Electronic Circuits Lec.3

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Uploaded By: Khaled Saleh

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Chapter 1 : Power Amplifiers

Class B Amplifier Operation

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Input (DC) Power (drawn from power supply

(Full wave rectified )

dcCCiIVdcP )(

)(2 pIIdc

)(

2)( pIVdcP CCi


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27 March 2011 5

Output (AC) Power

L

L

oR

rmsVacP

)()(

2

L

L

L

L

o R

pV

R

ppVacP

2

)(

8

)()(

22

Efficiency

%100)(

)(%

dcP

acP

i

o

%100

)(

4%100

)()/2(

2/)(%100

)(

)(%

2

CC

L

CC

LL

i

o

V

pV

pIV

RpV

dcP

acP


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27 March 2011 6

Power Dissipated by Output Transistors

acPdcPP oiQ 2

22Q

Q PP

CCCCL VVpV

2636.0)(

L

CCQ

R

VP

2

2

2

2maximum


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27 March 2011 7

EXAMPLE

For a class B amplifier providing a 20-V peak signal to a16- load (speaker) and a power supply of V = 30 V,

determine the input power, output power, and circuitefficiency.

Solution:

A 20-V peak signal across a 16- load provides a peakload current of:


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27 March 2011 8

The dc value of the current drawn from the power supply :

ARpVpIL

LL 25.116/20)()(

ApII Ldc 796.0)25.1(2)(2

The input power delivered by the supply voltage is:

WIVdcPdcCCi

9.23)796.0)(30()(


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27 March 2011 9

The output power delivered to the load is:

WR

pVacP

L

L

o 5.12)16(2

)20(

2

)()(

22

for a resulting efficiency of:

%3.52%1009.23

5.12%100

)(

)(%

dcP

acP

i

o


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Class B Amplifier Circuits

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ways to obtain phase-inverted signals from asingle input signal


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27 March 2011 13

Transformer-Coupled Push-Pull Circuits


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27 March 2011 14

Complementary-Symmetry Circuits


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27 March 2011 16

Complementary-symmetry push-pull circuit usingDarlington transistors.


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27 March 2011 18

Quasi-complementary push-pull

transformerless power amplifier


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27 March 2011 20

EXAMPLE (1.7)

Calculate the input power, output power, and

power handled by each output transistor and thecircuit efficiency for an input of 12 V rms.


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The peak input voltage is:

Since the resulting voltage across the load is ideally

the same as the input signal (the amplifier has,ideally, a voltage gain of unity),

And the output power developed across the load is:

VVrmsVpV ii 1797.16)12(2)(2)(

VpVL 17)(

WR

pVacP

L

L

O 125.36)4(2

)17(

2

)()(

22


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27 March 2011 23

ARpVPIL

LL 25.4

417

)()(

ApII Ldc 71.2)25.4(2

)(2

WIVdcP dcCCi 75.67)71.2)(25()(

WPPP

P OiQ

Q 8.1522

2

WdcP

acP

i

o3.53%100

75.67

125.36%100

)(

)(%


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Class C Amplifiers

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Is biased to conduct for much less than 180

Used as radio frequencies (RF) Amplifier

Basic class C amplifier operation(noninverting).


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Class C Amplifiers

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Basic Operation


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Class C Amplifiers

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Basic Operation


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Class C Amplifiers

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Basic Operationmaximum collector current

is Ic(sat),

Ideal minimum collectorvoltage is Vce{sat).


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Class C Amplifiers

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To avoid complex mathematics, we will use idealpulse approximations for the collector current andthe collector voltage during the ontime of thetransistor.

Power Dissipation


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Class C Amplifiers

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The transistor is on for a short time, ton, and offfor the rest of the input cycle T

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