Designing an Audio Amplifier

Document Sample
Designing an Audio Amplifier Powered By Docstoc
					                 Designing an Audio Amplifier
       Milos Cejkov                    Reeve Groman              Kathleen Sindoni     

1. Abstract                                            For our project, we set out to design an
    Amplifiers, which are devices that             audio amplifier. The inputs of our circuit
increase the gain of an audio signal,              were stereo signals from a portable music
dominate modern audio technologies. In             player. Although we used a low-power
this project, we designed and built our own        speaker,      we     needed      to   achieve
audio amplifier from scratch in order to           approximately three times gain over the
demonstrate that such a key device can be          entire circuit. In addition, the amplifier had
constructed      using      basic     electrical   to be produced at a low cost with available
engineering principles.                            materials. Before building the actual
    After     performing      major      circuit   amplifier, we realized that we had to design,
calculations by hand, we modeled our circuit       simulate, and test the circuit. Each step was
in PSpice, which is computer software that         necessary to understand the concepts
analyzes electrical circuits. In particular, we    involved in amplification.
studied the variance that using budget
electrical components introduced into the          3. Background
circuit overall by comparing three different           Before beginning the design process, it
amplifiers that we constructed.                    was necessary to understand several core
    We found the variance between our three        concepts of electrical engineering. When
amplifiers to be minimal, confirming our           designing      electronics,       three    main
method of building a low budget, low power         specifications govern all circuit components;
audio amplifier.                                   voltage [V; measured in volts (V)], current
                                                   [I; measured in amps (A)], and resistance
2. Introduction                                    [R; measured in ohms ( )]. These three
    The term amplifier refers to any device        concepts are connected by Ohm’s Law,
that increases the amplitude of a signal,          where 1V = 1 A * 1 .
usually measured in voltage or current. This           For amplifier circuits, it is also important
versatile device is used in a variety of           to consider both types of current in the
different electronic applications. Especially      design because both alternating and direct
in audio technology, a wide range of               current run through the system. Alternating
amplifiers can be produced based on product        current (AC) acts like a sinusoidal curve,
specifications (i.e. power, voltage, current).     providing the signal for the amplifier. On the
    Currently, there are many types of audio       other hand, direct current (DC) runs through
amplifiers available for consumers. Sound          the circuit as a voltage source. Used
signal amplification is used for instruments,      together, AC source creates the signal at the
such as the guitar or the bass. They are also      horizontal axis, which is determined by the
used commonly in home theater systems and          value of the DC source. Each is analyzed
with stereo speakers. The basic design             independently of the other, but without one
behind all of these amplifiers is derived          type of current, it is meaningless to include
from the simplest concepts of circuit design.      the other.
3.1 Circuit Components                                   integrity of the signal is maintained through
     Some basic components in amplification              the amplification process); however, this
are resistors, capacitors, and transistors.              amplifier topology is known to be very
Resistors produce a voltage based on the                 inefficient. In addition, Class A amplifiers
amount of current passing through the                    invert the signal (meaning the function is
circuit. Capacitors consist of two metal                 reflected over its axis).
plates separated by a weak conducting                        Class B amps are much less linear,
material. At DC, these devices temporarily               leading to higher distortion of the signal, but
store the charge. However, at AC, the                    they are much more efficient. Since a Class
frequency is high enough to complete the                 B amp only amplifies half of a signal, two
circuit. At this point, the capacitors act like          Class B amps are generally used in
wires. The main advantage of these devices               synchronization.
is the ability to block the direct current while
allowing the AC signal to flow through
     Transistors are the most important part
of amplifier circuits. Capable of controlling
an output signal in comparison to an input
signal, a transistor can produce gain. In
other words, the transistor is responsible for
the amplification component of the audio
                                                             Fig. 3.2 – Two Class B amplifiers being used to
amplifier. Although there are several types                  amplify a complete signal
of transistors, simple bipolar junction
transistors were acceptable. These devices                  The compromise between these two
consist of three terminals: the base, the                topologies is the Class AB amplifier. The
collector, and the emitter. Simply put, they             Class AB is more efficient than the Class A
are terms used for labeling measurements,                with lower distortion than the Class B. Often
calculations, and schematic diagrams.                    the different types of amplifiers are used in
                                                         combination with other amplifiers into order
                                                         to achieve the specifications of a particular

