Designing an audio amplifier

Designing of audio amplifier Academic session 2009/2010 Group 8

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MINI PROJECT

DESIGN OF AUDIO AMPLIFIER

2.0 ABSTRACT Amplifiers, which are devices that increase the gain of an audio signal, dominate modern audio

technologies. In this project, we designed and built our own audio amplifier from scratch in order

to demonstrate that such a key device can be constructed using basic electrical engineering

principles. After performing major circuit calculations by hand, we modeled our circuit in

Multisim, which is computer software that analyzes electrical circuits. In particular, we studied

the variance that using budget electrical components introduced into the circuit overall by

comparing three different amplifiers that we constructed. We found the variance between our

three amplifiers to be minimal, confirming our method of building a low budget, low power

audio amplifier. Human hearing extends from approximately 20 Hz to 20 kHz. Hence, in our

design, the frequency of 1kHz is chosen as it is fallen in this range. For our project, we set out to

design an audio amplifier. The inputs of our circuit were alternative signals from a function

generator. Although we used a low-power speaker, we needed to achieve approximately three

times gain over the entire circuit. In addition, the amplifier had to be produced at a low cost with

available materials. Before building the actual amplifier, we realized that we had to design,

simulate, and test the circuit. Each step was necessary to understand the concepts involved in

amplification.

3.0 INTRODUCTION

An audio signal is a representation of sound waves in a different form. Typically this is an

electrical voltage, but these signals can be expressed through alternative mediums such as

magnetic particles. An audio signal can be manipulated, stored, transmitted and reproduced in

ways that a sound wave cannot. Microphones convert sound pressure waves into voltage, an

electrical audio signal. Therefore the microphone sensitivity is as millivolts per pascal.

Loudspeakers or headphones convert an electrical audio signal into sound. Although many audio

signals have their origin as a sound wave, devices such as synthesisers are designed to create

audio signals.

The term amplifier refers to any device that increases the amplitude of a signal, usually measured

in voltage or current. This versatile device is used in a variety of different electronic applications.

Especially in audio technology, a wide range of amplifiers can be produced based on product

specifications (i.e. power, voltage, current). Currently, there are many types of audio amplifiers

available for consumers. Sound signal amplification is used for instruments, such as the guitar or

the bass. They are also used commonly in home theater systems and with stereo speakers. The

basic design behind all of these amplifiers is derived from the simplest concepts of circuit design.

http://en.wikipedia.org/wiki/Sound

http://en.wikipedia.org/wiki/Voltage

http://en.wikipedia.org/wiki/Microphone

http://en.wikipedia.org/wiki/Sound_pressure

http://en.wikipedia.org/wiki/Voltage

http://en.wikipedia.org/wiki/Loudspeaker

http://en.wikipedia.org/wiki/Headphones


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Audio power amplifiers are, in principle, very simple. There is an input voltage and an output

voltage, and at any instant of time these are both just numbers (with units of volts). If the input

voltage is 1 volt at a certain instant of time and the output voltage is then 20 volts, then the

amplifier has a gain of 20. In an ideal amplifier this number, 20, would be a complete description

of the performance, and the output voltage would always be simply 20 times the input voltage. In

a real amplifier with an input voltage varying with time the output voltage has a maximum level,

and a maximum rate of change, and the gain falls at high frequencies, but within these limits it is

still possible to approach the ideal behaviour.

4.0 BACKGROUND Before beginning the design process, it was necessary to understand several core concepts of

electrical engineering. When designing electronics, three main specifications govern all circuit

components; voltage [V; measured in volts (V)], current [I; measured in amps (A)], and

resistance [R; measured in ohms (Ω)]. These three concepts are connected by Ohm’s Law, where

1V = 1 A * 1 Ω. For amplifier circuits, it is also important to consider both types of current in the

design because both alternating and direct current run through the system. Alternating current

(AC) acts like a sinusoidal curve, providing the signal for the amplifier. On the other hand, direct

current (DC) runs through the circuit as a voltage source. Used together, AC source creates the

signal at the horizontal axis, which is determined by the value of the DC source. Each is

analyzed independently of the other, but without one type of current, it is meaningless to include

the other.

