1. UIT ‘RGPV’
PROJECT ON
OCSILLATOR
BY
AAKANKSHA CHOUBEY
2. 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.
4. 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.
5. 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
6. 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
7. RC PHASE SHIFT OSCILLATOR
A phase-shift oscillator is a simple electronic oscillator. It contains
an inverting amplifier, and a feedback filter which 'shifts' the phase of the
amplifier output by 180 degrees at a specific oscillation frequency.[1]
The filter produces a phase shift that increases with frequency. It must have
a maximum phase shift of considerably greater than 180° at high
frequencies, so that the phase shift at the desired oscillation frequency is
180°.
9. 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
10. The RC Oscillator which is also called a Phase Shift Oscillator, produces a
sine wave output signal using regenerative feedback from the resistor-capacitor
combination. This regenerative feedback from the RC network is due to the ability
of 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.
11. WEIN BRIDGE OSCILLATOR
A Wien bridge oscillator is a type of electronic oscillator that generates sine
waves. It can generate a large range of frequencies. The oscillator is based on
a bridge circuit originally developed by Max Wien in 1891.[1] The bridge comprises
four resis tors and two capacitors. The oscillator can also be viewed as a positive
gain amplifier combined with a band pass filter that provides positive feedback.
The modern circuit is derived from William Hewlett's 1939 Stanford
University master's degree thesis. Hewlett figured out how to make the oscillator
with a stable output amplitude and low distortion.[citation needed] Hewlett, along
with David Packard, co-founded Hewlett-Packard, and Hewlett-Packard's first
product was the HP200A, a precision Wien bridge oscillator.
The frequency of oscillation is given by:
13. HARTLEY OSCILATOR
The Hartley oscillator is an electronic oscillator circuit that uses
an inductor and a capacitor in parallel to determine the frequency. Invented
in 1915 by American engineer Ralph Hartley, the distinguishing feature of
the Hartley circuit is that the feedback needed for oscillation is taken from a
tap on the coil, or the junction of two coils in series.
14. Operation
A Hartley oscillator is essentially any configuration that uses two series-
connected coils and a single capacitor (see Colpitts oscillator for the equivalent
oscillator using two capacitors and one coil). Although there is no requirement for
there to be mutual coupling between the two coil segments, the circuit is usually
implemented this way.
It is made up of the following:
•Two inductors in series, which need not be mutual
•One tuning capacitor
Advantages 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 needed
Disadvantages include:
•Harmonic-rich content if taken from the amplifier and not directly from the LC
circuit.
Note that, if the inductance of the two partial coils L1 and L2 is given
15. COLPITTS OSCILLATOR
A Colpitts oscillator, invented in 1920 by American engineer Edwin H.
Colpitts, is one of a number of designs for electronic oscillatorcircuits using
the combination of an inductance (L) with a capacitor (C) for frequency
determination, thus also called LC oscillator. The distinguishing feature of
the Colpitts circuit is that the feedback signal is taken from a voltage
divider made by two capacitors in series. One of the advantages of this
circuit is its simplicity; it needs only a single inductor. Colpitts obtained US
Patent 1624537[1] for this circuit.
The frequency is generally determined by the inductor and the two
capacitors at the bottom of the drawing.
16. the amplification of the active component should be marginally larger
than the attenuation of the capacitive voltage divider, to obtain stable
operation. Thus, a Colpitts oscillator used as a variable frequency
oscillator (VFO) performs best when a variable inductance is used for
tuning, as opposed to tuning one of the two capacitors. If tuning by
variable capacitor is needed, it should be done via a third capacitor
connected in parallel to the inductor (or in series as in the Clapp
Oscillation frequency
The ideal frequency of oscillation for the circuits in Figures 1 and 2 are given by the
equation:
where the series combination of C1 and C2 creates the effective capacitance of the
LC tank.
Real circuits will oscillate at a slightly lower frequency due to junction capacitances
of the transistor and possibly other stray capacitances.
17. CLAPP OSCILLATOR
The Clapp oscillator is one of several types of electronic
oscillator constructed from a transistor (or vacuum tube) and a positive
feedback network, using the combination of an inductance (L) with
a capacitor (C) for frequency determination, thus also called LC oscillator.
It was published by James Kilton Clapp in 1948. According to
Vačkář, oscillators of this kind were independently developed by several
inventors, and one developed by Gouriet had been in operation at
the BBC since 1938.
18. Capacitors C1 and C2 form a voltage divider that determines the amount of feedback
voltage applied to the transistor input. The Clapp oscillator is a Colpitts oscillator that has
an additional capacitor placed in series with the inductor. The oscillation frequency in
hertz (cycles per second) for the circuit in the figure, which uses a field-effect transistor
(FET), is
A Clapp circuit is often preferred over a Colpitts circuit for constructing a variable
frequency oscillator (VFO). In a Colpitts VFO, the voltage divider contains the variable
capacitor (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 of
the desired frequency range. This problem is avoided in the Clapp circuit by using fixed
capacitors in the voltage divider and a variable capacitor (C0) in series with the inductor.