                                                         3.3 Negative Feedback
        PNP BJT                    NPN BJT
                                                             Another popular method of controlling
     Fig. 3.1 – Sample schematic representation of a
     bipolar junction transistor in two configurations
                                                         amplifier distortion is negative feedback. A
                                                         portion of the amplifier’s output is
                                                         transferred back to the input. Overall, this
3.2 Classes of Amplifiers                                method controls the gain of the amplifier
    Assembled in different configurations,               even when affected by outside factors (i.e.
resistors, capacitors, and transistors can               temperature). In addition, the recycled
create several classes of amplifiers that can            output signal reduces amplifier distortion.
be      distinguished    by     performance
characteristics. For our research, three                     In order to measure the success of an
economical designs were the most essential.              amplifier, designers use many tests for
Class A amps are very linear (meaning the                circuit   variables.  One    manner    of
representing the performance data is through    with approximately 1.7 times gain brought
a Bode plot. A logarithmic frequency scale      the output gain up to around 3 times after
spans the x-axis (measured in Hertz). The       the input stage. As the current flows into the
y-axis measured gain in decibels, which is      output stage, the voltage becomes irrelevant.
also a logarithmic measurement. Combined,       Instead, the designer needs to increase the
the two axes present the output gain of an      power gain in order to drive the speakers at
amplifier over a wide range of frequencies.     the output. Class AB amplifiers are capable
After a certain point, the gain reaches a       of producing power gain (at the slight
maximum level. At even higher frequencies,      expense of the previous voltage gain). A
gain becomes inversely related to frequency     Class AB amplifier consists of two PNP
as the performance drops off.                   common emitters and two NPN common
    This continues until the point that the     emitters in a loop.
gain drops with an increase in frequency.
This point is known as the 3dB point. For
optimal performance, the 3dB point of an
amplifier should fall beyond the amp’s                           A               A                AB
active range of frequencies.
                                                Fig. 4.3 – Block diagram of the circuit set-up.
4. Methodology
                                                    Audio signals in the modern music
4.1 Design Requirements                         industry are now broadcast almost
    From the very beginning of the design       exclusively in stereo sound. To account for
process, the design specifications were         the dual signal amplification, the entire
crucial to the choices for topologies and       circuit (Fig. 4) was repeated. The final
components. Our amplifier had to be able to     output of the amplifier was fed through a
amplify a signal from a portable music          low power audio speaker, completing the
player (a 0.8V – 1.1V supply voltage load).     circuit. Actual values for circuit components
In order to reach satisfactory amplitude, 1.7   are dependent on the DC based calculations
times gain was necessary for each section of    for our 12V source.
the input stage. Class A designs are capable
of this gain, and their high inefficiency was   4.2 Theoretical Design
not a major factor in the small scale of our        Given the design parameters for the
experiment. However, these amplifiers           amplifier circuit, theoretical values were
invert the input signal. The DC voltage         calculated using a system of DC biasing.
remains the same, but the AC signal reflects    Though several circuit calculations are
over its x-axis. As a result, two Class A       involved, the goal of DC biasing is to
amps were used to correctly orient the          balance the circuit components based on the
output signal and provide the necessary         supply voltage and current at each stage.
gain. Each Class A amplifier was a common       Incorrect resistors would shift the amplifier
emitter BJT. Together these two amplifiers      away the active region of operation.
constitute the input stage of the audio
    Considering a starting voltage of around
1VPP (1 volt peak to peak) , two amplifiers
                                                   base and the emitter. Since the emitter goes
                                                   to ground, the voltage drop between the
                                                   emitter and ground becomes 2.3V. The
                                                   complicated setup of the emitter leads to a
                                                   new equation for determining the resistance.

                                                                          βR L '
                                                                        Rπ + β R E1
                                                                   A = gain
                                                                    β ≈ 400
                                                                    RL ' = the sum of the
                                                                   resistors in parallel
                                                                   with RC1
      Fig. 4.4 – First stage of
      our audio amplifier                                           Rπ = β t