4.1 OUTPUT STAGES

Output stage is to provide the amplifier with a low output resistance so that it can deliver the

output signal to the load without loss of gain. Since the output stage is the final stage of the

amplifier, it usually deals with relatively large signals. Linearity remains a very important

requirement. In fact, a measure of goodness of the design of the output stage is the total

harmonic distortion (THD) it introduces. This is the rms value of the harmonic components of

the output signal, excluding the fundamental, expressed as a percentage of the rms of the

fundamental. A high-fidelity audio power amplifier features a TDH of the order of a fraction of a

percent.

4.2 CIRCUIT COMPONENTS Some basic components in amplification are resistors, capacitors, and transistors. Resistors

produce a voltage based on the amount of current passing through the circuit. Capacitors consist

of two metal plates separated by a weak conducting material. At DC, these devices temporarily

store the charge. However, at AC, the frequency is high enough to complete the circuit. At this

point, the capacitors act like wires. The main advantage of these devices 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 amplifier. Although there are several types of transistors, simple bipolar

junction transistors were acceptable. These devices consist of three terminals: the base, the

collector, and the emitter. Simply put, they are terms used for labeling measurements,

calculations, and schematic diagrams.


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4.3 CLASSES OF AMPLIFIERS

Assembled in different configurations, resistors, capacitors, and transistors can create several

classes of amplifiers that can be distinguished by performance characteristics. Output stages are

classified according to the collector current waveform that results when an input signal is applied.

4.2.1 Class A stage

Class A amplifiers are very linear (meaning the integrity of the signal is maintained through the

amplification process); however, this amplifier topology is known to be very inefficient. In

addition, Class A amplifiers invert the signal (meaning the function is reflected over its axis). A

stage conducts for entire cycle of the input signal; that is, the conduction angle is 360°(never

switch off) . Since Class-A amps are inefficient, generate lots of heat, and require a far more

complex power supply than conventional Class-AB amplifiers, there have to be some compelling

reasons to use this arrangement. The benefit of this is that the signal is subjected to

comparatively little amplification, resulting in an open loop (i.e. without feedback) gain which is

generally fairly low - probably less than 250 (48dB), and possibly as low as 50 or so (34dB).

This means that very little overall feedback is used, so stability and phase should be excellent

over the audio frequencies. A well designed Class-A amplifier should not require any frequency

compensation (or very little), so the open loop gain will remain reasonably constant over the

audio range. This results in superior transient response, and dramatically reduced "Transient

Intermodulation Distortion" (or TID, aka Dynamic Intermodulation Distortion), which is thought

by many designers to be caused by phase and time delays between the input and feedback signals.

It may be possible that this is the cause, although the existence of TID is virtually zero in any

competently designed amp.

Figure 1: Class A

4.2.2 Class AB stage

Class B amps are much less linear, leading to higher distortion of the signal, but they are much

more efficient. Output devices conduct for 180 degrees (1/2 of input cycle).Since a Class B amp

only amplifies half of a signal, two Class B amps are generally used in synchronization. The

compromise between these two topologies is the Class AB amplifier. The Class AB is more

efficient than the Class A with lower distortion than the Class B. It conducts halfway (or partway)

between the above two examples (181 to 200 degrees typical). Often the different types of

amplifiers are used in combination with other amplifiers into order to achieve the specifications

of a particular design.


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Figure 2: Class B

Figure 3: Class AB

4.2.3 Negative Feedback

Another popular method of controlling amplifier distortion is negative feedback. A portion of the

amplifier’s output is transferred back to the input. Overall, this method controls the gain of the

amplifier even when affected by outside factors (i.e.temperature). In addition, the recycled output

signal reduces amplifier distortion. In order to measure the success of an amplifier, designers use

many tests for circuit variables. One manner of representing the performance data is through a

Bode plot. A logarithmic frequency scale spans the x-axis (measured in Hertz). The y-axis

measured gain in decibels, which is also a logarithmic measurement. Combined, the two axes

present the output gain of an amplifier over a wide range of frequencies. After a certain point, the

gain reaches a maximum level. At even higher frequencies, gain becomes inversely related to

frequency as the performance drops off. This continues until the point that the 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 amplifier’s active range of frequencies.

