Audio Amplifiers
Basics, Circuits and Parameters
MSc. Karol Kropidłowski
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.
http://www.fuw.edu.pl/~pablo/s/projects/2004_2005/winda/stepmotor_idea.html
http://www.tme.eu
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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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http://www.edw.com.pl/ea/tranzystory/bipolarne.gif
We got transistor, what’s next?
Transistor to be able to work as amplifier must be properly
polarized. We will follow the correct polarity on the NPNtype 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
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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
Let’s assume transistor current gain β=100
We calculate Uce = Uz-Uce_sat=12v-0,9V=11,1V
We calculate maximum collector current Uce/Rc=11,1V/1kΩ=11,1mA
Let’s chose operating point in the middle of load line (Uwy=5,5V oraz Ic=5,5mA)
We calculate base current Ib=Ic/β =55uA
and value of base resistor Rb=(Uz-Ube)/Ib= 205,45kΩ
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http://www.edw.com.pl/ea/tranzystory/probc.gif
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.
http://www.edw.com.pl/ea/tranzystory/probc.gif
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Operating Point of amplifier
and the class
A-class input/output waveforms
Green – Input signal
Red – 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.
http://www.edw.com.pl/ea/tranzystory/probc.gif
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Operating Point of amplifier
and the class
B-class input/output waveforms
Green – Input signal
Red – 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 signal
Red – 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.
http://www.edw.com.pl/ea/tranzystory/probc.gif
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Operating Point of amplifier
and the class
C-class input/output waveforms
Green – Input signal
Red – 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 signal
Red – 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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http://www.bionovo.pl/images/artykoly/image_00547.JPG
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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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http://www.edw.com.pl/ea/tranzystory/probc.gif
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Operating Point of amplifier
and the class
2AB-class input/output waveforms
Green – Input signal
Red – 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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http://www.autocentrum.pl/gfx/opisyaut/827564672/maj_art1004_02032009_3_s_m.jpg
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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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http://carphotos.cardomain.com/ride_images/1/1412/501/3527750065_large.jpg
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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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http://www.proelectronic.rs/upload/thumbnails/400x400/7170_chip_tda1560q
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 power[W]
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
βo, 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:
http://www.national.com/ds/LM/LM124.pdf
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Operational amplifier
Puzzle time
http://www.national.com/ds/LM/LM124.pdf
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