Chapter 2 Power Amplifier

7/26/2019 Chapter 2 Power Amplifier

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DC Voltage Source

Signal SourceE.g.: CD player,

Human Voice

Small

Signal

Amplifier

Large

Signal

Amplifier

Load


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INTRODUCTION In small-signal amplifiers the main factors are:

Amplification

Linearity

Gain

Since large-signal, or power amplifiers handle relatively large voltagesignals and current levels, the main factors are:

Efficiency

Maximum power capability Impedance matching to the output device


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POWER AMPLIFIER TYPES CLASS A

CLASS B

CLASS AB

CLASS C

CLASS D


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CLASS A POWER AMPLIFIER The output of a class A power amplifier conducts for the full 360 of the cycle.

The Q-point is set at the middle of the load line so that the AC signal can swing a

full cycle.icic

ic sat

t

Vce

ic(cutoff)

ICQ

VCEQ VCE(cutoff)VCE(sat)


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LOAD LINE CONDITION 1

icic

ic sat

The Q-point is set at the middle of the load line so that the AC signal can swing a

full cycle.

Value of ic change from ICQ either increase to ic(sat) or decrease to ic(cutoff)

t

Vce

ic(cutoff)

ICQ

VCEQ VCE(cutoff)VCE(sat)


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LOAD LINE CONDITION 2 When the input signal larger than the Q-point limits, amplifier will operate in cut off

and saturation region.

ic(sat)

ICQ

ic

X- Operate in

Saturation region

Y- Operate in cut-

off region

ic(cutoff)

Vce(sat) Vce(cutoff)VCEQ

VCE


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LOAD LINE CONDITION 3 When the Q-point not in the middle of AC load line.

ic(sat)

Ic

ICQ

ic(cutoff)

Vce(sat) Vce(cutoff)VCEQ

VCE


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LOAD LINE IN POWER AMPLIFICATION Class A power amplifier


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DRAW DC LOAD LINE All capacitor will act as open circuit, AC source will short circuit.

() 0,

()

EC

CC

RR

V

+

()0, ()

Ic

VCE

ICQ

VCEQ

EC

CC

RR

V

+

VCC0

Ic(sat)

VCE(cutoff)


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DRAW DC LOAD LINE

E

BEBCQ

R

VVI

)(

21

2

ECCQCCCEQ

CCB

RRIVV

VRR

RV

+

+=


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DRAW AC LOAD LINEFrom the shown figure, equation 1:

Where

LCL

L

cecLcce

RRr

r

ViriV

//

@

=

==

Ic and Vce can be relates to thechanges in DC value, then, :

)3___(

)2_____(

CEQCEce

CQCc

VVV

IIi

=

=


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DRAW AC LOAD LINE

In saturation region, Ic=ic(sat) and Vce=Vce(sat)0v, thus

(2) (3) 1

L

CEQCECQCr

VVII

+

=

L

CEQ

CQsatcr

VIi +=)(

In cutoff region, VCE = Vce(cutoff) and Ic=ic(cutoff) 0A, thus

LCQCEQoffcutce rIVV += )(


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DRAW AC LOAD LINE

In saturation region, Ic=ic(sat) and Vce=Vce(sat)0v, thus

(2) (3) 1

L

CEQCECQCr

VVII

+

=

L

CEQ

CQsatcr

VIi +=)(

In cutoff region, VCE = Vce(cutoff) and Ic=ic(cutoff) 0A, thus

LCQCEQoffcutce rIVV += )(


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CLASS A POWER AMPLIFIER

Power delivered to load:

Power dissipated in transistor:

CQCEQDQ IVP =

( )2)(rmsV=

Power drawn from supply :

2

)(

)(

po

rms

L

VV =

CQCCdci IVP =)(


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CLASS A POWER AMPLIFIEREfficiency

100P

P

% i(dc)

o(ac)

=

Power Efficiency for CLASS A is between 25% to 50%

Continue with exercise 2.2


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CLASS A POWER AMPLIFIER with RC


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CLASS A POWER AMPLIFIER with RC The AC load line and DC load line overlap each other.

The Q-point is still in the middle of load line

.


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CLASS A POWER AMPLIFIER with RC Power consumption


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CLASS A POWER AMPLIFIER withTRANSFORMER COUPLED

This circuit uses a transformer to couple to the load. This improves the

efficiency of the Class A to 50%.


