chapter 9 output stages and power amplifiers

Microelectronic Circuits SJTU Yang Hua

Chapter 9 output stages and power amplifiers

Introduction9.1 Classification of output stages 9.2 Class A output stage9.3 Class B output stage9.4 Class AB output stage9.5 Biasing the class AB circuit

SJTU Yang HuaMicroelectronic Circuits

Power Amplifier

Small-signal approximation and models either are not applicable or must be used with care. THD (total harmonic distortion)

Deliver the power to the load in efficient manner. Ex. To provide the amplifier with a low output resistance so that it can deliver the output signal to the load without loss of gain.

Power dissipation is as low as possible.


Classification of Power Amplifier

Power amplifiers are classified according to the collector current waveform that results when an input signal is applied.

Conducting angle.



cC II ˆ

o360angleonduction cCollector current waveforms for transistors

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

o081angleonduction c



oo 360angleonduction 081 c

class AB class C

o180angleonduction c


CE amplifier is not proper for using as a power amplifier

)(21

2

2/)//(

''2

'LCQCQL

CQom

LCCQ

RIIRI

P

RRI

DC supply power: ICQVcc , the area of ABCORc power: ICQURc ， the area of QBCDCollector dissipation power: ICQUCEQ ， the area of AQDOLoad power:


Class A Output Stage

satCECCo

Lo

satCECCo

BEIo

VVv

IRv

VVvvvv

2min

min

1max

1

or


Figure 14.3 Transfer characteristic of the emitter follower. This linear characteristic is obtained by neglecting the change in vBE1 with iL. The maximum positive output is determined by the saturation of Q1. In the negative direction, the limit of the linear region is determined either by Q1 turning off or by Q2 saturating, depending on the values of I and RL.


Figure 14.4 Maximum signal waveforms in the class A output stage of Fig. 14.2 under the condition I = VCC /RL or, equivalently, RL = VCC /I.


Class A Output Stage

%25,ˆ whenˆ

41

2 ˆ

21

)power(Psupply

)power(P load:efficiency conversionpower

:ndissipatiopower

2

2

S

L

111

LCCoCCL

o

CCSL

oL

CCED

IRVVVIR

V

IVPRVP

ivp


Class B Output Stage

A class B output stage.

Complementary circuits.

Push-pull operation

Maximum power-conversion efficiency is 78.5%


Class B Output Stage

When Vi=0, QN and Qp are cut off, Vo=0When Vi>0.5V, QN is on and Qp is cut off, Vo=Vi-VBEN

When Vi>0.5V, Qp is on and QN is cut off, Vo=Vi-VBEP


Transfer Characteristic


Crossover Distortion

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Power –conversion efficiency

The load power

Maximum load powerL

oL R

VP2ˆ

21

L

CC

VVL

oL R

VRVP

CCo2

ˆ

21 2

ˆ

2

max



Total supply power

Maximum total supply power

CCL

os V

RVPˆ2

L

CC

VV

CCL

os R

VVRVP

CCo

2

ˆmax

2ˆ2

CCCEsatCC VVV



Power-conversion efficiency

Maximum power-conversion efficiencyCC

o

VV̂

4

%5.78ˆ

4 ˆmax

CCo VVCC

o

VV


Power Dissipation

Power dissipation

Maximum Power dissipationL

oCC

L

oLSD R

VVRVPPP

2ˆ

21ˆ2

L

CCDPDN

LL

CC

VVL

oCC

L

oD

RVPP

PR

V

RVV

RVP

CCo

2

2

maxmax

max2

2

2ˆ

2

max

2.02

ˆ

21ˆ2


Power Dissipation At the point of maximum power dissipation the

efficiency can be evaluated:%50

ˆ

4 2

CCVoVCC

o

VV


Example: 9.1

efficiency conversion-power theandpower,supply Io, Vcc, thedetermine

tage.output volpeak thean greater th 5V is Vccsuch that

selected issupply power the20 8 WPR LL


Reducing crossover distortion


Class AB Output Stage

BEPBENBBLPNBENBB

Io VVViiiVVvv 2


A Class AB Output Stage Utilizing Diodes for Biasing

The diode biasing has anImportant advantage:

Thermal stabilization of Quiescent current


A Class AB Output Stage Utilizing A VBE Multiplier for Biasing

)1()(

,

1

2121

11

1

RRVRRIV

IIR

VI

BERBB

BRBE

R


Homework:

May 25th, 2008

9.12; 9.13; 9.14; 9.15

chapter 9 output stages and power amplifiers

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