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SJTU Zhou Lingling 1
Chapter 8
Oscillator and
Power Amplifier
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SJTU Zhou Lingling 2
Outline
• Oscillator
• Power amplifier
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SJTU Zhou Lingling 3
Oscillator
• Basic principles of sinusoidal oscillator.
• The Wien-bridge oscillator
• The phase shift oscillator
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SJTU Zhou Lingling 4
Basic Principles of Sinusoidal Oscillator
• The oscillator feedback loop
• The basic structure of a sinusoidal oscillator.
• A positive-feedback loop is formed by an amplifier and a frequency-selective network.
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SJTU Zhou Lingling 5
Basic Principles of Sinusoidal Oscillator
• Feedback signal xf is summed with a positive sign
• The gain-with-feedback is
• The oscillation criterion
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1)()()( 000 jjAjL
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SJTU Zhou Lingling 6
Basic Principles of Sinusoidal Oscillator
• Nonlinear amplitude controlTo ensure that oscillations will start, the Aβ is
slightly greater than unity.As the power supply is turned on, oscillation
will grown in amplitude.When the amplitude reaches the desired level,
the nonlinear network comes into action and cause the Aβ to exactly unity.
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SJTU Zhou Lingling 7
A Popular Limiter Circuit for Amplitude Control
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SJTU Zhou Lingling 8
A Popular Limiter Circuit for Amplitude Control
Transfer characteristic of the limiter circuit;
When Rf is removed, the limiter turns into a comparator with the characteristic shown.
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SJTU Zhou Lingling 9
Oscillator Circuits
• Op Amp-RC Oscillator Circuits The Wien-Bridge Oscillator The phase-Shift Oscillator
• LC-Tuned Oscillator Colpitts oscillator Hareley oscillator
• Crystal Oscillator
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SJTU Zhou Lingling 10
The Wien-Bridge Oscillator
A Wien-bridge oscillator without amplitude stabilization.
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SJTU Zhou Lingling 11
The Wien-Bridge Oscillator
• The loop gain transfer function
• Oscillating frequency
• To obtain sustained oscillation
sCRsCR
RRsL
13
11)( 2
RC1
0
21
2 RR
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SJTU Zhou Lingling 12
The Wien-Bridge Oscillator
A Wien-bridge oscillator with a limiter used for amplitude control.
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SJTU Zhou Lingling 13
The Phase-Shift Oscillator
The circuit consists of a negative-gain amplifier and three-section RC ladder network.
Oscillating frequency is the one that the phase shift of the RC network is 1800
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SJTU Zhou Lingling 14
The Phase-Shift Oscillator
A practical phase-shift oscillator with a limiter for amplitude stabilization.
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SJTU Zhou Lingling 15
The LC-Tuned oscillator
Colpitts Oscillator
A parallel LC resonator connected between collector and base.
Feedback is achieved by way of a capacitive divider
Oscillating frequency is determined by the resonance frequency.
)(121
210 CC
CCL
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SJTU Zhou Lingling 16
The LC-Tuned oscillator
Hartley Oscillator
A parallel LC resonator connected between collector and base.
Feedback is achieved by way of an inductive divider.
Oscillating frequency is determined by the resonance frequency.
)(121
210 CC
CCL
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SJTU Zhou Lingling 17
Crystal Oscillators
A piezoelectric crystal. (a) Circuit symbol. (b) Equivalent circuit.
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SJTU Zhou Lingling 18
Crystal Oscillators
Crystal reactance versus frequency (neglecting the small resistance r, ).
A series resonance at
A parallel resonance at ss LC1
)(1ps
psp CC
CCL
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SJTU Zhou Lingling 19
Crystal Oscillators
A Pierce crystal oscillator utilizing a CMOS inverter as an amplifier.
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SJTU Zhou Lingling 20
Power Amplifier
• Small-signal approximation and models either are not applicable or must be used with care.
• Deliver the power to the load in efficient manner.
• Power dissipation is as low as possible.
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SJTU Zhou Lingling 21
Classification of Power Amplifier
• Power amplifiers are classified according to the collector current waveform that results when an input signal is applied.
• Conducting angle.
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SJTU Zhou Lingling 22
Classification of Power Amplifier
Collector current waveforms for transistors operating in (a) class A, (b) class B
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SJTU Zhou Lingling 23
Classification of Power Amplifier
class AB class C
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SJTU Zhou Lingling 24
Class B Output Stage
A class B output stage.
Complementary circuits.
Push-pull operation
Maximum power-conversion efficiency is 78.5%
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SJTU Zhou Lingling 25
Transfer Characteristic
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SJTU Zhou Lingling 26
Crossover Distortion
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SJTU Zhou Lingling 27
Power Dissipation
• The load power
• Maximum load power
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SJTU Zhou Lingling 28
Power Dissipation
• Total supply power
• Maximum total supply power
CCL
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SJTU Zhou Lingling 29
Power Dissipation
• Power-conversion efficiency
• Maximum power-conversion efficiency
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SJTU Zhou Lingling 30
Power Dissipation
• Power dissipation
• Maximum Power dissipation
L
oCC
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oD R
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SJTU Zhou Lingling 31
Class AB Output Stage
A bias voltage VBB is applied between the bases of QN and QP, giving rise to a bias current IQ . Thus, for small vI, both transistors conduct and crossover distortion is almost completely eliminated.
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SJTU Zhou Lingling 32
A Class AB Output Stage Utilizing Diodes for Biasing
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SJTU Zhou Lingling 33
A Class AB Output Stage Utilizing A VBE Multiplier for Biasing