Post on 10-Apr-2018
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Chapter 8
Oscillator and
Power Amplifier
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Outline
Oscillator
Power amplifier
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Oscillator
Basic principles of sinusoidal oscillator.
The Wien-bridge oscillator
The phase shift oscillator
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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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Basic Principles of Sinusoidal
Oscillator
Nonlinear amplitude control
To ensure that oscillations will start, the A 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 andcause the A to exactly unity.
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A Popular Limiter Circuit for
Amplitude Control
Transfer characteristic of the limiter circuit;
When Rfis removed, the limiter turns into a comparator with the
characteristic shown.
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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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The Wien-Bridge Oscillator
A Wien-bridge oscillator without amplitude stabilization.
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The Wien-Bridge Oscillator
The loop gain transfer function
Oscillating frequency
To obtain sustained oscillation
sCRsCR
RRsL
13
11)( 2
!
RC1
0 ![
21
2 !R
R
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The Wien-Bridge Oscillator
A Wien-bridge oscillator with a limiter used for amplitude control.
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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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The Phase-Shift Oscillator
A practical phase-shift oscillator with a limiter for amplitude stabilization.
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The LC-Tuned oscillator
Colpitts Oscillator
A parallel LC resonator
connected between collector and
base.
Feedback is achieved by way ofa capacitive divider
Oscillating frequency is
determined by the resonance
frequency.
)(121
210
CC
CCL
![
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The LC-Tuned oscillator
Hartley Oscillator
A parallel LC resonator
connected between collector and
base.
Feedback is achieved by way ofan inductive divider.
Oscillating frequency is
determined by the resonance
frequency.
)(121
210
CC
CCL
![
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Crystal Oscillators
A piezoelectric crystal. (a) Circuit symbol. (b) Equivalent circuit.
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Crystal Oscillators
Crystal reactance versus
frequency (neglecting the small
resistance r, ).
A series resonance at
A parallel resonance atss LC1![
)(1ps
psp
CC
CCL
![
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Crystal Oscillators
A Pierce crystal oscillator utilizing a CMOS inverter as an amplifier.
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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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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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Classification of Power Amplifier
Collector current waveforms for transistors operating
in (a) class A, (b) class B
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Classification of Power Amplifier
class AB class C
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Class B Output Stage
A class B output
stage.
Complementary
circuits.
Push-pull operation
Maximum power-
conversion efficiencyis 78.5%
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Transfer Characteristic
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Crossover Distortion
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Power Dissipation
The load power
Maximum load power
L
oL
R
VP
2
2
1!
L
CC
VVL
oL
R
V
R
VP
CCo 2
2
12
2
max !!
!
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Power Dissipation
Total supply power
Maximum total supply power
CC
L
os V
R
VP
2
T
!
L
CC
VV
CC
L
os
R
VV
R
VP
CCo
2
max
22
TT
!!
!
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Power Dissipation
Power-conversion efficiency
Maximum power-conversion efficiency
CC
o
V
V
4
TL !
%5.78
4 max !!
! CCo VVCC
o
V
VTL
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Power Dissipation
Power dissipation
Maximum Power dissipation
L
oCC
L
oD
R
VV
R
VP
2
2
12!
T
max2
2
2
2
maxmax
2.02
2
12
L
L
CC
VVL
oCC
L
oDPDN
PR
V
R
VV
R
VPP
CCo
!!
!!
!
T
TT
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Class AB Output Stage
A bias voltage VBB is applied between the bases ofQ and QP, giving rise to a bias
currentIQ . Thus, for small vI, both transistors conduct and crossover distortion is
almost completely eliminated.
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A Class AB Output Stage Utilizing
Diodes for Biasing
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A Class AB Output Stage Utilizing
A VBEMultiplier for Biasing