Class A power amplifiers

7/29/2019 Class A power amplifiers

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8. Class A power amplifiers(16.1 16.3)

Classification of Power AmplifiersSeries-fed Class A Power Amplifiers

Transformer Coupled Class A Amplifiers


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VCC

RCR2

R1RE

B

C

E

vo

IB

IC

vin

Q

IC

IBQ

ICQ

VCC

VCE

VCEQ

Class A operation

In Class A operation, IC should be flowing the whole cycle (2)Q point is at the center of the operating range. IBQ>0

Classification of Power Amplifiers

Here R1 and R2 provide required IBQ for class A operation. Q point isat the center of the operating range. IBQ>0


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VCC

RLR2

R1RE

B

C

E

vo

IB

IC

vin

VCC

RL

R2

R1RE

B

C

E

vo

IB

vin

n:1

Series-fed Class APower Amplifier Transformer CoupledClass A Power Amplifier

Class A Power Amplifiers

Here dc power from VCC is converted into ac output power in RL


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VCC

RC

R1RE

B

C

E

vo

IB

IC

vin

Q

IC

IBQ=0

ICQ

VCC

VCE

VCEQ

Class B operation

In Class B operation, IC should be flowing half cycle ()Q point is at cutoff (IC=0) of the operating range. IBQ=0

Here R1 provides required IBQ =0 for class B operation. Q point is atthe cutoff of the operating range. IBQ=0


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RL

R2

VCC

R2

OTL Class B PowerAmplifierOutput Transformer-Less

RL

R2

VCC

R2

-VCC

OCL Class B PowerAmplifierOutput Capacitor-Less

TransformerCoupled Class BPower Amplifier

Class B Power Amplifiers


Vin

RL

Q1

Q2

VCC

R2D


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VCC

RCR2

RE

B

C

E

vo

IBQ

ICQ

vin

D

Q

IC

IBQ

VCC

VCEICQ

180

>180

Class AB operation

In Class AB operation, IC

should be flowing more than half cycle (>)Q point is at small amount above cutoff (IC>0) of the operating range. IBQ>0Here R2 and diode Dprovides required small IBQ for class ABoperation. Q point is above the cutoff of the operating range. IBQ>0


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RL

R2

VCC

R2

D

D

OTL Class AB Power

AmplifierOutput Transformer-Less

RL

R2

VCC

R2

-VCC

D

D

OCL Class AB Power

AmplifierOutput Capacitor-Less

Transformer

Coupled Class ABPower Amplifier

Vin

RL

Q1

Q2

VCC

R2D

Class AB Power Amplifiers



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VCC

RC

R2

RE

B

C

E

voIBQ< 0

ICQ=0

vin

-VBB

Q

IC

IBQ< 0

VCEICQ= 0

VCC

Class C operation

In Class C operation, IC should be flowing less than half cycle (< )Q point is at below cutoff (IC< 0) of the operating range. IBQ< 0Here RB and VBB provides required below cutoff IBQ for class Coperation. Q point is below the cutoff of the operating range. IBQ


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VCC RA

RE

E

vo

IBQ=0

ICQ=0

vin

RB IC

Tuned Class C RFPower AmplifierRadio Frequency Amplifier

Class C Power Amplifiers



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t0

Amplifier

Low-pass

Filter

Feedback

t0

t0

Sawtooth

generator

Input signal

wave

Output signal

wave

IC conductingonlywhendigitalversionoftheinputwaveishigh ----CLASS-Doperation.

high

low

Efciencyisveryhigh = over90%digitalswitchingoftheinputwave

Class D Power Amplifiers


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VCC

RLR2

R1RE

B

C

E

vo

IB

IC

vin

Q

IC

IBQICQ

VCC

VCE

VCEQ

RLVCC/RL

Vo(PP) Maximum levelAny level

( )

dc

ac

CQCCdc

L

2

o

L

2o

ac

o

P

P

IVP

R8 )pp(VR 22

)pp(V

P

level)pp(VanyAt

=

=

=

=

%2525.0V

R2

R8

V

P

P(max)

R2

V

R2V

VIV(max)P

R8VR 22

)pp(V

(max)P

V)pp(Vlevel)pp(VimummaxAt

2

CC

L

L

2CC

dc

ac

L

2CC

L

CC

CCCQCCdc

L

2

CCL

2o

ac

CCoo

====

===

=

=

=

Series-fed Class A Power Amplifiers


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Transformer Coupled Class A Power AmplifiersVCC

RL

R2

R1RE

B

C

E

vo

IB

vin

n:1

Q

IC

IBQICQ

2VCC

VCE

VCEQ=VCC

n2RL

2VCC/n2RL

Vo(PP)

RE

( )

dc

ac

CQCCdc

L2

2o

L2

2o

ac

o

P

P

IVP

Rn8)pp(V

Rn22

)pp(V

P

level)pp(VanyAt

=

=

=

=( )

%505.0V

R

Rn8

V4

P

P(max)

R

V

R

VVIV(max)P

Rn8V2

R22

)pp(V

(max)P

V2)pp(Vlevel)pp(VimummaxAt

2CC

L

L2

2CC

dc

ac

L

2CC

L

CCCCCQCCdc

L2

2

CC

L

2o

ac

CCoo

====

===

=

=

=


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IC(pp)

VO(pp)

10V

R2

R1

B

C

EIB

vin

3:1vo(PP)

RL=8W

( )

%6.41416.015.1

4784.0

P

P

)e(

W15.1mA2

2525510IVP)d(

W4784.0838

7.13.18

Rn

22)pp(V

P)c(

V33.5

3

V6.16)pp(V

3

1

)pp(V

)pp(V)b(

V6.167.13.18)pp(V)a(level)pp(VanyAt

dc

ac

CQCCdc

2

2

L2

2o

ac

L

o

L

oo

====

=

==

=

=

=

===

==

Example: Transformer Coupled class A is operating with VCC=10V, RL=8, andtransformer turn ratio n=3. Measurements at VCE gives VCEmin=1.7V andVCEmax=18.3V,and measurements at IC gives ICmin=25mA and ICmax=255mA. Find (a)Vo(pp) ,(b) VL(pp) (c) Pac (d) Pdc (e) efficiency at that moment.

Documents

Class A power amplifiers