Polytechnic of Namibia School of Engineering

Electrical engineering department

Analogue electronics semester project report

Title: A 2 stage audio amplifier

By: Hiskiel Stephanus [201082616]Mentor: Mr KV Munthali

Abstract

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An audio amplifier is an amplifier that is used to amplify the audio input that is inputted into the circuit. Different designs are usually used to make up this amplifier. These are usually ranging from integrated circuit amplifiers to those using discrete components.

This project therefore requested for the students to design an audio amplifier with at least two stages that will be able to amplify audio input to at least be heard within the classroom range. The design was then limited to have an output of 0.5W and a load resistance of about 8Ω. This typical circuit was therefore built by the students using only discrete components and was fully operable.

Acknowledgements

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The students would like to dedicate a special vote of thanks to their lecturer mr KV Muntahli for always being there when they needed help on how to design their project. They would also like to thank him for all the very necessary knowledge he has passed on to them in order to help them theoretically understand their projects. On a very serious note, the students also wish to extend their gratitude to their very qualified store manager and lab technician for always being there when they needed advice on the practicality of the components they have to use in their project. You guys have done a very good job and we see ourselves as very well polished diamonds.

Table of contents

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Content page #

1. Introduction………………………………………………………………………………………………………………………5

2. Arrival upon the design of the project……………………………………………………………………………….5

3. Calculations ………………………………………………………………………………………………………………………7

4. Deviations and the practical values chosen………………………………………………………………………11

5. Operation of the circuit…………………………………………………………………………………………………….12

6. Precautions during operation………………………………………………………………………………………….. 13

7. Conclusions……………………………………………………………………………………………………………………..14

8. References ……………………………………………………………………………………………………………………..15

1.Introduction

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An audio amplifier is a very important device in the advancing world of technology. It is the device that is used to amplify and hence enlarge the input audio signal in order to output the desired output audio signal. The output signal is than supplied to necessary devices that output sound. These devices can be speakers or sometimes loudspeakers. These audio amplifiers are used mostly in entertainment facilities such as clubs,in shows and also in official meetings.

They can also be used in other devices such as cellphones to amplify the music in music players and in large conferences where one may not always hear what the main presenter at the conference is talking about.

In our project, the output of the amplifier was to be outputted by an 8Ω speaker. This implies that the amplifier we are dealing with is an ideal situation of an audio amplifier and larger ones may exist.

The following are therefore considered as aims and objectives of this experiment:

To help the students to understand about the different classes of amplifiers and how the outputs of these amplifiers should be.

It also aims at giving an insight of what the students should expect as the efficiencies of the different classes of amplifiers.

In a more practical sense, the students also got an enhancement on their practical abilities and were able to understand the difference between ideal and practical applications of different components.

The project experiment also aimed at creating a broader understanding in the students so that they know the difference between the calculated values and the usually available practical values of components that can be used in electronics circuits.

Because a transistor was to be used as one of the very basic components in the project, the project hence also aimed at creating understating on how transistors operate and their power dissipation capabilities as one of the basic properties to be considered for power amplifiers.

Overally, the project aimed at exposing the students to the ins and outs of designing a project that happens in the engineering industry where they are going to be employed once they have obtained their qualifications.

2. Arrival upon the design of the project

Before I arrived at my design, I have considered the following underlying facts about audio amplifiers with reference to the provided output conditions.

1. I firstly considered the number of stages that my audio amplifier is going to have and I decided that it is going to have two stages. This was for reasons because; as the stages of an amplifier circuit increase so does the power dissipated by the amplifier circuit. This in a way therefore decreases the efficiency of the amplifier circuit, therefore I opted not to use as many stages.

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2. I than considered the components that I was going to use as amplifying devices and have chosen transistors because transistors usually have high input impedance and they have a high bandwidth and sometimes power conversion efficiency. This are the traits usually needed in a good audio amplifying device and hence the characteristics of a good transistor.

3. I than also thought of the class of my amplifier that I was going to use. I than decided to use a class A amplifier due to its practicality and due to the fact that it has output signal for 360 degrees of the input signal. I also chose it because class B amplifiers which were my next available option has only an output signal for 180 degrees of the input signal and the class A-B amplifier has distortions such as the frequency distortion, amplitude distortion and crossover distortion that may affect my output audio signal even if they have really good efficiencies compared to class A.

