Different Types Of Oscillators
Hartley oscillator
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| Hartley oscillator |
The circuit diagram of a Hartley oscillator is shown in fig. It is an LC oscillator. This
oscillator contains a CE amplifier, feedback network, and a tank circuit made up of L, L, and C. The resistors R, and R, provide the necessary bias to the amplifier. The capacitors Ci and Co are used to block the DC components. The capacitor CE is a bypass capacitor. The resistor R provides negative feedback to the amplifier to improve its stability. The RF choke (RFC) provides a path for collector bias current but offers high impedance for oscillating signal
Principle of operation
When the supply is turned ON, the capacitor 'C is charged. When this capacitor is fully charged, it
discharges through the coils L and L, setting up an oscillation. The output voltage of the amplifier appears across L, and the feedback voltage appears across L. The voltage across L is 180° out of phase with the output voltage. It is the feedback signal. A phase shift of another 180 is produced by the CE amplifier. Hence the total phase shift between input and output is 180+ 180 360. This results in positive feedback which makes the oscillation continuously undamped.
COLPITTS OSCILLATOR
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| Colpitts oscillator |
The Colpitts oscillator is the same as the Hartley oscillator. The major difference between the two is that the Colpitts oscillator uses a tapped capacitor whereas the Hartley oscillator uses a tapped inductor. The circuit diagram of the Colpitts oscillator is shown in fig.
The tank circuit is made up of C, C, and L. The resistors R and R provide proper bias and, RE With CE provides stabilization. The RF choke (RFC) gives high impedance for high-frequency oscillating signals. The frequency of the oscillation is given by the Principle of operation When the supply is switched ON, the capacitor C. and C, are charged. Then these capacitors discharge through the coil L. So oscillations are produced. The oscillations across C, are applied to the input of the CE amplifier. The amplified output is available at the collector terminal of the transistor.
The amount of feedback depends upon the capacitance values of C and C. The capacitor
feedback circuit provides a 180 phase shift. transistor amplifier (CE) provides another 180 phase
shift. Hence the total phase shift is 360, which provides positive feedback. Therefore continuous
undamped oscillation is produced. Colpitts oscillators are widely used in signal generators and local oscillators of superheterodyne radio receivers.
RC PHASE SHIFT OSCILLATOR
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| RC phase shift oscillator |
The LC oscillators have two general drawbacks
(1) It cannot be used, for very low frequencies, because the coil becomes too bulky, expensive, and noisy.
(2)The frequency stability and waveform are very low.
The RC oscillators produce good frequency Stability signals and also operate at very low frequencies. The circuit diagram of the RC phase shift oscillator is shown in fig. The oscillator consists of three stages of RC networks (R, C,, R, C, and R, C). The resistor R provides bias, and RE With CE provides stabilization.
Principle of operation
When the supply is switched ON, the random variations of base current caused by noise variations in the transistor and voltage variations in the power source produce oscillation. This variation is amplified by the CE amplifier. The feedback network consists of three stages of RC networks. The three stages are identical. The feedback section provides 180 phase shifts because each RC network provides a 60" phase shift (3 x 60 180"). The CE amplifier provides another 180" phase shift. Hence the total phase shift is 360, which provides positive feedback. Therefore, continuous undamped oscillation is produced.
Advantages
Does not require transformers and inductors
(i) It can produce very low-frequency signals
(ii) The circuit provides good frequency stability
Disadvantages
(1) It is difficult to start oscillation
(ii) It gives a low power output
(iii) This RC oscillator is not suitable for tuned oscillators because the variation of capacitor and resistor_values also changes the phase shift of the RC networks.
CRYSTAL OSCILLATOR
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| Crystal oscillator |
Due to the following effects, the LC and RC oscillators do not produce more stable oscillations,
(1) The value of R, L, and C which are frequency-determining factors in these circuits, will change
with temperature.
2)If any component in the feedback network is changed, it will shift the operating frequency of
the oscillator.
To maintain a constant frequency, piezoelectric Crystals are used with the LC or RC oscillators (or) in
the place of LC or RC circuits. Oscillators of this type are called crystal oscillators.
The crystal has a peculiar property known as the piezoelectric effect, which is expressed as when mechanical stress is applied across its two faces, the crystal vibrates and an ac voltage is
produced.
(ii) when an ac voltage is applied across its crystal, the crystal vibrates at the frequency of the applied
voltage.
The frequency of the vibration is equal to the resonance frequency of the crystal.
The equivalent electrical circuit of the crystal is mentioned as
(i)When the crystal is not vibrating, it is equivalent to the capacitance Cm, known as
mounting capacitance
(ii) When a crystal vibrates, it is equivalent to an R-L-C series circuit.
The circuit diagram of a crystal oscillator is shown in fig. It is actually a Colpitts oscillator,
modified into a crystal oscillator. The crystal replaces the inductor (L) in the Colpitts oscillator. The resistors R, and R, provide base bias and RE with CE provide stabilization.
Principle of operation
The natural frequency of the LC circuit is made nearly equal to the natural frequency of the crystal.
When the supply is switched ON, the capacitor C is going to charge. When the capacitor C, is fully charged, it discharges through .crystal which oscillation. The frequency of the oscillation depends
upon the values of C,, C, and the RLC equivalent values of crystal. If the frequency of the oscillation is equal to produces its crystal resonant frequency, the circuit produces a more stable oscillation.
The crystal frequency is independent of temperature. The C, feedback network, provides a 180 phase
shift, and also the CE amplifier provides another 180 phase shift. Hence the total phase shift is 360,
which provides positive feedback. Therefore, continuous undamped oscillation is produced.
A crystal oscillator always generates high-frequency oscillations, range from 20KHz to 20MHz
Advantages:
(1)Excellent frequency stability
(2) Simple circuit
(3) High-quality factor
Disadvantages
(1)Crystal is a fragile type, hence it is used only in low
power circuits
(2) Not used as tuned oscillators
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