Sound amplification

Contents
 Preamplifiers Requirements
 Signal Voltage & Impedance Levels
 Preamplifier Stages
 Voltage Amplifier Design
 Constant-Current Sources
 Current Mirrors
 Performance Standards
 Power Amplifier Classes
 Thermal Dissipation Limits
 Single-Ended Versus Push–Pull Operation

Contents
 Switching Amplifiers
 Amplifier Grounding
 Cross Over Network
 Audio terminations: line in/out
 Audio terminations: aux in/out
 Audio terminations: mic in

Preamplifiers Requirements
 Due to vast number of audio equipment
manufacturers, the equipments are differing on the
standards of output impedance or signal voltage
 For this reason, it is conventional practice to use a
versatile preamplifier unit between the power
amplifier and the external signal sources to perform
the input signal switching and signal level adjustment
functions
 This preamplifier either forms an integral part of the
main power amplifier unit or, is a free-standing,
separately powered unit

Signal Voltage and Impedance
Levels
 For tuners and cassette recorders, the output is either
that of the German Deutsches Industrie Normal (DIN)
standard or the line output standard
 In (DIN) standard, the unit is designed as a current
source, which gives an output voltage of 1 mV for each
1000 ohms of load impedance
 The line output standard, designed to drive a load of
600 ohms or greater, at a mean signal level of 0.775 V
rms

Levels
 Units having DIN type interconnections, of the styles
shown in Figure will conform to the DIN signal and
impedance level conventions
Din Plug 4 Pin

Levels
 The connectors having “ phono ” plug/socket outputs,
of the form shown in Figure
 The permissible minimum load
impedance will be within the
range 600 to 10,000 ohms
 The mean output signal level will
commonly be within the range
0.25–1 V rms

Voltage Amplifier Design
 The nonlinearity in a bipolar junction transistor
characteristics affects the performance of an amplifier
circuit
 The principal nonlinearity in a bipolar device is that
due to its input voltage/output current characteristics
 If the device is driven from a high impedance source,
its linearity will be substantially greater, since it is
operating under conditions of current drive

 For example, for the circuit shown in figure the Q2
transistor is driven by Q1 which provides a very high
impedance

 The input transistor, Q1 , is only required to deliver a
very small voltage drive signal to the base of Q2 so that
the signal distortion due to Q1 will be low
 Q2 ,is driven from a relatively high source impedance,
composed of the output impedance of Q1 parallel with
the base-emitter resistor, R4
 To study the effects of feedback, we can connect the
collector of Q2 to emitter of Q1

 Due to feedback the THD(Total Harmonic Distortion)
is 0.01% at 1 kHz

 An improved version of this simple two-stage amplifier
circuit is shown in Figure

 In this, if the two-input transistors are reasonably well
matched in current gain and if the value of R3 is
chosen to give an equal collector current flow through
both Q1 and Q2 , the DC offset between input and
output will be negligible,
 This will allow the circuit to be operated over a
frequency range extending from DC to 250 kHz or
more

Constant-Current Sources and
Current Mirrors
 The Common emitter
configuration is often used
for constant current source
with constant base current
at its input
 R1 and R2 form a potential
divider to define the base
potential of Q1
 This configuration can be
employed with transistors of
either PNP or NPN types

Current Mirrors
 An improved, two-transistor,
constant current source is
shown as below
 In this, R1 is used to bias Q2
into conduction, and Q1 is
employed to sense the
voltage developed across R2

Current Mirrors
 This voltage is proportional to emitter current, and to
withdraw the forward bias from Q2 when that current
level is reached at which the potential developed
across R2 is just sufficient to cause Q1 to conduct
 The performance of this circuit is greatly superior to
that with single transistor, in that the output
impedance is about 10 greater
 The circuit is insensitive to the potential, Vref. applied
to R1 , so long as it is adequate to force both Q2 and Q1
into conduction

Performance Standards
 The performance characteristics of any audio amplifier
is dependant on ability of human ear to detect small
differences in sound
 Thus a proper standard is difficult to make as the
response of human ear changes from human to human
 Therefore, the focus is given to achieve more gain with
less distortion within the audio amplifier
 Earlier, the use of ICs was an important criterion for
design engineers to determine the performance of an
audio amplifier

Use of ICs
 Many engineers were of opinion that the ICs are less
preferable than the discrete components
 This is because of the fabrication method allows
multiple PN junctions to be laid side by side
 This leads to reverse diode leakage currents associated
with every component on the chip
 Additionally, there were quality constraints in respect
to the components formed on the chip surface that
also impaired the circuit performance

Use of ICs
 In recent IC designs, considerable ingenuity has been
shown in the choice of circuit layout to avoid the need
to employ unsatisfactory components in areas where
their shortcomings would affect the end result
 Substantial improvements, both in the purity of the
base materials and in diffusion technology, have
allowed the inherent noise background to be reduced
to a level where it is no longer of practical concern