4. I than thought for a while and decided to DC isolate the two stages of the audio amplifiers and decided to connect the two stages of the amplifier circuit using a coupling capacitor. This was done to ensure that the DC voltage from the first stage does not affect the biasing of the transistor in the second stage.

5. I than again also considered the coupling method that I am going to use and have decided to use capacitor coupling because this type of coupling does not only couple the signals into the two transistors in the two stages but also blocks the DC voltage (available in the audio signal) from the two transistors as to keep the biasing technique of the two transistors as intact as possible.

6. I also thought of the type of biasing technique that I was going to use to bias the transistors. I have decided to use common emitter biasing because this type of biasing has the following benefits.

This type of biasing produces a gain that is positive The gain produced using this type of biasing is also greater than unity. This type of biasing also produces excellent signal characteristics when operated accordingly. This type of biasing provides pre-amplification of weak input signals and makes them stronger

and therefore finds very useful applications in audio amplifiers.

Comparing this type of biasing with other types of biasing, this type of biasing has more positive and useful characteristics for audio amplifiers and is therefore the one I picked.

7. I than considered the stability of the circuit and hence its ability to withstand oscillation. For this application I have decided the capacitors I use in this experiment be large enough to withstand this effects.

8. I also considered the fact that since the amplifier is an audio amplifier, it must be prone noise and there must be an available mechanism to filter out this noise. I have decided that the coupling capacitor at the input stage will be large enough as to act as a filter capacitor to filter out the noise at 50Hz of the input audio signal.

9. Another important consideration that I thought of was how I was going to switch on and off my circuit and how I was going to vary the volume of my amplification. I then decided to use a digital switch to switch the transistors on and off and I used a relatively small variable resistor to

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vary the volume of my amplified signal. This will be varied to increase and decrease the output signal flowing through it and hence controls the volume.

10. The final consideration was therefore the input DC voltage which was the most difficult decision I have made in this project. I finally decided that I will use Vcc=10V because considering that I will use common emitter biasing I will have to consider the voltage drop across the biasing network resistors which is equal to the voltage driving the base of the transistors to be more than 0.7V( the cut in voltage for a silicon transistor).

With these in mind, I than drew up a picture of an approximate circuit of how my audio amplifier will look like. This is shown in the figure1 below:

Figure 1 [showing an approximate picture of how my circuit design will look like]

After having a clear picture in mind of how my circuit will look like, I then calculated the values of my components.

3. Calculations

When calculating the values of the components one always starts with the output stage. In my amplifier circuit my output stage was stage two and is the stage with transistor T1. Hence there follows my calculations below:

Stage 2

1. Vcc=10V (the reasons for choosing this voltage are explained briefly in the previous sections of this report.

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2. R9=8.2Ω( this is because this resistance must be roughly equal to the load resistance and because there is no 8Ω available in the laboratory shop, I could only pick the 8.2Ω to be my R9)

3. VR9 =40% of the VCC. (This is a standard condition for the operation of a class A amplifier operating at saturation).=0.4×10V=4V

4. IC1(the current through the collector of transistor T1)=Vr9R9

=4V8.2Ω

=0.488A5. IE1(emitter current for transistor T1)= IC1+IB1

But since IC1>>IB1

IE1≈IC1=0.488A6. The power dissipated by the transistor T1=PT1=IC1×VR9

=0.488A×4V=1.95Watts

7. VR8=10% of Vcc (a standard assumption that is taken for a class A transistor at saturation)=0.1×10V=1V

8. R8=VR8IE 1

=1V

0.488 A=2.05Ω

9. |Xc1|=1

C1×2πF where in Namibia F=50Hz

|Xc1|=R8

C1=1

2×π×50Hz×2.05Ω=1553.4µF

10. |Xc2|=1

C2×2πF where in Namibia F=50Hz

|Xc1|=R9

C1=1

2×π×50Hz×8.2Ω=388.17µF

11. Hfe=100 for the transistor I have decided to use, hence IB=Ic 1Hfe

=0.488 A100

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=4.88mA12. IR6=11×IB (This is also a standard assumption for class A amplifiers)

=11×4.88mA=53.68mA

13. IR7=10×IB (This is also a standard assumption for class A amplifiers)=10×4.88mA=48.8mA

14. By kirchoff’s voltage law,VR7=VBE+VR8

=0.7V+1V=1.7V

15. R7=VR7IR7

=1.7V48.8mA

=34.84Ω16. VCC=VR7+VR6

VR6=VCC- VR7

=10V-1.7V=8.3V

17. R6=VR6IR 6

=8.3V

53.68mA=154.6Ω

18. Hence it was time calculate the input resistance to the second stage of the amplifier circuit

RIN1=1

1R7

+ 1R6

+ 1Hie

Where Hie for the transistor T1 is assumed to be 1000.