4.2.4 Emitter Degeneration Configuration

Emitter degeneration is even more useful in the CE amplifier. With the exception of gain

reduction, these characteristics represent performance improvements. Indeed, the reduction in

gain is the price paid for obtaining the other performance improvements. In many cases this is a

good bargain; it is the underlying motive for the use of negative feedback. If for some reason the

collector current increase, the emitter current also will increase, resulting in an increased voltage

drop across Re. Thus the emitter voltage rises, and the base-emitter voltage decrease. The latter

effect causes the collector current to decrease, counteracting the initially assumed change, an

indication of the presence of negative feedback. Negative feedback action of gives the name

emitter degeneration resistance.

Rin=(β+1)re + (β+1)

This equation indicates that present of reduces the effect of on increasing Rin.

This is because shunt away some of the current that would have flowed through .

Open circuit voltage gain:

= -


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That is ,the open circuit voltage gain obtained with a relatively small remain very closed to the value

without .

The output resistance is :

= (1+ )

Gm =

EMITTER WITHOUT DEGENERATION

For input resistance,

=

Voltage gain:

AV = - RC

Output resistance of the amplifier:

= RC

Gm =gm(RC||RL||ro)

Practically , <

as =

4.2.5 Common Collector Configuration

For emitter follower configuration, there is no phase inversion and the output is approximately

the same amplitude as the input.

Figure 4: Emitter-follower

Voltage gain

As in all amplifiers, the voltage gain is

. The capacitive reactance is assumed to be

negligible at the frequency of operation. For the emitter-follower, as shown in Figure 4,


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Where is but in Figure 4 =0 and

Notice that the gain is always less than 1.If , then a good approximation is

Since the output voltage is at the emitter, it is in phase with the base voltage, so there is no

inversion from the input to output. Because there is no inversion and because the voltage gain is

approximately 1, the output voltage closely follows the input voltage in both phase and

amplitude; thus the term emitter-follower is commonly used.

Input resistance

The emitter-follower is characterized by a high input resistance; this is what makes it a useful

circuit. Because of the high input resistance, it can be used as a buffer to minimize loading effect

when a circuit is driving a low-resistance load.

Output Resistance

The output resistance is very low, making the emitter-follower useful for driving low-resistance

loads.

Where is the resistance of the input source.

4.2.6 DC Operating Point

DC Bias

Bias establishes the dc operating point (Q-point) for proper linear operation of an amplifier. If an

amplifier is not biased with correct dc voltage on the input and output, it can go into saturation or

cutoff when an input signal is applied.

DC Load Line

The dc operation of the transistor circuit can be described graphically using a dc load line. This is

a straight line drawn on the characteristic curves from the saturation value where on

the y-axis to the cutoff value where on the x-axis.

Linear Operation

The region along the load line including all points between saturation and cutoff is generally

known as the linear region of the transistor’s operation. As long as the transistor is operated in

this region, the output voltage is ideally a linear reproduction of the input.

Waveform Distortion


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Under certain input signal conditions, the location of the Q-point on the load line can cause one

peak of the waveform to be limited or clipped. In the case that when the input signal is too

large for the Q-point location and is driving the transistor into cutoff or saturation during a

portion of the input cycle. When both peaks are limited, the transistor is being driven into both

saturation and cutoff by an excessively large input signal. When only the positive peak is limited,

the transistor is being driven into cutoff region. When only the negative peak is limited, the

transistor is being driven into saturation region.

Figure 5: Graphical load line

illustration of a transistor being

driven into saturation and /or cutoff

4.2.7 Crosstalk Distortion

Crossover distortion is a type of distortion which is caused by switching between devices driving

a load, most often when the devices (such as a transistor) are matched. It is most commonly seen

in complementary, or "push-pull", Class-B amplifier stages, although it is occasionally seen in

other types of circuits as well.The term crossover signifies the "crossing over" of the signal

between devices, in this case, from the upper transistor to the lower and vice-versa. The term is

not related to the audio crossover—a filtering circuit which divides an audio signal into

frequency bands.

