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POWER AMPLIFIER

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AGENDA

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Power amplifierSignals / definitionsDC power supplyAC signal powerEfficiencyClassesClass A type CEClass A type CCPush Pull

Cross over distortionVbe MultiplierThermal runawayEmitter resistorsTemperature dependentBias voltageShort circuit protection

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Power Amplifier

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Power amplifier = current amplifierRloudspeaker = 8 Ohm, for 50 Watt >> I = 2,5 Ampere

Power IN = DC powerPower out = AC power

2P I R

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Signals / definitions 1/3 DC supply power

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DC-power =

DC value for supply voltages,

And Average for current waves

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Signals / definitions 2/3 AC Signal power

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2

U

U signal

2signal

II

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Signals / definitions 3/3 efficiency

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= efficiency / for definition see below

So,

POWER SUPPLY delivers DC or average power

And the

LOAD gets SIGNAL power (use root mean square value)

AC signalpower delivered to load

DC power from DC source

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ɳ

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Classes

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Class-A Output device(s) conduct through 360 degrees of input cycle (never switch off) - A single output device is possible. The device conducts for the entire waveform

Class-B Output devices conduct for 180 degrees (1/2 of input cycle) - for audio, two output devices in "push-pull" must be used

Class-AB Halfway (or partway) between the above two examples (181 to 200 degrees typical) - also requires push-pull operation for audio.

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Classes

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Class-C Output device(s) conduct for less than 180 degrees (100 to 150 degrees typical) - Radio Frequencies only - cannot be used for audio! This is the sound heard when one of the output devices goes open circuit in an audio amp!

Class-D Quasi-digital amplification. Uses pulse-width-modulation of a high frequency (square wave) carrier to reproduce the audio signal.

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Class A type CE

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The configuration is a common emitter

The load / loudspeaker is in the collector

If there is no signal there is a quecient current to adjust Vc = ½ Vcc for symmetrical use of voltage range

If there is no signal, the power supply should deliver power to the circuit

The efficiency is very low

The lost power is just dissipation: heating of components: so you need to cool for high power

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Class A

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Vcc = 40 Volt

Rload = 8 Ohm

Quesientpoint

Uc = 20 volt

I bias = 2,5 A

I signal max = 2,5 A 1

22 2

ccq

V IAC signalpower

cc qDC power V I 25,04

1

qcc

qcc

IV

IV

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Class A type CC

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The configuration is a common collector

The load / loudspeaker is connected to the emitter

If there is no signal there is a quecient current to adjust Ve = ½ Vcc for symmetrical use of voltage range

If there is no signal, the power supply should deliver power to the circuit

The efficiency is very low

The lost power is just dissipation: heating of components: so you need to cool for high power

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Class A

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Vcc = 40 Volt

Rload = 8 Ohm

Quesientpoint Uc = 20 volt

Isignal max = 2,5 Ampere

25,041

qcc

qcc

IV

IV

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2 2

ccq

V IAC signalpower

cc qDC power V I

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Push Pull idea

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If you need water; open Q1

If you deliver water;

Open Q2

Q1 and Q2 never opens at the same time

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Push Pull with transistors

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Here the BJT are complementary (NPN and PNP)

Each device amplify the opposite halves of the input signal

At the output you get the total signal.

excellent efficiency

But small mismatch between the two halves of the signal

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Push Pull in realisation

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Vcc = +20 Volt

Vee = -20 Volt

Rload = 8 Ohm

Uin = 0 Volt

Uout = 0 Volt

So no bias current

Vcc

Vee

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Push Pull in realisation

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Vcc

Vee

I

VP ccdc 2

22

IV

P ccsignal

250,78

32

20 2,525

2 2Watt

2,524 32Watt

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Cross-over distortion

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Dead zone = 1,4 Volt

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Eliminating Cross-OverDistortion

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Shift NPN 0,7 Volt to left

Shift PNP 0,7 Volt to right

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Eliminating Cross-OverDistortion

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Eliminating Cross-OverDistortion

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Shift

1,4 Volt

Or

Shift

2,8 Volt

Or

Something else …

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Vbe multiplier

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Base current is negligible, so:

221 RV

II BERR

VBE

1 2

2

0.7bias

R RV

R

11 2 1

2

BER R

V RV I R

R

1 1 21 2

2 2

BEbias R R BE BE

RV R RV V V V V

R R

Inverse

voltage divider !!!

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Vbe multiplier

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Thermal runaway

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Fit a bigger heatsink.

Use series emitter-resistors.

Use a temperaturedependent bias voltage.

The latter two are preferred methods. Both

introduce negative feedback.

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Emitter resisters

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REEBBEbias VVVV 22 21

So, if IC rises, VBE falls and IC is reduced.

Note RE should be small compared with RL to minimise power wasted.

By symmetry:

ECbias

REbiasEBBE

RIV

VVVV

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Temperaturedependent bias voltage

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If junction temperature rises but IC stays the same, VBE must fall causing Vbias to fall also.

Negative thermal feedback achieved if the transistor is in close contact with the output devices.

Especially suitable for integrated circuits where close thermal contact is guaranteed.

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Short circuit protection

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Current limitation

Rx is a current measuring resistor

If URx> 0,7 the T1 switches ON

Then output current in limited !!

T1 Rx