POWER AMPLIFIER
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Transcript of POWER AMPLIFIER
2
AGENDA
Bollen
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
Bollen
Power amplifier = current amplifierRloudspeaker = 8 Ohm, for 50 Watt >> I = 2,5 Ampere
Power IN = DC powerPower out = AC power
2P I R
4
Signals / definitions 1/3 DC supply power
Bollen
DC-power =
DC value for supply voltages,
And Average for current waves
6
Signals / definitions 3/3 efficiency
Bollen
= 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
8
Classes
Bollen
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
Bollen
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
Bollen
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
Bollen
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
Bollen
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
Bollen
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
Bollen
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
Bollen
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
Bollen
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
Bollen
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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Vbe multiplier
Bollen
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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Thermal runaway
Bollen
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
Bollen
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
21
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Temperaturedependent bias voltage
Bollen
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.