Modern Standards
 The standard of performance that is now obtainable in
audio applications is frequently of the same order as that of
the best discrete component designs, but with considerable
advantages in other respects, such as cost, reliability, and
small size
 The designer of equipment will seek to attain standards
substantially in excess of those that he supposes to be
necessary
 This means that the reason for the small residual
differences in the sound quality among the hifi systems is
the existence of malfunctions of types that are not
currently known or measured

General Design Considerations
 Three major design considerations are listed as below
 Economic considerations
 Requirements of reliability
 Nature of IC design
 The first two of these factors arise because both the
manufacturing costs and the probability of failure in a
discrete component design are directly proportional to
the number of components used
 Therefore it is better to use less components in a
circuit

 In an IC, both the reliability and the expense of
manufacture are affected only minimally by the
number of circuit elements employed
 Still the discrete component circuits have the
advantage of higher voltage swing where the ICs are
limited to small voltage operations
 It is a difficult matter to translate a design that is
satisfactory at a low working voltage design into an
equally good higher voltage system

 The reasons are as stated below
● increased applied potentials produce higher thermal
dissipations in the components for the same operating
currents
● device performance tends to deteriorate at higher
inter-electrode potentials and higher output voltage
changes
● available high voltage transistors tend to be more
restricted in variety and less good in performance than
lower voltage types

Power Amplifier Classes
 The Class of an amplifier refers to the design of the
circuitry within the amp
 For audio amplifiers, the Class of amp refers to the
output stage of the amp
 In practice there may be several classes of signal level
amplifier within a single unit
 The more common amplifier classes are : Class A, Class
B, Class AB, Class C, Class D, Other classes

Power Amplifier Classes: Class A
 Class A amplifiers have very low distortion (lowest
distortion occurs when the volume is low) however they are
very inefficient and are rarely used for high power designs
 The distortion is low because the transistors in the amp are
biased such that they are "on" when the amp is idling
 As a result of being on at idle, a lot of power is dissipated in
the devices even when the amp has no music playing
 Class A amps are often used for "signal" level circuits
(where power requirements are small) because they
maintain low distortion

Power Amplifier Classes: Class B
 Class B amplifiers are used in low cost designs or
designs where sound quality is not that important.
 Class B amplifiers are significantly more efficient than
class A amps, however they suffer from bad distortion
when the signal level is low
 Class B is used most often where economy of design is
needed
 Before the advent of IC amplifiers, class B amplifiers
were common in pocket transistor radios and other
applications where quality of sound is not that critical

Power Amplifier Classes: Class AB
 Class AB is probably the most common amplifier class
currently used in home stereo and similar amplifiers
 Class AB amps combine the good points of class A and
B amps.
 They have the improved efficiency of class B amps and
distortion performance that is a lot closer to that of a
class A amp.
 With such amplifiers, distortion is worst when the
signal is low, and generally lowest when the signal is
just reaching the point of clipping.

Power Amplifier Classes: Class AB
 Class AB amps (like class B) use pairs of transistors,
both of them being biased slightly ON so that the
crossover distortion (associated with Class B amps) is
largely eliminated

Power Amplifier Classes: Class C
 They are commonly used in RF circuits
 Class C amplifiers operate the output transistor in a
state that results in tremendous distortion (it would be
totally unsuitable for audio reproduction)
 However, the RF circuits where Class C amps are used
employ filtering so that the final signal is completely
acceptable
 Class C amps are quite efficient
 Class C amps are not used in audio circuits

Other classes
 There are a number of other classes of amplifiers, such
as G, H, S, etc
 Most of these designs are actually clever variations of
the class AB design, however they result in higher
efficiency for designs that require very high output
levels

Thermal Dissipation Limits
 The BJT suffers the problem of thermal runaway
 The potential barrier of a P-N junction (that voltage
that must be exceeded before current will flow in the
forward direction) is temperature dependent and
decreases with temperature
 Because there will be unavoidable non-uniformities in
the doping levels across the junction, this will lead to
non-uniform current flow through the junction
sandwich, with the greatest flow taking place through
the hottest region

 If the ability of the device to conduct heat away from
the junction is inadequate to prevent the junction
temperature rising above permissible levels, this
process can become cumulative
 This will result in the total current flow through the
device being funneled through some very small area of
the junction, which may permanently damage the
transistor
 This malfunction is termed secondary breakdown
 Field effect devices do not suffer from this type of
failure

 The operating limits imposed by the need to avoid this
failure mechanism are shown in Figure

Single-Ended Versus Push–Pull
Operation
 A transistor can
also act as switch
other than
amplifier
 Shown in figure is
the arrangement in
which a transistor
can be operated

Operation
 If we consider first the single-ended layout of Figure
when Q1 is O/C, the current flow into R2 is only
through R1 and i2 = V /( R1 + R2 )
 If Q1 is short circuited, S/C, then

