ECE 334 Lecture 27 Ocilators

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Chapter Fifteen

Applications and Design of

Integrated Circuits

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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.

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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.

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oscillations

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The oscillation frequency of the phase shift amplifier is given by

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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

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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.

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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

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Comparator

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Comparators in which crossover voltage is a

function of resistor ratios

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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.

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As input voltage increases

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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.

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Need for Hysteresis and applications

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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.

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(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)

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Astable multivibrator

From the analysis of the astable circuit, the period of the oscillation T is given by

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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.

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(a) An op-amp monostable circuit. (b) Signal waveforms in the circuit of(a).

(a)

Trigger circuit

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