Chapter_8 Power Amplifier

8/8/2019 Chapter_8 Power Amplifier

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SJTU Zhou Lingling 1

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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A Wien-bridge oscillator without amplitude stabilization.


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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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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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Collector current waveforms for transistors operating

in (a) class A, (b) class B


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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

Chapter_8 Power Amplifier

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