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Colpitts Oscillator:Working andApplications

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Electronic Oscillator:

An electronic oscillator is an electronic circuit that producesa periodic, oscillating electronic signal, often a sine wave ora square wave.

Oscillators convert direct current (DC) from a power suppl

to an alternating current signal. !he are widel used inman electronic devices.

Oscillators can "e classi#ed into di$erent tpes generall"ased on their output frequenc.

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Introduction:

%lectronic oscillators can "e termed as voltagecontrolled oscillators as their frequenc of oscillationscan "e controlled " their input voltage.

&oremost electronic voltage controlled oscillators can"e considered as two tpes namel: 'inear Oscillatorand onlinear Oscillator.

'C oscillators can "e designed " using di$erent

methods. !he well nown 'C oscillators are *artleoscillator and Colpitts oscillator.

Among these two, the frequentl used design is ColpittsOscillator designed " and named after an American

%ngineer %dwin * Colpitts in ++-.

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Colpitts Oscillator :

Oscillator is an ampli#er with the positive feed"ac and it converts

DC input signal into AC output waveform with certain varia"lefrequenc drive and certain shape of output waveform (lie sinewave or square wave, etc) " using the positive feed"ac instead ofinput signal.

Oscillators which utilies the inductor ' and capacitor C in theircircuit are called as 'C oscillator which is a tpe of linear oscillator.

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olpitts Oscillator Theory:

t consists of a tan circuit which is an 'C resonance su"circuit made of two series capacitors connected in parallelto an inductor and frequenc of oscillations can "edetermined " using the values of these capacitors andinductor of the tan circuit.

 !his oscillator is almost similar to *artle oscillator.

t is termed as electrical dual of *artle oscillator and isdesigned for the generation of high frequenc sinusoidaloscillations with the radio frequencies tpicall ranging

from +0 1* to 2003*.

 !he ma4or di$erence "etween these two oscillators is that ituses tapped capacitance, whereas the *artle oscillatoruses tapped inductance.

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Circuit Explanation:

%ver other oscillator circuit which generates sinusoidalwaveforms utilies the 'C resonant circuit e5cept a fewelectronic circuits such 6C oscillators, 7ien86o"insonoscillator and a few crstal oscillators which don9t requireadditional inductances for this purpose.

t can "e realied " using gain device such as ipolar ;unction !ransistor(;!), operational ampli#er and #eld e$ecttransistor(&%!) as similar in other 'C oscillators also.

 !he capacitors C+ < C= forms potential divider and this

tapped capacitance in the tan circuit can "e used as thesource for feed"ac and this setup can "e used to provide"etter frequenc sta"ilit compared to the *artle oscillatorin which tapped inductance is used for feed"ac setup.

6% resistor in the a"ove circuit provides sta"iliation forcircuit against variations in temperature.

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Circuit Diagram:

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 !he capacitor C% connected in the circuit which isparallel to the 6%, provides low reactive path to theampli#ed AC signal acting as pass capacitor.

 !he 6esistors 6+ and 6= form voltage divider for circuit

and provide "ias to the transistor.

 !he circuit consists of a 6C coupled ampli#er withcommon emitter con#guration transistor.

 !he coupling capacitor Cout"locs DC " providing anAC path from the collector to the tan circuit.

ircuit Explanation(Continue…

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olpitts Oscillator Working:

7henever power suppl is switched on, the capacitors C+and C= start charging and after the capacitors get fullcharged, the capacitors starts discharging through theinductor '+ in the circuit causing damped harmonicoscillations in the tan circuit.

 !hus, an AC voltage is produced across C+ < C= " theoscillator current in the tan circuit.

7hile these capacitors get full discharged, the electrostatic

energ stored in the capacitors get transferred in the form ofmagnetic >u5 to the inductor and thus inductor getscharged.

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?imilarl, when the inductor starts discharging, thecapacitors start charging again and this process of energcharging and discharging capacitors and inductor continuescausing the generation of oscillations and the frequenc ofthese oscillations can "e determined " using the resonantfrequenc of the tan circuit consisting of inductor andcapacitors.

 !his tan circuit is considered as the energ reservoir orenerg storage. !his is "ecause of frequent energ chargingand discharging of the inductor, capacitors that part of 'C

networ forming the tan circuit.

 !he continuous undamped oscillations can "e o"tained fromthe arhausen criterion.

Working(continue…

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&or sustained oscillations, the total phase shift must "e 2@00

or 00. n the a"ove circuit as two capacitors C+ < C= arecenter tapped and grounded, the voltage across capacitorC= (feed"ac voltage) is +-00 with the voltage acrosscapacitor C+ (output voltage).

 !he common emitter transistor produces +-00 phase shift"etween the input and output voltage. !hus, from thearhausen criterion we can get undamped continuousoscillations.

Working(continue…

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 !he resonant frequenc is given "

  rB+/(=('+EC))

7here r is the resonant frequenc

C is the equivalent capacitance of series com"ination of C+ andC= of the tan circuit.

t is given as

  CB (C+EC=)/ ((C+FC=))

  '+ represents the self8inductance of the coil.

!ormulas:

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Applications :

t is used for generation of sinusoidal output signals withver high frequencies.

 !he Colpitts oscillator using ?A7 device can "e used as thedi$erent tpe of sensors such as temperature sensor. As thedevice used in this circuit is highl sensitive to

pertur"ations, it senses directl from its surface.

t is frequentl used for the applications in which ver widerange of frequencies are involved.

Gsed for applications in which undamped and continuousoscillations are desired for functioning.

 !his oscillator is preferred in situations where it is intendedto withstand high and low temperatures frequentl.

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 !he com"ination of this oscillator with some devices(instead of tan circuit) can "e used to achieve greattemperature sta"ilit and high frequenc.

t is used for the development of mo"ile and radio

communications.

t has man applications used for the commercialpurposes.

Applications(Continue…

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Conclusion:

7e e5plained a"out the Colpitts oscillator theor, woring,applications and along with its tan circuit.

 !he Colpitts oscillator is ver useful to %lectrical and%lectronic ?tudents.

7e hope that this e5planation will clear our dou"ts and itis the "est presentation on Colpitts oscillator.

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