RIN1=1

134.84Ω

+ 1154.66Ω

+ 11000

=27.65Ω19. R5=RL=8Ω (but because of availability, I have used it to be 8.2Ω)

Stage 1

1. R4=RIN1=24.65Ω2. VR4= 40% of VCC

=0.4×10V=4V

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3. IC2=VR 4R4

=4V

24.65Ω=162.27mA

4. IE2=IC2+IB2

But IC2>>IB2

Hence IE2≈ IC2

=162.27mA5. The power dissipated by transistor T2 is given the equation= IC2×VR4

=4V×162.27mA=0.64908W

6. VR3=10% of VCC

=0.1×10V=1V

7. R3=VR3IE 2

=1V

162.27mA=6.163Ω

8. C4=1

2π ×50Hz×R3

=1

2π ×50Hz×6.613Ω=516.49μF

9. C5=1

2π ×50Hz×RIN 1

=1

2π ×50Hz×2765Ω=115.12μF

10. IB2=IC 2Hfe

Where Hfe=100 for the transistor that I have chosen.

=IB2=162.27mA100

=1.623mA11. IR1=11×IB2

=11×1.623mA=17.85mA

12. IR2=10×IB2

=10×1.623mA=16.23mA

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13. R2=VR2IR 2

=1.7V

16.23mA=104.75Ω

14. VCC=VR2+VR1

VR1= VCC- VR2

=10V-1.7V=8.3V

15. R1=VR1IR1

=8.3V

17.85mA=464.99Ω

16. RIN2=1

1R1

+ 1R2

+ 1Hie

RIN2=1

1464.99

+ 1104.75

+ 11000

=78.76Ω

17. C3=1

2πF×RIN 2

C3=1

2π ×50Hz×78.76Ω=40.4µF

18. C6=1

2π ×50Hz×R5

=1

2π ×50Hz×8.2Ω=388.18µF

4. Deviations and the practical values chosen

The values that I calculated above are theoretical values and may not always be available practically. Considering that our lab store is a very small store and may not always be equipped with all the demanded components, the table below therefore shows the list of calculated values and the corresponding values as advisable by the store manager.

Resistance values

Resistor

Calculated values Values obtained from the store

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R1 464.99Ω 470Ω

R2 104.75Ω 100Ω

R3 6.613Ω 4.7Ω

R4 24.65Ω 20Ω

R5 8Ω 8.2Ω

R6 154.6Ω 160Ω

R7 34.84Ω 30Ω

R8 2.05Ω 1.8Ω

R9 8Ω 8.2Ω

Table1 [calculated and used values of resistances]

Also there were deviations in the values of the capacitors used as shown in the table that follows. Please also note that the input capacitor C3 was made very large as to eliminate noise and did not follow the calculated value.

Capacitor valuesCapacitor Calculated values Values obtained from the storeC1 1553.4 µF 2200 µFC2 388.18 µF 470 µFC3 40.4 µF 2200 µFC4 516.49 µF 680 µFC5 115.12 µF 220 µFC6 388.18 µF 470µFTable 2 [calculated and used capacitance values]

5. Operation of the circuit

The operation of this circuit is explained using the statements below.

This circuit has two stages namely the input stage (stage 1) and the output stage (stage2). All the two stages each has a single transistor (BD243C) that acts as the amplifying device in the circuit.

Each of these transistors is supplied with dc current so that their base emitter junctions will be forward biased. This is supplied as VCC in the circuit.

When the base emitter junctions of the two npn transistors is forward biased,a supply voice signal is than supplied through the microphone or appropriately an audio jack for constant supply of voice signals.( advantages of the audio jack lie in the fact that it allows for power measurements when a constant value of input signal is applied to the circuit.)

