Audio Amplifiers Basics, Circuits and Parameters

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Audio AmplifiersBasics, Circuits and Parameters

• MSc. Karol Kropidlowski

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Presentation Plan

• Transistor what is it?
• Polarization and classes
• Pros and cons of classes
• Applications
• Basic Parameters of amplifiers
• Differential amplifier what is it?
• Operational amplifiers

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Transistor what is it?

• Transistor is a semiconductor device used to
  amplify and switch electronic signals.

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How transistor works?
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Symbol and Diode Model

• Diode equivalent circuit is very simplistic and
  does not explain the operation of a transistor
  but it gives some idea of ??the voltages that
  exist between the electrodes.

Bipolar transistor symbols
Diode model citcuits
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We got transistor, whats next?

• Transistor to be able to work as amplifier must
  be properly polarized. We will follow the correct
  polarity on the NPN-type transistor  - the
  potential of the collector must be higher than
  the potential of emitter (we need to feed power
  to the transistor) -  diode in the base-emitter
  junction must be polarized in conducting
  direction, and the collector-base diode in
  opposite direction (we apply voltage to the base
  of transistor higher than the threshold voltage
  and lower than the supply voltage)

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Polarization

• Polarization example for npn transistor

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Operating Point and Load Line
Operating point is a point on transistor output
characteristic in which it active region and
parameters such as Uce and Ic can be
designated. Load line shows all the possible
operating points when different values of base
current are applied. It also shows how will
voltages and current change when we feed input
signal to base of transistor.
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Setting up Operation Point

• Lets assume transistor current gain ß100
• We calculate Uce Uz-Uce_sat12v-0,9V11,1V
• We calculate maximum collector current
  Uce/Rc11,1V/1kO11,1mA
• Lets chose operating point in the middle of load
  line (Uwy5,5V oraz Ic5,5mA)
• We calculate base current IbIc/ß 55uA and
  value of base resistor Rb(Uz-Ube)/Ib 205,45kO

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Operating Point of amplifier and the class
Setting up operating point in the middle of load
line we get amplifier working in A-class. P point
on output characteristics.
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Operating Point of amplifier and the class
A-class input/output waveforms
Green Input signalRed Output signal
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A-class Pros and Cons

• Pros
• No crossover distortion
• Very low distortion
• Low construction cost (for small powers)
• Cons
• Low efficiency of less than 50 (up to 20)
• At higher powers problem with the heat
  dissipation from the transistors
• The need for precise setting of the operating
  point
• The need to compensate for transistor parameters
  change according to temperature.
• Weight, size and price of the amplifier increases
  exponentially with the output power

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A-class applications

• Oldish mobile players (eg. walkman)
• Impedance matching (input circuits)
• Preamplifiers (audio and not only audio)
• Audiophile audio power amplifiers
• Studio listenings

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Operating Point of amplifier and the class
Setting up operating points in way that collector
current is minimal (Iceo) We obtain B-Class
amplifier. Point A on output characteristics.
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Operating Point of amplifier and the class
B-class input/output waveforms
Green Input signalRed Output signal
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B-class Pros and Cons

• Pros
• High efficiency (theoretically 78,5)
• Very low quiescent current in idle.
• Cons
• Huge signal distortion
• Only one half of signal is amplified, other half
  is cut.

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Operating Point of amplifier and the class
Adding second transistor working with second half
of the signal we get 2B-Class amplifier.
Green Input signalRed Output signal
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2B-class Pros and Cons

• Zalety
• High efficiency (theoretically 78,5)
• Very low quiescent current in idle.
• Wady
• Crossover distortion
• metallic sound of this kind of amplifier

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2B-class applications

• -Outputs of TTL digital gates
• Very rarely used in audio practice, because of
  nonlinear distortion.
• Sometimes encountered in budget constructions.

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Operating Point of amplifier and the class
Setting up operating points in way that there is
no collector current. We obtain C-Class
amplifier. Point A on output characteristics.
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Operating Point of amplifier and the class
C-class input/output waveforms
Green Input signalRed Output signal
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C-class Pros and Cons

• Pros
• High efficiency
• No quiescent current, no power draw without
  signal.
• Cons
• Huge signal distortion, bigger than in B-class

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Operating Point of amplifier and the class
Adding second transistor working with second half
of the signal we get 2C-Class amplifier.
Green Input signalRed Output signal
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2C-class Pros and Cons

• Pros
• High efficiency
• No quiescent current, no power draw without
  signal.
• Cons
• Crossover distortion bigger than in 2B-class

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2C -class applications

• Sometimes seen in megaphones
• Ultrasonic cleaners
• Some car alarms itp.
• Used in D-Class amplifiers

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Operating Point of amplifier and the class
Setting up operating points of 2B-class in way
that collector current is just above minimal
(Iceo) We obtain 2AB-Class amplifier. Point
between A and P on output characteristics.
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Operating Point of amplifier and the class
2AB-class input/output waveforms
Green Input signalRed Output signal
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2AB-class Pros and Cons

• Pros
• High efficiency (theoretically 78,5)
• Low quiescent current
• Low signal distortion
• Cons
• It requires precise settings of operating points
  that the quiescent current of both transistors is
  identical.

