Power Amps 4

7/29/2019 Power Amps 4

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Output Transistors Current gain / input impedance is a vital

parameter of a power amplifier.

In the class A analysis, the load impedance isscaled by a factor ofb.

High power transistors often have a value ofbof the order of tens rather than hundreds.


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Multiple Transistor Units

To realise a high current gain with low b transistors,use two cascaded emitter follower stages.

12112222 BCBC IIII bbbb

21i.e. bbb Total


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Darlington Pair

Darlington Pairconfiguration is

available in a single integratedpackage.VBE is double that of a normaltransistor.b is typically between 1000 &

10000.


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Efficiency The efficiency of a power amplifier is the ratio

between the power delivered to the load and

the power drawn from the supplies. Power supply requirements and transistor

power dissipation ratings depend on theefficiency.


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Class A Amplifier Efficiency

L

LLoutL

inout

R

A

RtARvP

tAvv

2/sin/

:loadtodeliveredpowerAverage

sinAssume

2222

To calculate efficiency, must calculateload power, PL, and the suppliedpower, PS.


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ESCSveSIVIVP

)(

:supplyvefromdrawnpowerAverage

LESESLS

L

RIV

A

IVR

A

P

P

42

1

2

22

Amplifier efficiency :

ESveSIVP

)(

:supplyvefromdrawnpowerAverage

EStotalSIVP 2)(


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LES RIV

A

4

2

Amplifier efficiency :

So, peak efficiency is when A is at its maximum.Worst efficiency is when A = 0.

%254

1

and

max

LES RIAVA

Maximum output voltage swing is VS.Maximum output current swing is IE.So :


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Class A Efficiency In practice, the theoretical peak value ofA

would not be reached without distortion.

Practical maximum efficiency is between 10and 20 %.

Very low figure, only suitable for low powerapplications.


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Power Dissipation All power which is not delivered to the load

must be dissipated by the output device(s)

in the form of heat. As a result, the temperature of the device

rises.

Temperature rise changes the properties ofa transistor and may, in extreme cases,destroy it.


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Case, TC

Transistor Power Dissipation

Junction, TJ

Ambience / Air, TA

Powerdissipation

Heat only travels from a hot to a cold body TJ > TC > TARate of transfer is proportional to temperature difference

(and vice versa)


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Thermal Resistance

Temperature difference between the junction andambience depends on the power dissipation and the

thermal resistance between them.

DJAAJPTT

JA is the thermal resistance between the junction andambience measured in C per Watt.


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Power-Derating CurveJunction temperature must not exceed Tjmax. Also,power dissipation must not exceed PD0. Combiningthese limitations gives the power derating curve.

PDmax

PD0

0TA0 TJmax TA

JA

AJ

D

TTP

maxmax


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Using the Power Derating CurveExample : TIP 120

PDmax [W]

0TA[C]25 150

2

50

1.6W6.150

W225

max

max

DA

DA

PT

PT


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Increasing Power Dissipation The only fixed point on the power-derating

curve is TJmax.

To increase power dissipation, slope ofpower-derating curve must be steeper.

Thermal resistance must be lowered.


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Heat-Sinks

JAcan be broken down into a pair of series resistances.

CAJCJA

JC = thermal resistance between junction and case (fixed)CA= thermal resistance between case and ambience.

CAcan be lowered by increasing the surface area of thetransistor case, i.e. by adding a heat-sink.


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Example

TIP 120PDmax [W]

0TA[C]25 150

2

PDmax [W]

0TA[C]25 150

5

No heat-sink, JA= 62.5 C/W With heat-sink, JA= 25 C/W


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Summary High current gain transistors can be

synthesised using the Darlington pair.

Class A amplifiers may be simple and linear,they are also highly inefficient.

Inefficiency leads to power dissipation potential thermal problems.

Power dissipation must always be consideredwhen designing power amplifiers.

Power Amps 4

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