4.1.1 The First Stage
     Once we developed the general design,              The current (IC) in this equation is
we created a schematic of our actual circuit.      dependent on the circuit and Vt is provided
The base of the first common emitter BJT           by the manufacturer. Solving this equation
receives the AC signal input. At this point,       for a gain of about 1.7 will produce the
two resistors provide the DC voltage. The          resistor value for the first resistor on the
output of the first amplifier, at the collector,   emitter. From there, the other resistors can
is connected through a negative feedback           be determined using the V=IR equation. The
loop. Two resistors and a capacitor at the         entire process of determining resistor values
emitter ground the circuit.                        is what constitutes DC biasing.
     From the layout of the schematic the               All of the capacitors in the first stage are
resistor values can be chosen to match the         equal to 100µF (microfarads), except for the
specifications of voltage and current. At the      capacitor involved with the negative
first stage, the required voltage going into       feedback, which is 1000pF. These values
the base was 3V. The parallel resistor values      were chosen because they are large enough
at this point have to be in a 3:1 ratio going      to be completely ignored at DC and they
from the DC source to ground, giving us the        quickly become ‘shorts’ at AC.
15k and 5k values at RB11 (refers to the first
resistor at the base of the first transistor)and
RB12 (refers to the second resistor at the base
of the first transistor
     Within the transistor, the calculations
became more difficult. Ideally, there should
be a drop of .7V between the 3V input at the
                                                                  Fig. 4.6 – Third stage of our audio

       Fig. 4.5 – Second stage of our audio amplifier
                                                        4.1.3 The Output Stage
                                                             The original design for our amplifier
4.1.2 The Second Stage                                  called for two Class A common emitters
    The second stage of our amplifier was               followed by a Class AB output stage. Since
very similar to the first stage of the circuit.         the output stage of our amplifier consisted of
Since they were both Class A amplifiers in a            an entirely different amplifier, the schematic
common       emitter    configuration,     this         diagram of the final stage bears only a faint
schematic looks very similar to the first half          resemblance to the input stage design.
of the input stage. The largest difference in               The resistors connected to the input
the design is the lack of a negative feedback           bases of the output stages are arranged in a
loop.                                                   parallel circuit as seen in the first and second
    For this section of the circuit, the ideal          stages of the amplifier, but the rest of the
voltage entering the base of the transistor             Class AB follows another design entirely.
was 3V. The current increases from 1µamp                Instead of a single transistor, it consists of
in the first Class A amplifier to 5µA in the            four transistors connected in a loop. In the
second. Even though this will result in                 diagram above, the top-left and bottom-right
different resistor values, the equations                transistors are NPN transistors. The other
involved in the calculations are the same in            two are PNP transistors. All the resistor
both instances.                                         values in this circuit can be found by
                                                        manipulation of the equation V=IR. At this
                                                        stage of the amplifier, the current was 20µA.
                                                        Using the V=IR equation and the ideal
                                                        voltage drops, we found the resistor values
                                                        at the DC bias points.
    After the entire circuit was biased, we                     The time-domain simulation in Figure
discovered that certain resistors required for               4.9 shows how the signal changes over time.
our circuit were either not manufactured                     In the first stage of our amplifier, which is
values or not available to us. We were                       class A, the signal is inverted. The signal
forced to make some changes to the circuit,                  was reoriented in the second stage. The last
replacing the unavailable resistors with                     stage, which is a class AB amplifier,
others that were close to our calculated                     maintains the integrity of the signal.
values.                                                         8.0V



      Theoretical                     Actual                   10.0V

     Resistor Values              Resistor Values               7.5V


           5k                          4.7k                    10.0V


           180                          200                    SEL>>

           1.1                         1.2k                      0V

           280                          270                   -500mV
                                                                                                  0.5ms                       1.0ms                          1.5ms           2.0ms              2.5ms                      3.0ms


    Fig. 4.7 – Difference between calculated resistors and   Fig. 4.9 – Time-Domain Simulation (each graph is scaled differently)
    similar available resistors                                First stage Second stage     Output/third stage Input signal

    The adjusted circuit design needed to be                     The dB test graph (Figure 4.10)
biased again in order to account for the                     emphasizes the changes in gain between the
changes in resistor values. From the second                  stages. In the first stage, the output signal
bias, we obtained our ideal bias points for                  displayed significant gain. But the output of
the circuit involving our actual components.                 the second stage presents an even higher
                                                             gain as the signal is amplified again. The
                                                             output of this amplifier has slightly lower
4.3 Computer Simulation                                      gain than the output of the second stage. The
    The computer simulation plays an                         Class AB amplifier used sacrifices minimal
important role in the process of building an                 voltage gain for significant power gain.
audio amplifier. It gives more accurate                      Represented by the gain drop off, the 3dB
results than the theoretical design and it also              point (195.019kHz) falls after our ideal
allows circuit designers to picture the                      range of operation. The human ear can here
changes of the signal through the amplifier                  between 20Hz and 20kHz and our range of
as it passes through different stages. By                    operation from 30Hz to the 30kHz is
performing the computer simulation, we                       acceptable.
could see if the amplifier we designed was
working properly. If it does not produce                        8.0

expected results, changes can be made
before physical components have been used.
                                                                                                                                      First Stage

