1. OSCILLATORS Dr.S.SUJA Associate Professor
Department oF Electrical and Electronics Engineering,
Coimbatore Institute of Technology,
Coimbatore
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2. CONTENTS OSCILLATOR BASICS TYPES OF OSCILLATORS
RC PHASE SHIFT OSCILLATORS
WEIN BRIDGE OSCILLATORS
LC OSCILLATOR
COLPITTS OSCILLATOR
HARTLEY OSCILLATOR
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3. OSCILLATOR BASICS
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4. OSCILLATOR
A change that occurs repeatedly and regularly with reference to the mean value
The parameters related to oscillators are
wave-shape
frequency
amplitude
distortion
stability
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5. THE OSCILLATOR
Oscillators are electronic circuits that generate an output signal (oscillating in nature) without the necessity of an input signal .
produces a periodic waveform (time domain) on its output with only the DC supply voltage as an input.
They convert DC power into AC signal power.
Signal generation implies production of self-sustained oscillations.
The output voltage can be either sinusoidal or nonsinusoidal, depending on the type of oscillator.
Different types of oscillators produce various types of outputs including sine waves, square waves, triangular waves, and sawtooth waves.
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6. Simple Oscillator Circuit
Oscillators are electronic circuits that generate an output signal (oscillating in nature) without the necessity of an input signal .
produces a periodic waveform on its output with only the DC supply voltage as an input.
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7. Basic elements of a feedback oscillator
Feedback oscillator operation is based on the principle of positive feedback.
Feedback oscillators are widely used to generate sinusoidal waveforms.
A feedback oscillator consists of an amplifier for gain (A) (either a discrete transistor or an op-amp) and a positive feedback circuit (β) that produces phase shift and provides attenuation
The voltage gain around the closed feedback loop, is the product of the amplifier gain, and the attenuation, of the feedback circuit.
Loop gain A β
After oscillations are started, the loop gain is maintained at 1.0 to maintain oscillations.
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8. Positive Feedback In positive feedback, a portion of the output voltage of an amplifier is fed back to the input with no net phase shift, resulting in a strengthening of the output signal.
The feedback oscillator, returns a fraction of the output signal to the input with no net phase shift, resulting in a reinforcement of the output signal.
After oscillations are started, the loop gain is maintained at 1.0 to maintain oscillations.
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9. OSCILLATION
The in-phase feedback voltage is amplified to produce the output voltage, which in turn produces the feedback voltage.
That is, a loop is created in which the signal maintains itself and a continuous sinusoidal output is produced.
This phenomenon is called oscillation.
In some types of amplifiers, the feedback circuit shifts the phase and an inverting amplifier is required to provide another phase shift so that there is no net phase shift.
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10. Necessary Conditions for Oscillation
Two conditions, are required for a sustained state of oscillation:
1. The phase shift around the feedback loop must be effectively Zero.
2. The voltage gain, around the closed feedback loop (loop gain) must be equal to 1 (unity).
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11. Conditions for Oscillation
If a sinusoidal wave is the desired output, a loop gain greater than 1 will rapidly cause the output to saturate at both peaks of the waveform, producing unacceptable distortion.
To avoid this, some form of gain control must be used to keep the loop gain at exactly 1 once oscillations have started.
For example, if the attenuation of the feedback circuit is 0.01, the amplifier must have a gain of exactly 100 to overcome this attenuation and not create unacceptable distortion .
An amplifier gain of greater than 100 will cause the oscillator to limit both peaks of the waveform.
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12. Vo-output voltage of Amplifier Vi-input voltage of Amplifier
OSCILLATOR OPERATION
Vo-output voltage of Amplifier
Vi-input voltage of Amplifier
Vf-output voltage of feedback Amplifier
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13. OSCILLATOR CONDITIONS NOT MET
If the feedback signal is not positive or the gain is less than one, then the oscillations will dampen out.
If the overall gain is greater than one, then the oscillator will eventually saturate.
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14. Start-Up Conditions
The requirements for the oscillation to start when the dc supply voltage is first turned on.
the unity-gain condition must be met for oscillation to be maintained.
For oscillation to begin, the voltage gain around the positive feedback loop must be greater than 1 so that the amplitude of the output can build up to a desired level.
The gain must then decrease to 1 so that the output stays at the desired level and oscillation is sustained.
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15. TYPES OF OSCILLATORS
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16. Types of Oscillator in communication systems
Most electronic communication systems operate with sources of sinusoidal electrical waves.
Many classes of oscillator circuits are used to produce these sinusoids.
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17. Types of Oscillator-Waveform Generated
According to the types of waveforms produced oscillators can be classified into one of four generic types:
Harmonic oscillators: used for sine-wave generation.
Saw tooth oscillators: used for the generation of exponential or linear saw tooth waves.
