2. 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
3. Contents
Switching Amplifiers
Amplifier Grounding
Cross Over Network
Audio terminations: line in/out
Audio terminations: aux in/out
Audio terminations: mic in
4. 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
5. 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
6. Signal Voltage and Impedance
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
7. Signal Voltage and Impedance
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
8. 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
9. Voltage Amplifier Design
For example, for the circuit shown in figure the Q2
transistor is driven by Q1 which provides a very high
impedance
10. Voltage Amplifier Design
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
12. Voltage Amplifier Design
An improved version of this simple two-stage amplifier
circuit is shown in Figure
13. Voltage Amplifier Design
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
14. 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
15. Constant-Current Sources and
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
16. Constant-Current Sources and
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
17. 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
18. 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
19. 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
20. 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
21. 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
22. General Design Considerations
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
23. General Design Considerations
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
24. 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
25. 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
26. 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
27. 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.
28. 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
29. 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
30. 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
31. 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
32. Thermal Dissipation Limits
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
33. Thermal Dissipation Limits
The operating limits imposed by the need to avoid this
failure mechanism are shown in Figure
34. 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
35. Single-Ended Versus Push–Pull
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
36. Single-Ended Versus Push–Pull
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
37. Single-Ended Versus Push–Pull
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
38. 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
39. 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
40. 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
41. Power Amplifier Classes: Class D
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)
43. 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
44. 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
46. 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
47. 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
49. 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
51. 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,
52. 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