    After designing the amplifier in OrCad
Capture, a schematic creation software, we                        0

were able to perform the simulations and see
if our amplifier was capable of operating

near theoretical values. In order to perform                      10Hz
                                                                                                                                       1.0KHz            3.0KHz

                                                                                                                                                                     10KHz   30KHz   100KHz          300KHz       1.0MHz

simulations, we decided to use the program                   Fig. 4.10 – dB test
PSpice.                                                         First stage Second stage                                                                 Output/third stage              Input stage
4.4 Lab Work                                      output. We tested for this by inputting a
    Using      available     resistors,   BJTs,   source directly to a non-amplified speaker
capacitors, and wire, we constructed our          and then running the signal through the
amplifier circuits on breadboards, which are      amplifier
devices often used for prototyping circuits.
In this lab setting, we tested the voltages and   5. Results
calculated the actual values for the gain and         In the lab, we used a digital multi-meter
the 3dB point. We powered our amplifier           to find the following voltages at the bases,
with a 12V DC power supply and supplied           collectors, and emitters of each transistor
an AC input signal with a function                contributing to the final gain in each of the
generator. We used a digital multimeter to        three team members’ amplifiers. In Figure
measure the voltage at the base, collector,       5.1, the variables VB1, VE1, VC1 stand for the
and emitters of each of the Class A amps,         voltage at base one, emitter one, collector
and the base and emitters of the Class AB         one and so on. In the final column of Figure
amp. We used an oscilloscope to read the          5.1, we calculated the average of our three
gain at output of each amplifier. Using           values for each voltage.
these values, we measured the 3dB point of
the amplifier. Several problems arose while       Voltages      Reeve         Kate         Miloš       Average
we were building and testing the circuits.
There were several occasions in which the           VB1         2.83V        2.85V         2.83V        2.84V
components were not correctly oriented in
the circuit. The current flowing through            VE1         2.15V        2.17V         2.15V        2.16V
them in the wrong direction overloaded the
                                                    VC1         7.79V        7.75V          7.8V        7.78V
misplaced devices. Burned components
needed to be replaced.                              VB2         2.75V        2.76V         2.75V        2.75V

4.5 Final assembly                                  VE2         2.03V        2.04V         2.04V        2.04V
    With a working physical model of our
amplifier design, we had circuit boards             VC2         7.69V        7.68V         7.70V        7.70V
produced to fit our models. Circuit boards
                                                    VB3         5.97V        5.96V         5.57V        5.97V
are desired because they mount the circuit
components neatly and permanently. To do            VE3         6.57V        6.58V         6.57V        6.57V
this, our design was entered into a circuit
board layout program. From there, our board         VE4         5.32V        5.33V         5.33V        5.33V
went through an extensive manufacturing
process.                                            VE5         5.94V        5.95V         5.95V        5.95V
    Upon receiving the board, we soldered
the components onto the printed circuit               Fig. 5.1 – Actual voltages at different stages
board. After completing the amplifier, we
connected a speaker to the output using an           Figure 5.2 displays a comparison of the
1/8” cable. We sent an audio signal through       theoretical values, the computer simulated
the input using a portable music player           values, and the actual lab tested values.
device. The sound signal from the audio           Note that the theoretical values are based on
player was indeed amplified through the           the actual resistor values used in the
physical model, not the original resistor                                        Reeve           Kate           Miloš             Average
values designed for our amplifier.
                                                                                  1.56          1.562           1.562              1.561
Voltages   Theoretical       Simulated        Actual       ΔV
                                                                                  2.44          2.406           2.063              2.303
  VB1         2.86V             2.84V         2.84V       0.02V   Stage

  VE1         2.16V             2.18V         2.16V        0V     3rd(AB)
                                                                                  2.16          2.188           2.063              2.137
  VC1          7.3V             7.77V         7.78V       0.48V

  VB2         2.86V             2.77V         2.75V       0.11V   x 0.708         1.56          1.549            1.46              1.523