Distortion mechanism

The image shows a typical class-B emitter-follower complementary output stage. Under no

signal conditions, the output is exactly mid-way between the supplies (i.e., at 0 V). When this is

the case, the base-emitter bias of both the transistors is zero, so they are in the cut-off region

where the transistors are not conducting.

Consider a positive going swing: As long as the input is less than the required forward VBE drop

(≈ 0.65 V) of the upper NPN transistor, it will remain off or conduct very little - this is the same

as a diode operation as far as the base circuit is concerned, and the output voltage does not

follow the input (the lower PNP transistor is still off because its base-emitter diode is being

reverse biased by the positive going input). The same applies for the lower transistor but for a

negative going input. Thus, between about ±0.65 V of input, the output voltage is not a true

replica or amplified version of the input, and we can see that as a "kink" in the output waveform

near 0 V (or where one transistor stops conducting and the other starts). This kink is the most

http://en.wikipedia.org/wiki/Distortion

http://en.wikipedia.org/wiki/Transistor

http://en.wikipedia.org/wiki/Class_B_amplifier#Class_B_and_AB

http://en.wikipedia.org/wiki/Audio_crossover

http://en.wikipedia.org/wiki/Common_collector

http://en.wikipedia.org/wiki/Transistor

http://en.wikipedia.org/wiki/Diode


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pronounced form of crossover distortion, and it becomes more evident and intrusive when the

output voltage swing is reduced.

Less pronounced forms of distortion may be observed in this circuit as well. An emitter-follower

will have a voltage gain of just under 1. In the circuit shown, the NPN emitter-follower and the

PNP emitter-follower will generally have very slightly different voltage gains, leading to slightly

different gains above and below ground. Other more subtle forms of crossover distortion,

stemming from slight differences between the PNP and NPN devices, exist as well.

5.0 METHODOLOGY

5.1 DESIGN REQUIREMENT

From the very beginning of the design process, the design specifications were crucial to the

choices for topologies and components. Our amplifier had to be able to amplify a signal from a

portable music player (a 0.8V – 1.1V supply voltage load). In order to reach satisfactory

amplitude, 1.7 times gain was necessary for each section of the input stage. Class A designs are

capable of this gain, and their high inefficiency was not a major factor in the small scale of our

experiment. However, these amplifiers invert the input signal. The DC voltage remains the same,

but the AC signal reflects over its x-axis. As a result, two Class A amps were used to correctly

orient the output signal and provide the necessary gain. Each Class A amplifier was a common

emitter BJT. Together these two amplifiers constitute the input stage of the audio amplifier.

Considering a starting voltage of around 1VPP (1 volt peak to peak) , two amplifiers with

approximately 1.7 times gain brought the output gain up to around 3 times after the input stage.

As the current flows into the output stage, the voltage becomes irrelevant. Instead, the designer

needs to increase the power gain in order to drive the speakers at the output. Class AB amplifiers

are capable of producing power gain (at the slight expense of the previous voltage gain). A Class

AB amplifier consists of two PNP common emitters and two NPN common emitters in a loop.

Crossover distortion

http://en.wikipedia.org/wiki/File:Crossover_distortion.png


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5.2 THEORETICAL DESIGN

The First Stage

Once we developed the general design, we created a schematic of our actual circuit. The base of

the first common emitter BJT receives the AC signal input. At this point, two resistors provide

the DC voltage. The output of the first amplifier, at the collector, is connected through a negative

feedback loop. Two resistors and a capacitor at the emitter ground the circuit. From the layout of

the schematic the resistor values can be chosen to match the specifications of voltage and current.

At the first stage, the required voltage going into the base was 3V in order to get the optimum

biasing. The parallel resistor values at this point have to be in a 3:1 ratio going from the DC

source to ground, giving us the 15k and 5k values at (refers to the first resistor at the base of

the first transistor)and (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 0.7V

between the 3V input at the base and the emitter. Since the emitter goes to ground, the voltage

drop between the emitter and ground becomes 2.3V.

