Post on 04-Jun-2018
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Oscillator Circuits
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II. Oscillator Operation
For self-sustaining oscillations:
the feedback signal must positive
the overall gain must be equal to one (unity gain)
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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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Types of Oscillator Circuits
A. Phase-Shift Oscillator
B. Wien Bridge Oscillator
C. Tuned Oscillator Circuits
D. Crystal Oscillators
E. Unijunction Oscillator
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A. Phase-Shift Oscillator
Frequency of the oscillator: (the frequency where the phase shift is 180)
Feedback gain = 1/[152 j (63) ] where = 1 / (2fRC)Feedback gain at the frequency of the oscillator = 1 / 29
The amplifier must supply enough gain to compensate for losses. The overall gain must be
unity. Thus the gain of the amplifier stage must be greater than 1/, i.e. A > 29
The RC networks provide the necessary phase shift for a positive feedback. They also
determine the frequency of oscillation.
62
10
RCf=
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Example of a Phase-Shift Oscillator
FET Phase-Shift Oscillator
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Example 1
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BJT Phase-Shift Oscillator
C
Cfe
R
R
R
Rh 42923
iehRR
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Phase-shift oscillator using op-amp
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B. Wien Bridge Oscillator
So frequency of oscillation is 22110 21
CRCRf=
1
1
1
1
4
3
2
143
4
21
2
===
R
R
Z
ZRR
R
ZZ
Z
V
VV
V
V
a
bd
o
i
2and,2
1
thenandWhen
4
30
2121
=
R
R
RC f
CCCRRRfrequencynoscillatiotheatphasezerohaveshouldi.e.,,2
1
21
2
Z
Z
ZZ
Z
1
2
2
1
4
30C
C
R
R
R
R
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Calculate the resonant frequency of the Wien bridge oscillator shown above
Hz7.3120)101)(1051(2
1
2
1930
== RCf
Example 2
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C. Tuned Oscillator Circuits
Tuned Oscillators use a parallel LC resonant circuit (LC tank) to provide the oscillations.
There are two common types:
Colpitts The resonant circuit is an inductor and two capacitors.
Hartley The resonant circuit is a tapped inductor or two inductors and one
capacitor.
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Colpitts Tuned Oscillator Circuit
FET based Colpitts Oscillator
21
21where,2
10
CC
CCC
LCf eq
eq =Frequency of oscillations:
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Transistor Colpitts oscillator.
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Op-amp Colpitts oscillator.
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Hartley Tuned Oscillator Circuit
FET Hartley Oscillator
1221 2where,2
10 LLLL
CLf eq
eq
Frequency of oscillations:
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BJT based Hartley Oscillator
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D. Crystal Oscillators
The crystal appears to the rest of the circuit as a resonant circuit.
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Crystal Resonant Frequencies
The crystal has two resonant frequencies:
Series resonant: RLC determine the resonant frequency. The crystal has a low impedance.Parallel resonant: RL and CM determine the resonant frequency. The crystal has a high
impedance.
The series and parallel resonant frequencies are very close, within 1% of each other.
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Series Resonant Crystal Oscillator
FET Crystal Oscillator
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Parallel Resonant Crystal Oscillator
BJT Crystal Oscillator
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Crystal Oscillator using Op-amp
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E. Unijunction Oscillator
Frequency of oscillations:
where = 0.4 to 0.6
is a rating of the unijunction transistor.
)(CRf TT = 11ln1
0
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Unijunction Oscillator Waveforms
The unijunction oscillator (or relaxation oscillator) produces a sawtooth waveform.
DBBBP
BBBB
VVVV
VVV
112
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Unijunction transistor (UJT): basic construction.
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UJT Unijunction Transistor
The UJT is also basically a switching device.
Schematic Symbol:
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UJT Basic Operation
Even though the UJT is a switching device it works very differently from the SCR variety
of devices.
The equivalent circuit indicates that the UJT is like a diode and a resistive voltage divider
circuit. The resistance exhibited by RB1 is variable; it is dependent on the value of current
IE.
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