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Uploaded By: Khaled Saleh

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Electronic Circuits

Dr. Farid Shawki Al-Hosarey

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Contents

Power Amplifiers

Chapter

1

Class A power Amplifies

Transformer-coupled class

A amplifier

Class B Amplifier Operation

Amplifier Distortion

Class C Power Amplifier

Troubleshooting

System Application

Thyristors

Chapter

2

The Basic 4-Layer Device

The Silicon-Controlled Rectifier

SCR Applications

The Diac and Triac

The Silicon-Controlled Switch

Optoelectronics

Chapter

3

The Phototransistor

The Light-Activated SCR

Optical Couplers

Fiber Optics

System Application

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Programmable Logic DevicesChapter

4


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Chapter 1 : Power Amplifiers

Class A power Amplifies

Amplifier operating classes

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Class C is usually not used for delivering large amounts of power, and

thus the efficiency is not given here.

A AB B C D

Operating

Cycle

360 180 to

360

180 Less than

180

Pulse

operation

Power

Efficiency

25 % to

50%

Between

25% (50%)and 78.5%

78.5% Typically

over 90%

Comparison of Amplifier Classes

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Series-Fed Class A power Amplifies

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B

CCB

R

VI

7.0

BC II

CCCCCE RIVV

The dc bias set by VCC and RB fixes the dc base-biascurrent at:

with the collector current then being:

with the collector-emitter voltage then:

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Transistor characteristic showing load line and Q-point13 March 2011 9


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AC Operation13 March 2011 10


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AC Operation13 March 2011 11


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Power Considerations

The power into an amplifier is provided by the supply. Withno input signal, the dc current drawn is the collector biascurrent ICQ The power then drawn from the supply is:

CQCCi IVdcP )(

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Output PowerUsing RMS signals.

The ac power delivered to the load (RC) may be expressedusing:

)()()( rmsIrmsVacP CCEo

CCo RrmsIacP )()(2

C

CE

oR

rmsVacP

)()(

2

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Output PowerUsing Peak-to-peak signals.

The ac power delivered to the load (RC) may be expressedusing:

8

)()()(

ppIppVacP CCEo

8)()(

2

CCo RppIacP

C

CEo

R

ppVacP

8

)()(

2

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Power Efficiency

%100)(

)(% dcP

acP

i

o

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Maximum Power Efficiency

maximum VCE(p-p) = VCC

For the current swing it is

Using the maximum voltage swing yields:

C

CC

R

Vpp )(Imaximum C

C

CCCCCCC

R

VRVVac

88

/)(Pmaximum

2

o

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C

CCCCCCCCCR

VRVVVdc 22

/Imaximum)(Pmaximum

2

Ci

%25%1002/

8/%100

)(Pmaximum

)(Pmaximum%maximum

2

2

i

o CCC

CCC

RV

RV

dc

ac

The maximum power input can be calculated using thedc bias current set to one-half the maximum value:

We can then calculate the maximum efficiency:

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power loss considered here is that dissipated by the powertransistor and calculated using:

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acPdcPP oiQ


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Example:

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Calculate the input power, output

power, and efficiency of the amplifiercircuit in Figure 1-5 for an inputvoltage that results in a base currentof 10 mA peak.


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Example:

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Solution:

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VRIVV

AmAII

mAR

VI

CCCCQCE

BQC

B

CC

QB

4.14)20)(48.0(20

48.05.482)3.19(25

3.19101

7.0207.03


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Solution:

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acPdcPP oiQ = 8.975 W


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Transformer-Coupled Class A Amplifier

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Voltage Transformation

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Current Transformation

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Impedance Transformation

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Operation of Amplifier Stage

DC Load lineQ-PointAC Load line

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Signal Swing and Output AC Power

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minmax

minmax

)(

)(

CCC

CECECE

IIppI

VVppV

The ac power developed across the transformer primarycan then be calculated using:

8)(

minmaxminmax CCCECEo

IIVV

acP

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1

1

2

2 VN

NVVL

L

L

LR

rmsVP

)(2

CL

IN

NII

2

1

2

LLLRrmsIP )(

2

The voltage delivered to the load can be calculated using

The power across the load can then be expressed as:

The the load current yields:

The output ac power then calculated using:


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Maximum Theoretical Efficiency

%50%

2

minmax

minmax

CECE

CECE

VV

VV


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EXAMPLE

Calculate the efficiency of a transformer-

coupled class A amplifier for a supply of 12 Vand outputs of:

V(p) = 12 V.V(p) = 6 V.V(p) = 2 V.


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Since VCEQ = VCC = 12 V, the maximum andminimum of the voltage swing are,respectively, resulting in

%50%024

02450%

2

VpVVV

VpVVV

CEQnCE

CEQnCE

01212)(

241212)(

min

max

resulting in

electronic circuits lec.1 & lec.2

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