The current (IC) in this equation is dependent on the circuit and Vt is provided by the

manufacturer. Solving this equation for a gain of about 1.7 will produce the resistor value for the

CLASS A

AMPLIFIER

CLASS A

AMPLIFIER

CLASS A

AMPLIFIER

CLASS A

AMPLIFIER

CLASS A

AMPLIFIER

CLASS AB

AMPLIFIER


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first resistor on the emitter. From there, the other resistors can be determined using the V=IR

equation. The entire process of determining resistor values is what constitutes DC biasing.

The value of must be fixed in the range of plus minus 6V in order for the output waveform of

stage one to fall in the centre of the DC load line to avoid any distortion on the cutoff or

saturation region. The resistor chosen must be big enough in order to let most of the to flow to

the second transistor. In this case, resistor value is chosen to be 4.7kΩ as it is suitable to

provide gain for the first stage transistor and avoid large loading effect. Resistor value that is

greater than 4.7kΩ will not provide any positive amplification but negative amplification.

All of the capacitors in the first stage are equal to 220μF (microfarads), except for the capacitor

involved with the negative feedback, which is 1000pF. These values were chosen because they

are large enough to be completely ignored at DC and they quickly become ‘shorts’ at AC.

The Second Stage

The second stage of our amplifier was very similar to the first stage of the circuit. Since they

were both Class A amplifiers in a common emitter configuration, this schematic looks very

similar to the first half of the input stage. The largest difference in the design is the lack of a

negative feedback loop. For this section of the circuit, the ideal voltage entering the base of the

transistor was 3V. The current increases from 1μamp in the first Class A amplifier to 5μA in the

second. Even though this will result in different resistor values, the equations involved in the

calculations are the same in both instances.

is fixed at 6V for the same reason as previous. However, the value of is chosen to be 1kΩ

in order to take into consideration tolerance in the circuit.

The Output Stage

The original design for our amplifier called for two Class A common emitters followed by a

Class AB output stage. Since the output stage of our amplifier consisted of an entirely different

amplifier, the schematic diagram of the final stage bears only a faint resemblance to the input

stage design. The resistors connected to the input bases of the output stages are arranged in a

parallel circuit as seen in the first and second stages of the amplifier, but the rest of the Class AB

follows another design entirely. Instead of a single transistor, it consists of four transistors

connected in a loop. In the diagram above, the top-left and bottom-right transistors are PNP

transistors. The other two are NPN transistors. All the resistor 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. The resistor value of both must set to be the same in order to force the Q-

point in the central point. The resistor value of both must also set to be the same for

the same purpose.


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After the entire circuit was biased, we discovered that certain resistors required for our circuit

were either not manufactured values or not available to us. We were forced to make some

changes to the circuit, replacing the unavailable resistors with others that were close to our

calculated values.

Theoretical Resistor Values Actual Resistor Values

5k 4.7k

180 200

1.1k 1.2k

280 270

The adjusted circuit design needed to be biased again in order to account for the changes in

resistor, values. From the second bias, we obtained our ideal bias points for the circuit involving

our actual components.


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5.3 SIMULATION AND ANALYSIS

5.3.1 Analysis of stage 1 and stage 2

(a) R1 = R6 = 15 k, R2 = R3 = R7 = 4.7 k


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(b) R1 = R6 = 10 k, R2 = R3 = R7 = 3.9 k


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(c) R1 = R6 = 15 k, R2 = R3 = R7 = 5.1 k


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(d) R1 = R6 = 10 k, R2 = R3 = R7 = 3.3 k


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After analysis of some values of resistors by simulation, we found that the 1st and the 3

rd

combination will give higher gain than others. Since these two combinations give the slightly

difference gain, we choose the 1st combination as our circuit design. Though the 1

st combination

have lower gain than the 3rd

combination, but the 1st combination has more lower resistance (it is

only 4.7 k), so the low power dissipation make the device works more efficiency.

5.3.2 Analysis of stage 3

We have done some analysis on the R13 and R14 to choose the best value in order to make the

circuit run effectively. The optimum quiescent point in our circuit is 6V. From the table above,

we can notice that all resistors give the values near to the optimum quiescent point. So, we check

the output voltage to obtain the maximum gain as possible. Hence, we choose the first five

resistors to be tested at the next analysis.

