of rejecting interference.
Need for Hysteresis and applications
If the signal is havinginterference superimposed on it,then comparator would thus changestate a number of times at eachof the zero crossing points (it shouldchanage only one time) andour count would obviouslyBe in error.
7/27/2019 ECE 334 Lecture 27 Ocilators
7/8
25
Need for Hysteresis and applications
26
Astable multivibrator
It is also called free running relaxationoscillator. It has no stable state but only
two quazi stable (half stable) between
which it keeps oscillating continuously of
its own accord without any external
excitation.
27
(a) Connecting a bistable multivibrator with inverting transfer characteristic in a feedbackloop with an RC circuit results in a square-wave generator. (b) The circuit obtained when
the bistable multivibrator is modified (c) Waveforms at the various nodes of the circuit in
(b). This circuit is called an astable multivibrator.
(a)
(b)
28
Astable multivibrator
From the analysis of the astable circuit, the period of the oscillation T is given by
7/27/2019 ECE 334 Lecture 27 Ocilators
8/8
29 30
In some applications the need arises for a pulse of known height and width
Generated in response to a trigger signal. Such a standardized pulse can be
generated by monostable multivibrators.
31
(a) An op-amp monostable circuit. (b) Signal waveforms in the circuit of(a).
(a)
Trigger circuit
32