  VE2         2.16V             2.07V         2.04V       0.12V    3dB           140.4          157.6
                                                                                                              188.3kHz            162.1kHz
                                                                  Point           kHz            kHz
  VC2            7V             7.64V        7.69V 0.69V
                                                                           Fig. 5.3 – Gain at different stages in the amplifier
  VB3            6V             6.01V        5.83V 0.17V
                                                                       Figure 5.4 displays the comparison of
  VE3          6.7V             6.57V         6.57V       0.13V    the theoretical, simulated, and actual gain
                                                                   values of each stage of the amplifier.
  VE4          5.3V             5.46V         5.33V       0.03V
                                                                   Stage          Theoretical         Simulated        Actual        ΔGain
  VE5            6V             6.03V         5.95V       0.05V
                                                                                       1.7               1.55           1.56         .14
    Fig. 5.2 – Comparison of theoretical, simulated and           Stage
    actual values at different stages of the amplifier
                                                                                       1.7               1.65           1.56         .14
    Figure 5.3 displays the gain at each stage
of the amp. The first stage is the first class A                   Final
amp, the second stage is the second class A                                             3                2.21           2.14         .86
amp, and the third stage is the class AB
amp. We multiplied the final gain of each                                  Fig. 5.4 – Comparison of theoretical, simulated, and
amp by 0.708 to calculate the gain drop off                                actual gain in each stage
point in order to find the 3dB point. Using                          After conducting our lab tests, we returned
the function generator, we increased                               to the computer simulation and altered the
frequency of the signal of the input until the                     feedback capacitor value at the first stage.
oscilloscope read the gain drop off point we                       We noted that the higher the value of the
had calculated. This frequency is the 3dB                          feedback capacitor the lower the 3dB point,
point, which is where the signal is no longer                      and the lower the value of the feedback
amplified.                                                         capacitor, the higher the 3dB point. Because
                                                                   of this, it can be inferred that the feedback
                                                                   capacitor value and the 3dB point are
                                                                   inversely related.
                                                  helping us in our project and guiding us with
6. Conclusion                                     our research paper and presentation.
    Upon finishing our circuit according to       However, this amazing summer program
the initial specifications, our design was        would have been impossible had it not been
successful in amplifying an audio signal.         for the efforts of the NJ Governor’s School
The measured gain of 2.14 was close to our        of Engineering and Technolgoy, (Donald M.
expected gain of 3. But no electronic device      Brown, Director, and Blase Ur, Program
is perfect. Components and wires are mass         Coordinator), the Rutgers University School
produced, leading to differences between          of Engineering (Dr. Yogesh Jaluria,
“identical”      circuits.     Although     we    Outgoing Interim Dean, and Dr. Thomas
constructed three circuits from the same          Farris, Dean), the NJ Governor's School
design, each amplifier presented slightly         Board of Overseers, Kristin Frank, and the
different values. This is because the rapid       counselors of Governor’s School. We would
production rate of these devices creates          also like to thank our sponsors (Rutgers
flaws in the individual components and            University, the Rutgers University School of
variability in their actual values.               Engineering, the Motorola Foundation,
    In our research, we observed that             Morgan Stanley, PSEG, Silver Line
changing the value of the capacitor in the        Building Products, and the families of 2001-
negative feedback loop alters the 3dB             2008 program alumni) for providing the
inversely. Changing the value of resistors in     funds in order to make this unique summer
the circuit can also alter the gain. In our       program special and enjoyable for the most
endeavors, we had trouble finding the exact       talented students in the State of New Jersey
resistors values we wanted. We had to             and the engineers of the future. Once again,
substitute these with resistors that were close   we would like to thank everyone who made
to our ideal values. Overall, they had an         the 2009 Governor’s School of Engineering
impact on the gain, but it was not enough to      and Technology a truly memorable
prevent the amplifier from operating.             experience.
Though we may have gotten results closer to
our theoretical values had we used the exact      8. References
resistor values, the gain produced was            [1] Singmin, Davis, Patronis, Watkinson,
acceptable for our experiment and proved             Self, Brice, Duncan, Hood, Sinclair.
that we can amplify sound.                           Audio Engineering – Know It All. 2009,
7. Acknowledgments
    First of all, we would like to thank Ilya     [2] Douglas Self. Audio Power Amplifier
Chigirev for teaching us about audio                 Design Handbook Third Edition. 2002,
amplifiers, helping us in our design and also        MA.
taking time off his schedule in order to help
us put this project together. We would also       [3] John Linsley Hood. Audio Electronics.
like to thank our RTA Daniel Hogan for                1999, MA.