NO R13 = R14 / ohm VB3 = VB4 VE3 VE4 Vout3

1. 51 5.970 6.813 5.157 475.3m

2. 68 5.975 6.804 5.177 471.3m

3. 82 5.978 6.799 5.187 470.1m

4. 100 5.981 6.795 5.199 467.2m

5. 200 5.989 6.778 5.232 455.3m

6. 510 5.994 6.755 5.267 433.2m

7. 1k 5.996 6.737 5.288 417.9m

8. 5.1k 5.998 6.695 5.336 370.3m

NO R13 = R14 / ohm Vout(all)

1. 51 0.970

2. 68 1.030

3. 82 1.050

4. 100 1.068

5. 200 1.088


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When the resistor above is connected to the whole audio amplifier circuit, they give the different

output voltage. From the data obtained, we found that resistor 200 ohm is the best value of

resistor to get the maximum output voltage and gain.

6.0 RESULT AND ANALYSIS

6.1 SIMULATION

6.1.1 Overall result

Circuit Schematic


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Input voltage = 0.5 Vpk

Output voltage = 1.088 Vpk

Av =

=

= 2.176

Phase shift, = 00

6.1.2 OUTPUT VALUE AT FIRST STAGE WHEN ONLY STAGE 1 IS CONNECTED ( CLASS A )


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Av =

=

= 3.772

Phase shift, = 1800

6.1.3 OUTPUT VALUE AT SECOND STAGE WHEN ONLY STAGE 2 IS CONNECTED (CLASS A )


21



Av =

=

= 4.738

Phase shift, = 1800

6.1.4 OUTPUT VALUE AT SECOND STAGE WHEN BOTH STAGE 1 AND STAGE 2 ARE

CONNECTED


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Av =

=

= 7.244

Phase shift, = 00

6.1.5 OUTPUT VALUE AT THIRD STAGE WHEN ONLY STAGE 3 IS CONNECTED ( CLASS AB )


23


Output voltage = 451.461 mVpk

Av =

=

= 0.903

Phase shift, = 00


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6.2 PRACTICAL RESULT

Picture 1: Complete Circuit

Picture 2: Output waveform of first stage when only first stage is connected


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Picture 2: Output waveform of second stage when only second stage is connected

Picture 3: Output waveform of second stage when only first and second stage are connected


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Picture 4: Output waveform of third stage when only third stage is connected

Picture 5: Output waveform of first stage when all three stages are connected


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Picture 6: Output waveform of second stage when all three stages are connected

Picture 7: Output waveform of final stage when all three stages are connected


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Picture 8: power spectrum

1. Output value of first stage when only first stage is connected,

Phase shift, = 180°

2. Output value of second stage when only second stage is connected


3. Output value at second stage when both first and second stage are connected

Phase shift, = 0°/360°


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4. Output waveform at third stage when only third stage is connected


5. Output waveform at the first stage when all stages are connected


6. Output waveform at the second stage when all stages are connected


7. Output waveform at the final stage when all stages are connected


8. Cutoff input voltage for the overall circuit =0.68V


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9. Table for potting the power spectrum

F (HZ) I (mA) V(V) P(W)

50 50 0.4 0.02

100 90 0.7 0.063

150 110 0.85 0.0935

200 115 0.90 0.1035

250 120 0.90 0.108

300 125 1.00 0.125

400 130 1.00 0.13

500 130 1.00 0.13

600 130 1.00 0.13

700 132.5 1.00 0.1325

800 132.5 1.00 0.1325

900 132.5 1.00 0.1325

1K 132.5 1.00 0.1325

1.5K 140.0 1.00 0.14

2K 140.0 1.00 0.14


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6.3 ANALYSIS

6.3.1. Comparison between the practical, theoretical and simulation result for DC bias point:

Voltage Simulation

Result(V)

Practical

Result(V)

∆V

2.80 2.80 0.00 0.00%

2.10 2.20 0.10 4.76%

7.76 7.80 0.04 0.52%

2.77 2.80 0.03 1.08%

2.07 2.00 0.07 3.38%

7.63 7.80 0.17 2.23%

5.98 6.00 0.02 0.33%

6.77 6.70 0.07 1.03%

5.98 6.00 0.02 0.33%

5.22 5.30 0.08 1.53%

6.01 6.40 0.39 6.49%


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6.3.2. Comparison between actual and practical value for the component used