Operation
 If all resistors are 10 Ω in value, when Q1 is S/C, Vx will
be equal to V , and there will be no current flow in R2
 For Q1 in O/C, the current i2 will be ( V /20)A
 If R1 and R2 are 10 Ω in value and R3 is 5 Ω , then the
current flow in R2 , when Q1 is O/C, will still be (
V/20)A
 Whereas when Q1 is S/C, the current will be (–0.25 V
/10)A and the change in current will be (3 V /40)A

Operation
 By comparison, for the push–pull system the change in
current through R2 , when this is 10Ω and both R1 and
R3 are 5 Ω in value, on the alteration in the conducting
states of Q1 and Q2 , will be (2V/15)A, which is nearly
twice as large
 The increase in available output power from similar
output transistors when operated at the same V line
voltage in a push–pull rather than in a single-ended
layout is the major advantage of this arrangement

Switching Amplifiers
 Conventional (audio-) amplifiers are class A or class AB
amplifier
 These amplifiers operate their output devices in the
analogue domain
 This means the resistance of the devices is controlled
directly by the strength of the music signal
 As a result, the devices are neither fully 'on' nor fully 'off';
effectively they are variable resistors
 The simultaneous voltage across- and the current through
the devices in this mode results in dissipation in the power
stage of the amplifier and therefore a low efficiency

Switching Amplifiers
 Switching or class-D amplifiers operate the output
devices as switches which are turned either 'on' or 'off',
making the resistance either zero or infinite
 Operated in this way, the devices are almost lossless
because either the voltage across- or the current
through the device is zero
 Thus the efficiency is high, typically more than 90%
for high- as well as low output power

Power Amplifier Classes: Class D
 In a Class D amplifier, the input signal is compared
with a high frequency triangle wave, resulting in the
generation of a Pulse Width Modulation (PWM) type
signal
 This signal is then applied to a special filter that
removes all the unwanted high frequency by-products
of the PWM stage
 The output of the filter drives the speaker
 Class D amps are (today) most often found in car audio
subwoofer amplifiers

 The major advantage of Class D amplifiers is that they
have the potential for very good efficiency (due to the
fact that the semiconductor devices are ON or OFF in
the power stage, resulting in low power dissipation in
the device as compared to linear amplifier classes)
 One notable disadvantage of Class D amplifiers: they
are fairly complicated and special care is required in
their design (to make them reliable)

 Following is the class D block diagram

Amplifier Grounding
 The grounding system of an amplifier must fulfill
several requirements, among which are:
 1) The definition of a star point as the reference for all
signal voltages
 2) In a stereo amplifier, grounds must be suitably
segregated for good cross talk performance
 - A few inches of wire as a shared ground to the output
terminals will probably dominate the cross talk
behavior

Amplifier Grounding
 3) Unwanted AC currents entering the amplifier on the
signal ground, due to external ground loops, must be
diverted away from the critical signal grounds, that is,
the input ground and the ground for the feedback arm
 - Any voltage difference between these two grounds
appears directly in the output
 4) Charging currents for the power supply unit (PSU)
reservoir capacitors must be kept out of all other
grounds

Amplifier Grounding
 Reservoir capacitor is a capacitor that is used to
smooth the pulsating DC from an AC rectifier

Cross Over Network
 Audio crossover networks are a class of electronic filter
used in audio applications
 Most loud speaker could work in limited portion of the
audio spectrum
 So most hi-fi speaker systems use a combination of
multiple loudspeakers drivers, each catering to a
different frequency band
 Crossovers split the audio signal into separate
frequency bands that can be separately routed to
loudspeakers optimized for those bands

Cross Over Network
 Active crossovers allow drivers covering different
frequency ranges to be powered by separate amplifiers
 Passive crossover simply route the frequencies to their
respective speakers

Cross Over Network
 The capacitor has lower impedance for high
frequencies. It acts to block low frequencies and let
high frequencies through
 The inductor has a lower impedance for low
frequencies. It acts to block high frequencies and let
low frequencies through
 A capacitor and inductor in series act to block both
very high and very low frequencies

Audio terminations: line in/out
 Consumer electronic devices concerned with audio
often have a connector labeled "line in" and/or "line
out“
 Line out provides an audio signal output and line in
receives a signal input
 The signal out or line out remains at a constant level,
regardless of the current setting of the volume control
 The impedance is around 100 Ω, the voltage can reach
2 volts peak-to-peak with levels referenced to -10 dBV
(300 mV) at 10 kΩ,

Audio terminations: line in/out
 This impedance level is much higher than the usual 4 -
8 Ω of a speaker or 32 Ω of headphones, such that a
speaker connected to line out essentially short circuits
the op-amp
 Line in expects the kind of voltage level and impedance
that line out provides
 The line out connector of one device can be connected
with the line in of another
 A line input has a high impedance of around 10 kΩ, as
is often labeled as "Hi-Z" input

Sound amplification

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Sound amplification