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The input signal is than filtered for any DC voltage and noise outside the bandwidth that capacitor C3 is operating. It is than supplied to transistor T2.

Since transistor T2 is forward biased, it will amplify this input signal by a certain constant and the amplified signal is than filtered by the output capacitor of STAGE 1 for any DC components and is than coupled to the base of transistor T1.

The already initially amplified signal from the first stage of the circuit is amplified again by the transistor T1 which is already turned on by the common emitter voltage across its base and is coupled to the output by capacitor C2 which also filters the DC component of this secondly amplified signal out.

The output signal is hence fed into the speaker which converts the electrical energy of the signal into sound energy so that it will be heard by the audience.

To add more functionality to the circuit, a switch is added so that it will be able to cut-off the supply of VCC to the transistors and hence the base emitter junction of the transistors will not be forward biased anymore and hence no amplification takes place and hence no output signal since the transistors will not conduct.

The circuit also has a volume control option whereby a small variable resistor is connected in parallel with resistor R9 in order to control the output signal that reaches the loudspeaker and hence also loudness of the output sound waves.

During the operation of this circuit, a multimeter was also used to measure the values of currents and voltage and hence calculations were made for the power dissipation for the transistors. This are all shown in the table below

value Calculated Measured VR9 4V 3.90VIC1 0.488A 0.402AIE1 0.488A 0.414APT1 1.95W 1.9WIB1 4.88mA 4mAIR6 53.68mA 43mAIR7 48.8mA 38mAVR4 4V 3.81VIC2 162.27mA 190.2mAIE2 162.27mA 192.1mAPT2 0.64908 0.742WIR1 17.85mA 20.9mAIB2 1.623mA 1.9mAIR2 16.23mA 19.04mATable 3 [showing the measured and calculated values of the currents and voltages]

From this values one can hence calculate the overall power conversion efficiency of the amplifier

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Theoretical efficiency=0.5W4.88W ×100%

=10.3%

Practical efficiency=0.5W4W ×100%

=12.5%

6. Precautions during operation

The following were taken as precautions during the operation of the amplifier so that the operation of the circuit will not cause inconveniences.

The current of the power supply was kept to a maximum of 0.4A as to prevent the excess heating up of the resistors and transistors that will cause internal damage of the components.

Due to the fact that transistor T2 is suppose to dissipate large amounts of power, a heat sink was loaded onto this transistor as to conduct away the heat produced due to the power dissipated by this very device.

Short circuits were thoroughly avoided as they cause instant burning of the resistors and other components.

Due to the fact that the students wanted to have a constant voice signal being produced at the output, the students thus used an audio jack that should be connected to a smartphone. Care was thus take an to reduce the noise in the output signal by replacing the calculated value of capacitor C3 by 2200µF.

7. Conclusion Applications of audio amplifiers are very wide with the developing world of today. For example people would want to make life easier for themselves and would not want to talk long speeches loudly using so much effort but would use audio amplifiers to simply amplify their voice and a little whisper will be heard out loudly.Audio amplifiers hence are considered a very effective invention as they make life easier for human beings in almost all walks of life.With focus on the particular one that I have just designed, it has the following advantages due to the chosen method of operation

For this amplifier, the output DC offset is always zero (unless the output capacitor is leaky).

This type of amplifier is very simple to design and understand and may be applied in industries for a variety of useful applications.

Since this amplifier is RC coupled, there will hardly be any need for protection against the DC faults because the filter capacitors will have to do what they do best.

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This makes this type of amplifier cheap compared to DC amplifiers because costs for DC faults protection are cut short.

Through the use of the capacitors used in the coupling of signals, the amplifier is very easy to make short circuit proof.

AC coupled amplifiers circuits also do not require output inductors for stability as they already give good stability output.

Even though the class of amplifier used only has a maximum efficiency of 25%, this amplifier has the best practicability compared to other types of amplifiers and I very suited for applications in audio power amplifiers. The circuit I designed therefore worked according to the requested specifications.

8. References

1. Newnes ; Audio engineering,1st edition; amsterdan ,London.2. Douglas self;audio amplifier design handbook;second edition;oxford, boston.3. Lecture notes by mr Munthali;updated 2012;

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