1. Lecture 3 Oscillator Introduction of Oscillator Linear Oscillator
Wien Bridge Oscillator
RC Phase-Shift Oscillator
LC Oscillator
Stability
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2. Oscillators
Oscillation: an effect that repeatedly and regularly fluctuates about the mean value
Oscillator: circuit that produces oscillation
Characteristics: wave-shape, frequency, amplitude, distortion, stability
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3. 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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4. Linear Oscillators Wien Bridge Oscillators RC Phase-Shift Oscillators
LC Oscillators
Stability
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5. Integrant of Linear Oscillators
For sinusoidal input is connected
“Linear” because the output is approximately sinusoidal
A linear oscillator contains:
- a frequency selection feedback network
- an amplifier to maintain the loop gain at unity
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6. 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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7. Wien Bridge Oscillator
Frequency Selection Network
Let
and
Therefore, the feedback factor,
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8. 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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9. 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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10. RC Phase-Shift Oscillator
Using an inverting amplifier
The additional 180o phase shift is provided by an RC phase-shift network
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11. Applying KVL to the phase-shift network, we have
Solve for I3, we get Or
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12. 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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13. 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:
Colpitts Oscillator
Harley Oscillator
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14. For the equivalent circuit from the output
Therefore, the amplifier gain is obtained,
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15. If the impedance are all pure reactances, i.e.,
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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16. Hartley Oscillator Colpitts Oscillator Ref:06103104HKN
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17. Colpitts Oscillator 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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18. 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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19. Imaginary part = 0, we have
Real part = 0, yields
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20. 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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21. 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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22. Wien-bridge oscillator with bulb stabilization
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23. Wien-bridge oscillator with diode stabilization
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24. Twin-T Oscillator
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25. Bistable Circuit
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26. A Square-wave Oscillator
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