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A transformer improves the efficiency because it is able to transform

the voltage, current, and impedance

Voltage RatioVoltage Ratio

22 NV

Current RatioCurrent Ratio

Impedance RatioImpedance Ratio

11 NV

2

1

1

2

N

N

I

I====

2

2

2

1

2

1

L

La

N

N

R

R

R

R====

========


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DC Load LineDC Load Line

As in all class A amplifiers the Q-point is

established close to the midpoint of the

DC load line.

The saturation point (ICmax) is at Vcc/RLand the cutoff point is at V2 (the

secondary voltage of the transformer).

This increases the maximum output

swing because the minimum and

maximum values of IC and VCE are spreadfurther apart.

Continue with exercise 2.3


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CLASS B POWER AMPLIFIER

Power Amplifier class A :

has a low efficiency due to power dissipated by the transistor itself. This willeffect the power consumption at the load/output.

Improve the situation in Power Amplifier Class B where the transistor work

only in cut-off region and work only for 180 of the input signal and the

transistor dissipated only small amount of power.

With this a lot more power can be transmit to load and eventually increase the

efficiency of power amplification at the circuit.


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CLASS B POWER AMPLIFIER

In class B, the transistor is

biased just off. The AC signal

turns the transistor on.

The transistor only conducts

when it is turned on by one-half

of the AC cycle.

In order to get a full AC cycle out

of a class B amplifier, you need

two transistors:

An npn transistor that provides thenegative half of the AC cycle

Apnp transistor that provides the

positive half.


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CLASS B POWER AMPLIFIER: PUSH-PULL

CIRCUIT

OUTPUT SIGNAL ATPOSITIVE CYCLE

ON

OFF

OUTPUT SIGNAL AT

NEGATIVE CYCLE

ON

OFF


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CLASS B POWER AMPLIFIER: TRANSFORMERCOUPLE PUSH-PULL CIRCUIT

The center-tapped

transformer on the input

produces opposite polarity

signals to the two transistor

inputs.

The center-tappedtransformer on the output

combines the two halves of

the AC waveform together.


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CLASS B POWER AMPLIFIER: TRANSFORMERCOUPLE PUSH-PULL CIRCUIT

During the positive

half-cycle of the ACinput, transistor Q1(npn) is conducting and

Q2 (pnp) is off.

During the negativehalf-cycle of the AC

input, transistor Q2(pnp) is conducting and

Q1 (npn) is off.


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CLASS B POWER AMPLIFIER: CrossoverCrossoverDistortionDistortion

If the transistors Q1 and Q2 do not

turn on and off at exactl the

same time, then there is a gap inthe output voltage.


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CLASS AB POWER AMPLIFIER

Problem in Class B Power Amplifier:

1. Crossover Distortion

2. Thermal Runaway

Crossover Distortion problem:

1. Due to ON and OFF situation in ush- ull circuit.

2. Since the transistor in work in cut-off region only, thus the input voltage mustovercome the VBE voltage which usually 0.7v.

3. To improve this, add class A principle in class B power amplifier by adding

voltage divider at the base of the transistor and usually the R1 and R2 were

design to provide 1.4 v at the base.

4. The Class AB power efficiency for this case from 50% to 75%.


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CLASS AB POWER AMPLIFIER: CROSSOVERDISTORTION


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CLASS AB POWER AMPLIFIERThermal Runaway problem:

1. To maintain Q point in Class AB is difficult due to changes in VBE effected by

temperature.2. When temperature high, VBE decrease, increase and Ic increase.

3. This situation will damage the transistor.

4. Improve by adding diode at the base since diode share the same work

rinci le at base-emitter unction.

Figure 1: Graph of Ic versus VBE with temperature change


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CLASS AB POWER AMPLIFIER: THERMALRUNAWAY


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CLASS C POWER AMPLIFIER

A class C amplifier conducts for less than

180. In order to produce a full sine wave

output, the class C uses a tuned circuit (LC

tank) to provide the full AC sine wave.

radio communications circuits.


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CLASS D POWER AMPLIFIERA class D amplifier amplifies

pulses, and requires a pulsed

input.

There are many circuits that can

convert a sinusoidal waveform to a

pulse, as well as circuits that

convert a pulse to a sine wave.

This circuit has applications in

digital circuitry.

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Chapter 2 Power Amplifier