UIT ‘RGPV’ PROJECT ON OCSILLATOR BYAAKANKSHA CHOUBEY
OSCILLATORS Oscillator are basically ac generators used to generates ac voltage of desired shape at desired frequency. oscillators operates on the principle of positive feedback. When original input signal and feedback signal are in phase, the feedback is called as "positive feedback”. Hence oscillator is an amplifier with positive feedback.
DEFINITION Definition : oscillator is basically an amplifier which does not have any input but it operates on the principle of positive feedback to generates an ac signal at its output. An amplifier act as an oscillator if and only if it satisfied BARKHAUSEN CRITERION.
BAKHAUSEN CRITERIA An oscillator will operate at that frequency at which the total phase shift introduced ,as the signal proceeds from the input terminals, through amplifier and feedback network and back to input is zero,360degree or integral multiple of 360. The magnitude of the product of open loop gain of the amplifier and the magnitude of the feedback factor is unity. That is │Aβ│=1
CLASSIFICATION OF OSCILLATORS OSCILLATOR NON-SINUSODIAL SINUSODIAL MULTIVIBRATOR UJT OSCILLATOR HIGH LOW FREQUENCY-- FREQUENCY— HARTLEY RC PHASE COLPITT’S SHIFT CLAPP WIEN BRIDGE CRYSTAL TWIN-T
RC PHASE SHIFT OSCILLATORA phase-shift oscillator is a simple electronic oscillator. It containsan inverting amplifier, and a feedback filter which shifts the phase of theamplifier output by 180 degrees at a specific oscillation frequency.The filter produces a phase shift that increases with frequency. It must havea maximum phase shift of considerably greater than 180° at highfrequencies, so that the phase shift at the desired oscillation frequency is180°.
The amount of actual phase shift in the circuit depends upon the values of the resistor and the capacitor, and the chosen frequency of oscillations with the phase angle ( Φ ) being given as: Phase Angle
The RC Oscillator which is also called a Phase Shift Oscillator, produces asine wave output signal using regenerative feedback from the resistor-capacitorcombination. This regenerative feedback from the RC network is due to the abilityof the capacitor to store an electric charge,ie.If all the resistors, R and the capacitors, C in the phase shift network are equal in value,then the frequency of oscillations produced by the RC oscillator is given as:•Where:•ƒ is the Output Frequency in Hertz•R he Resistance in Ohms•C is the Capacitance in Farads•N is the number of RC stages.
WEIN BRIDGE OSCILLATORA Wien bridge oscillator is a type of electronic oscillator that generates sinewaves. It can generate a large range of frequencies. The oscillator is based ona bridge circuit originally developed by Max Wien in 1891. The bridge comprisesfour resis tors and two capacitors. The oscillator can also be viewed as a positivegain amplifier combined with a band pass filter that provides positive feedback.The modern circuit is derived from William Hewletts 1939 StanfordUniversity masters degree thesis. Hewlett figured out how to make the oscillatorwith a stable output amplitude and low distortion. Hewlett, alongwith David Packard, co-founded Hewlett-Packard, and Hewlett-Packards firstproduct was the HP200A, a precision Wien bridge oscillator.The frequency of oscillation is given by:
HARTLEY OSCILATORThe Hartley oscillator is an electronic oscillator circuit that usesan inductor and a capacitor in parallel to determine the frequency. Inventedin 1915 by American engineer Ralph Hartley, the distinguishing feature ofthe Hartley circuit is that the feedback needed for oscillation is taken from atap on the coil, or the junction of two coils in series.
OperationA Hartley oscillator is essentially any configuration that uses two series-connected coils and a single capacitor (see Colpitts oscillator for the equivalentoscillator using two capacitors and one coil). Although there is no requirement forthere to be mutual coupling between the two coil segments, the circuit is usuallyimplemented this way.It is made up of the following:•Two inductors in series, which need not be mutual•One tuning capacitorAdvantages of the Hartley oscillator include:•The frequency may be adjusted using a single variable capacitor•The output amplitude remains constant over the frequency range•Either a tapped coil or two fixed inductors are neededDisadvantages include:•Harmonic-rich content if taken from the amplifier and not directly from the LCcircuit.Note that, if the inductance of the two partial coils L1 and L2 is given
COLPITTS OSCILLATORA Colpitts oscillator, invented in 1920 by American engineer Edwin H.Colpitts, is one of a number of designs for electronic oscillatorcircuits usingthe combination of an inductance (L) with a capacitor (C) for frequencydetermination, thus also called LC oscillator. The distinguishing feature ofthe Colpitts circuit is that the feedback signal is taken from a voltagedivider made by two capacitors in series. One of the advantages of thiscircuit is its simplicity; it needs only a single inductor. Colpitts obtained USPatent 1624537 for this circuit.The frequency is generally determined by the inductor and the twocapacitors at the bottom of the drawing.
the amplification of the active component should be marginally largerthan the attenuation of the capacitive voltage divider, to obtain stableoperation. Thus, a Colpitts oscillator used as a variable frequencyoscillator (VFO) performs best when a variable inductance is used fortuning, as opposed to tuning one of the two capacitors. If tuning byvariable capacitor is needed, it should be done via a third capacitorconnected in parallel to the inductor (or in series as in the ClappOscillation frequencyThe ideal frequency of oscillation for the circuits in Figures 1 and 2 are given by theequation:where the series combination of C1 and C2 creates the effective capacitance of theLC tank.Real circuits will oscillate at a slightly lower frequency due to junction capacitancesof the transistor and possibly other stray capacitances.
CLAPP OSCILLATORThe Clapp oscillator is one of several types of electronicoscillator constructed from a transistor (or vacuum tube) and a positivefeedback network, using the combination of an inductance (L) witha capacitor (C) for frequency determination, thus also called LC oscillator.It was published by James Kilton Clapp in 1948. According toVačkář, oscillators of this kind were independently developed by severalinventors, and one developed by Gouriet had been in operation atthe BBC since 1938.
Capacitors C1 and C2 form a voltage divider that determines the amount of feedbackvoltage applied to the transistor input. The Clapp oscillator is a Colpitts oscillator that hasan additional capacitor placed in series with the inductor. The oscillation frequency inhertz (cycles per second) for the circuit in the figure, which uses a field-effect transistor(FET), isA Clapp circuit is often preferred over a Colpitts circuit for constructing a variablefrequency oscillator (VFO). In a Colpitts VFO, the voltage divider contains the variablecapacitor (either C1 or C2). This causes the feedback voltage to be variable as well,sometimes making the Colpitts circuit less likely to achieve oscillation over a portion ofthe desired frequency range. This problem is avoided in the Clapp circuit by using fixedcapacitors in the voltage divider and a variable capacitor (C0) in series with the inductor.