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2AB-class applications

• End stages of power amplifiers (audio,
  generators, radio transmitters, TV transmitters)
• It occurs in operational amplifiers, headphone
  amplifiers from 0.1 W thru car-audio to live
  performance amplifiers 4kW

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Other amplifier classes
By setting polarity of the transistors as in the
2C-Class and controlling them by PWM we obtain a
D-Class amplifier.Idea of converting the audio
signal to the PWM control signal is shown in the
following figure
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Other amplifier classes
Exemplary schematic of an amplifier operating in
D-class
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D-class Pros and Cons

• Pros
• High efficiency (theoreticaly 100)
• At high frequency switching distortion is
  minimal.
• Cons
• Large number of elements
• Expensive
• Higher the switching frequency more power losses
  occur.
• Complicated transistor control.

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D-class applications

• End stages of power amplifiers(audio, DC and BLDC
  motor drivers)
• Portable devices(mp3, phones,where efficiency is
  critical)
• It occurs in headphone amplifiers from 0.1W thru
  car-audio 250W to live performance amplifiers
  1,5kW

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Other amplifier classes
Class H For small signals, the amplifier operates
as a normal 2AB amplifier but as we approach the
maximum output signal, built-in
converter increases voltage for end stage in
power amplifier which allows to obtain even
4-fold increase in output power, at the same
power supply voltage. It occurs most often in
power amplifiers used in cars where the
limitation is the low voltage (12V). 
It is worth noting that the supply voltage is
increased in the rhythm of the signal only in the
channel, and only when necessary.
An example of such amplifier is TDA1560Q
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H-class Pros and Cons

• Pros
• Such as2AB
• The ability to achieve higher power output at the
  same supply voltage
• Cons
• Such as 2AB

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H-class applications

• End stages in power amplifiers
• It occurs in car-audio and performance amplifiers
  1,5kW

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Basic power amplifiers parameters

• Power gain (Kp)
• Output powerW
• THD- Total Harmonic Distortion
• Input/output impedance
• Bandwidth Hz
• Efficiency
• Output noise voltage mV
• Quiescent Current mA

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Basic parameters

• Power gain of amplifier is the power output
  divided by power input.
• Output power its power that amplifier can
  produce on nominal load impedance at a given
  frequency or frequency range, without exceeding a
  specific distortion ratio within 10 minutes.

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Basic parameters

• Nonlinear distortions (THD) rely on the formation
  of harmonic and combined frequencies signal. The
  signal at the output of the device contains
  additional components that were not present in
  the input signal. If harmonic distortions are
  less than 10 they are practically unnoticeable
  for the typical listener.

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Basic parameters

• Input impedance of the amplifier is impedance,
  which the input of the amplifier represents for
  the rated operating conditions.
• Output impedance determines the value of the load
  impedance, which may be attached at a specific
  efficiency of the amplifier.

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Basic parameters

• Bandwidth is the range of frequencies transmitted
  by the amplifier. Frequency is determined for the
  3dB rolloff to flat part of frequency
  characteristics.According to PN-74 / T-06251/07
  for Hi-Fi amplifiers minimum bandwidth should be
  40Hz - 16kHz.

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Basic parameters

• Energy efficiency is defined as the percentage
  ratio of the output power of the amplifier to a
  power drawn from the power source. This is an
  important criterion for assessing the quality of
  the power amplifier.

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Basic parameters

• Output noise voltage at the output is the
  maximum amplitude of the noise at the amplifier
  output with input shorted down.Quiescent
  current is the current drawn by the amplifier
  from the power source when there is no signal
  connected to the input.

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Input Circuits of Power Amplifiers

• Historically the A-class amplifier was used as
  the input circuit of the power amplifier because
  it ensured low distortion and impedance matching
  between input and output. High input and low
  output impedance.However, the A-class amplifier
  input circuit was replaced by a differential
  amplifier.

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Differential amplifier

• Amplifier whose output voltage depends on the
  voltage difference between the inputs of the
  amplifier.
• In its simplest version is composed of two
  transistors coupled together via an emitter
  resistor Re.
• This resistor stabilizes the operating points of
  the two transistors and forces the Ie emitter
  current flowing in a common circuit which is
  equal to the sum of the currents of both
  transistors.
• At high resistance Re, current Ie does not
  depend on the intensity of the currents at the
  inputs, and is constant.

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Differential amplifier

• Exemplary schematic of differential amplifier

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Differential amplifier

• In the construction of differential amplifiers we
  look for high differential gain Kur, high control
  signal damping factor Hs, high input resistance
  and small signal unbalance and small drift.
  Transistors should have parameters as close as
  possible to each other.

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Differential amplifier

• Improving the performance of the amplifier
  requires an increase in transistor current gain
  ßo, increase in the resistance Re and increase in
  resistance Rc. Instead of a resistor R and Rc
  current sources are used, it is easier to produce
  a current source and current mirror in a single
  chip than the resistor.

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Differential amplifier

• Differential amplifiers are used as the input
  circuit of power amplifiers and operational
  amplifiers. An example of an operational
  amplifier internals

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Operational amplifier

• Puzzle time

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Bibliografia

• http//www.edw.com.pl/ea/bipolarne.html
• http//pl.wikipedia.org/wiki/Punkt_pracy
• http//wazniak.mimuw.edu.pl/index.php?titleLabora
  torium_wirtualne_2/ModuC582_2_-_C487wiczenie_2
  
• http//pl.wikipedia.org/wiki/Wzmacniacz_rC3B3C5
  BCnicowy
• http//www.national.com/ds/LM/LM124.pdf
• Elektronika dla wszystkich 11/96
• Praktyczny elektronik 1/2001