COMPONENTS ACTUAL

VALUE

PRACTICAL

VALUE

(Value)

R1 15K 14.91K 0.09 0.6m%

R2 4.7K 4.639K 0.061 1.3m%

R3 4.7K 4.633K 0.067 1.43m%

R4 1.2K 1.171K 0.029 2.42m%

R5 1.2K 1.169K 0.031 2.58m%

R6 15K 14.87K 0.13 0.87m%

R7 4.7K 4.639K 0.061 1.3m%

R8 1K 977.3 22.7 2.27%

R9 200 198.4 1.6 0.8%

R10 270 268.65 1.35 0.5%

R11 1K 987.3 12.7 1.27%

R12 1K 984.7 15.3 1.53%

R13 200 194.42 5.58 2.79%

R14 200 199.72 0.28 0.14%

C1 220uF 164.6uF 55.4uF 25.18%

C2 1000pF 987.24uF 12.76uF 1.276%

C3 220uF 184.6uF 35.4uF 16.1%

C4 220uF 169.3uF 50.7uF 23.05%

C5 220uF 170.4uF 49.6uF 22.55%

C6 220uF 166.9uF 53.1uF 24.14%

C7 220uF 168.4uF 51.6uF 23.45%


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6.3.3 Comparison between actual and practical value for the component used

VOLTAGE GAIN SIMULATION

RESULT PRACTICAL RESULT

(only first stage is

connected)

3.772 3.6 4.56%

(only second stage is

connected)

4.738 4.6 2.91%

(at second stage when both first and second stage are connected)

7.244 6.8 6.13%

(third stage when only third

stage is connected)

0.903 0.8 11.4%

7.0 DISCUSSION 1. Since the percentage error of practical and theoretical result is less than 5%, our project is

considered success.

2. The problem we encountered throughout the project

(a) The waveform is not exactly what we predicted although the simulation results show that the

circuit works.

(i) This problem can be solved by first checking all the components used to detect the faulty

device.

-The transistor can be checked in the following way:

For NPN BJT

-ve +ve

TERMINAL TERMINAL RESISTANCE

B E 5KΩ

B C 5KΩ

E C ∞


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For NPN BJT, low resistance are detected when the negative terminal of the multimeter are

connected to base terminal and the positive terminal of the multimeter are connected to the

emitter and collector terminal.

For PNP BJT

+ve -ve

TERMINAL TERMINAL RESISTANCE

B E 5kΩ

B C 5KΩ

E C ∞

For PNP BJT, low resistance are detected when the positive terminal of the multimeter are

connected to base and emitter terminal and the negative terminal of the multimeter are connected

to the collector terminal.

-The value of resistors and capacitor can be checked by using the measurement machine

provided in lab

(ii) Secondly, check all the connection to make sure all jumper and devices are properly

connected.

(iii) If the above three method still cannot solve the problem, then must proceed to

troubleshooting the circuit by changing the value of the passive element used.

3. Class A amplifier is more linear if compared to others. It will maintain all the information

carried by the signal which is one of the requirements in designing the audio amplifier. Hence it

is used to provide the amplification in our audio amplifier. However, the output waveform of

class A stage is 180° out of phase if compared to the input signal. Hence, two class A stages are

required in order to provide 0° or 360° out of phase for the output waveform.

4. The efficiency of the class A stage is very low. It conducts for entire cycle of the input signal;

that is, the conduction angle is 360°(never switch off). Hence, if only class A stage is used, the

power consumption will be very high. Hence, a class AB is included to increase the efficiency

and reduce the power consumption.

8.0 CONCLUSION

After completing our mini project, we found that all the results obtained are almost the same as

the expected results that had proposed. The design of audio amplifier based on the theory of

analog electronic is really working. Besides, throughout the project, we have encountered many

problems that will enhance our practical skills. Moreover, upon the completion of this project,

we can see how well the practical and theoretical result agreement. This enhances our

understanding towards analog electronic and thus improving the concept. Besides, the skills we


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inculcate in troubleshooting and overcome the problems faced will definitely be beneficial in our

life.

Documents

Designing an audio amplifier