Relaxation oscillators: used for current or voltage pulse generation with negative resistance devices.
Astable multivibrators: used for the generation of rectangular or square waves.
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18. Harmonic oscillator in S Domain
The Positive Feedback Approach for a Harmonic Oscillator
With positive feedback, the harmonic oscillator can be represented in block diagram form of HO, where G(s) is the Laplace transform of the open-loop voltage-gain function of the amplifier stage and H(s) is the transfer function of the passive feedback network.
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19. Relaxation Oscillators
A second type of oscillator is the relaxation oscillator.
Instead of feedback, a relaxation oscillator uses an RC timing circuit to generate a waveform that is generally a square wave or other nonsinusoidal waveform.
Typically, a relaxation oscillator uses a Schmitt trigger or other device that changes states to alternately charge and discharge a capacitor through a resistor.
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20. RC PHASE OSCILLATORS
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21. Integrant of Linear Oscillators
A linear oscillator contains:
- a frequency selection feedback network
- an amplifier to maintain the loop gain at unity
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22. Basic Linear Oscillator
and
If Vs = 0, the only way that Vo can be nonzero
is that loop gain A=1 which implies that
(Barkhausen Criterion)
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23. RC Phase-Shift Oscillator
BJT Amplifier-3 RC cells- Each cell with a phase shift of 60 deg
Using an inverting amplifier
The additional 180o phase shift is provided by an RC phase-shift network
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24. Equivalent Circuit of RC phase Shift Oscillator
BJT – current Amplifier Loop gain-A(s)=Ib/Ib’
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25. Applying KVL to the phase-shift network, we have
Solve for I3, we get Or
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26. Hence the transfer function of the phase-shift network is given by,
The output voltage,
Hence the transfer function of the phase-shift network is given by,
For 180o phase shift, the imaginary part = 0, i.e.,
Note: The –ve sign mean the
phase inversion from the
voltage
and,
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27. Wien Bridge Oscillators
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28. Wien Bridge Oscillator
Frequency Selection Network
Let
and
Therefore, the feedback factor,
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29. For Barkhausen Criterion, imaginary part = 0, i.e.,
 can be rewritten as:
For Barkhausen Criterion, imaginary part = 0, i.e.,
Supposing,
R1=R2=R and XC1= XC2=XC,
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30. Example By setting , we get Imaginary part = 0 and
Due to Barkhausen Criterion,
Loop gain Av=1
where
Av : Gain of the amplifier
Wien Bridge Oscillator
Therefore,
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31. LC OSCILLATORS
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32. LC OSCILLATORS
The frequency selection network (Z1, Z2 and Z3) provides a phase shift of 180o
The amplifier provides an addition shift of 180o
Two well-known Oscillators: are
Colpitts Oscillator
Harley Oscillator
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33. For the equivalent circuit from the output
Therefore, the amplifier gain is obtained,
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34. It indicates that at least one reactance must be –ve (capacitor)
The loop gain,
If the impedance are all pure reactances, i.e.,
The loop gain becomes,
The imaginary part = 0 only when X1+ X2+ X3=0
It indicates that at least one reactance must be –ve (capacitor)
X1 and X2 must be of same type and X3 must be of opposite type
With imaginary part = 0,
For Unit Gain & 180o Phase-shift,
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35. HARTLEY OSCILLATOR and COLPITTS OSCILLATOR
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36. Colpitts Oscillator Equivalent circuit
In the equivalent circuit, it is assumed that:
Linear small signal model of transistor is used
The transistor capacitances are neglected
Input resistance of the transistor is large enough
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37. Apply KCL at node 1, we have
where,
Apply KCL at node 1, we have
For Oscillator V must not be zero, therefore it enforces,
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38. Imaginary part = 0, we have
Real part = 0, yields
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39. Hartley Oscillator
Colpitts Oscillator
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40. HARTLEY OSCILLATOR
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41. Frequency Stability
The frequency stability of an oscillator is defined as
Use high stability capacitors, e.g. silver mica, polystyrene, or teflon capacitors and low temperature coefficient inductors for high stable oscillators.
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42. Amplitude Stability
In order to start the oscillation, the loop gain is usually slightly greater than unity.
LC oscillators in general do not require amplitude stabilization circuits because of the selectivity of the LC circuits.
In RC oscillators, some non-linear devices, e.g. NTC/PTC resistors, FET or zener diodes can be used to stabilized the amplitude
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43. RC PHASE OSCILLATOR
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44. A Square-wave Oscillator
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45. Application of Oscillators
Oscillators are used to generate signals, e.g.
Used as a local oscillator to transform the RF signals to IF signals in a receiver;
Used to generate RF carrier in a transmitter
Used to generate clocks in digital systems;
Used as sweep circuits in TV sets and CRO.
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46. THANK YOU
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