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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

• Feedback signal xf is summed with a positive sign

• The gain-with-feedback is

• The oscillation criterion

)()(1

()(

ssA

sAsAf

）

1)()()( 000 jjAjL

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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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A Popular Limiter Circuit for Amplitude Control

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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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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 RR

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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 of a 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 of an 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 at ss 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 efficiency is 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

2ˆ

2

1

L

CC

VVL

oL R

V

R

VP

CCo

2

ˆ

2

1 2

ˆ

2

max

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Power Dissipation

• Total supply power

• Maximum total supply power

CCL

os V

R

VP

ˆ2

L

CC

VV

CCL

os R

VV

R

VP

CCo

2

ˆ

max

2ˆ2

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Power Dissipation

• Power-conversion efficiency

• Maximum power-conversion efficiency

CC

o

V

V̂

4

%5.78ˆ

4 ˆ

max CCo VVCC

o

V

V

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Power Dissipation

• Power dissipation

• Maximum Power dissipation

L

oCC

L

oD R

VV

R

VP

2ˆ

2

1ˆ2

max2

2

2ˆ

2

maxmax

2.02

ˆ

2

1ˆ2

LL

CC

VVL

oCC

L

oDPDN

PR

V

R

VV

R

VPP

CCo

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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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A Class AB Output Stage Utilizing Diodes for Biasing

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A Class AB Output Stage Utilizing A VBE Multiplier for Biasing