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

2

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.

http://

www.edw.com.pl/ea/tranzystory/bipolarne.gif

Bipolar
transistor
symbols

Diode model
citcuits

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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 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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http://www.edw.com.pl/ea/tranzystory/probc.gif

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

Let’s assume transistor current gain β=100
We calculate U

ce

= 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

http://www.edw.com.pl/ea/tranzystory/probc.gif

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Operating Point of
amplifier and the class

http://www.edw.com.pl/ea/tranzystory/probc.gif

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 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

http://bonifaczuk.pl/retro3.html

http://sound.eti.pg.gda.pl/student/elearning/sysnagkomwz
m.htm

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Operating Point of
amplifier and the class

http://www.edw.com.pl/ea/tranzystory/probc.gif

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 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

http://www.edw.com.pl/ea/tranzystory/probc.gif

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 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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http://nokautimg4.pl/p-37-86-37864b1c2422b059b32b72aca64db8f5500x500/megafon-bezprzewodowy-
monacor-txm-48.jpg

2C -class applications

-Sometimes seen in megaphones

-Ultrasonic cleaners

-Some car alarms itp.

-Used in D-Class amplifiers

http://www.bionovo.pl/images/artykoly/image_005
47.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.

http://www.edw.com.pl/ea/tranzystory/probc.gif

http://zseii.edu.pl/archive/dydaktyka/elektronika/ua/t
20.html

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Green – Input signal
Red – Output signal

2AB-class input/output waveforms

Operating Point of
amplifier and the class

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+

http://www.autocentrum.pl/gfx/opisyaut/827564672/maj_art1004_02032009_3_s
_m.jpg

http://www.linear.biz.pl/zdjecia/4094.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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%202GB.jpg

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). 

http://www.proelectronic.rs/upload/thumbnails/400x400/7170_chip_
tda1560q.jpg

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+

http://www.ads.com.pl/files/hxi_3000_big.jpg

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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]

37

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.

44

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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http://www.national.com/ds/LM/LM124.
pdf

Operational amplifier

Puzzle time

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