projects 2 - audio amplifier

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CAPE PENINSULA UNIVERSITY OF TECHNOLOGY DEPARTMENT OF ELECTRICAL ENGINEERING PROJECTS 2 Design and implementation of an audio amplifier for a cellphone Student: Kurt Arries Student no: 213254301 Lecturer: Dr. W. Fritz Date: 17/03/2014

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Projects 2 - Audio Amplifier

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March 17, 2014

CAPE PENINSULAUNIVERSITY OF TECHNOLOGY

DEPARTMENT OF ELECTRICAL ENGINEERING

PROJECTS 2

Design and implementation of an audio amplifier for a cellphone

Student:Kurt Arries

Student no:213254301Lecturer:Dr. W. Fritz

Date:17/03/2014

I swear that this is the original work of the author(s). All information obtained directly or indirectly from other sources has been fully acknowledged.

Signed:Dated:

Table of Contents

Introductioon3Statement of the problem4Methodology5Results7Conclusion10References11

IntroductionThe aim of the task is to design and implement a suitable audio amplifier (free of distortion or noise) which should amplify the input signal from the phone and output an amplified signal to the loud speaker.

This document will discuss the design procedure and experimental results of a class A audio amplifier. The technicalities that it will entail are calculations, circuit diagrams and the input/output waveform.

Statement of the problemAn audio amplifier must be designed that will amplify the input signal from a cellphone to a loud speaker as output.

As described by Georgia State University, The task of an audio amplifier is to take a small signal and make it bigger without any changes in it. (Georgia State University)

The following diagram graphically shows the difference between the input signal and the amplified output signal:Figure 1: Audio Amplification Waveforms

The audio frequency band ranges from 20Hz to 20kHz this level falls within the human range for hearing. Frequencies below 20Hz are generally felt more than being heard. There are many low-powered power amplifier circuit types to use when designing it. With that being said, there are certain design parameters that need to be followed as well. They are:

Gain Distortion Frequency response

MethodologyThe amplifier that has been designed is a class A audio amplifier. The loudspeaker used is an 8 31W speaker that can be driven by power as low as 70mW.

The amplifier has been designed to use two stages. The first stage is a voltage-divider biased common emitter stage with a swamping resistor to control the gain of the first stage and thus also the gain of the second stage. The second stage consists of a Darlington voltage-follower configuration. The speaker is thus the load.

The reason that 2 stages are needed is primarily that if only one stage was used, there would be a lot more attenuation instead of gain. Attenuation is a gain less than 1 and is the exact opposite of amplification. The second stage is driven by the first stage and provides the gain needed for an amplified signal. The correct resistor and capacitor values, as well as the correct transistor must be used in the design of this amplifier so that the distortion is eliminated as much as possible.

The following circuit diagram, similar to Example 7-1 (Floyd), was designed and simulated with Multisim 11.0:

Figure 2: Circuit Design

Resistor (R5) is the swamping resistor in the first stage and has been chosen to be 300 to ensure that the speaker is designed to play with as little distortion as possible on maximum volume.

Capacitor (C4) connects the first stage to the second stage. In the simulation, a 2N2222 transistor was used because Multisim didnt have the 2N3503 that was used in the actual circuit. This 2N2222 has a hfe similar to that of the average hfe of the 2N3503.

Q3 is chosen as a different NPN transistor as Q1 in the second stage because it provide more gain and a larger output waveform owing to a higher power gain. However, Q3 is a 2N3904 NPN transistor which can only handle a maximum of 300mA whereas Q1 and Q2 (2N3503) can handle a maximum of 700mA. That is the reason that Q1 is chosen to be at the output of the second stage because it can handle more current than the 2N3904 transistor which would instantly melt.

At the emitter of Q3, a 30 5W resistor has been used since it needs to handle a lot of current and power going through it. As the volume is increased, a higher Alternating Current (AC) is applied. A signal is a variation between voltage and current.

The circuit is designed to operate between 12-15V. However, a 9V battery applied as Vcc to the actual circuit.

ResultsIt is noted that the AC collector resistance of stage 1 (R2) is in parallel with the input resistance to the second stage (R1 and R7).

Rc1 = R2||(R1||R7) = 4.7k||5.6k||22k = 2.29k

Rc1 is the gain of the first stage with the loading of the second stage being taken into account.

Firstly, the following variables must be calculated: VB, IE, re (Q2).

VB: = 1.818V

IE: re: The value of re is then used to calculate the voltage gain of the first stage with the loading of the second stage taken into account.

Av1:

The negative sign in front of the voltage gain of the first stage is for the inversion.Furthermore, the bias resistors that are in parallel with the ac resistance at the base of Q3, is equal to the total input resistance of the first stage.

The total input resistance of the first stage is therefore:

ac(Q3):ac(Q3) = [min hfe = 50; max hfe = 250]= 111.803 Rin(tot):Rin(tot) = R3|| R4|| ac(Q3)(R5+ re(Q3))= 56k|| 10k||111.803(300+ 19.23)= 6.855kSecond StageThe voltage gain of the Darlington pair emitter-follower is approximately equal to 1, thus, the overall voltage gain of the amplifier is the product of the first and second stage voltage gains.

Av(tot):The power gain of the amplifier can be determined as follow:

Ap:= 43609.759

The following input and output waves were observed in the simulation using Multisim 11.0:

Channel A (Input)Channel B (Output)

Figure 3: Amplifier Input / Output waveforms

ConclusionThe amplifier built, provides really good amplification. The input signal is notably amplified so much so that the measured results agree with the calculated results. There is no distortion when the amplifier is turned to maximum volume. This is due to the swamping resistor R5 that has been chosen to be 300 which controls the gain of the first stage and so too, the gain of the amplifier. The amplifier has this been designed to operate at maximum volume with no distortion as long as 12V-15V is applied to the circuit which allows the correct amount of current to flow through the transistors and to the speaker.

References

Davis, U. (2011). Lab 9: Power Amplifiers. Retrieved from Physics Department, University of California: http://nuclear.ucdavis.edu/~cebra/classes/phys116/Lab_09_v2011.pdfFloyd, T. L. (n.d.). Electronic Devices (Conventional Current Version) (Ninth Edition). New Jersey: Pearson.Georgia State University. (n.d.). Audio Amplifiers. Retrieved March 2014, from Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/hph.htmlMetha, R. (n.d.). Design BJT Amplifiers For Given Gain & Impedences. Retrieved May 11, 2013, from Electronics Info Desk: http://electronicsinfodesk.blogspot.com/

8 |Audio Amplifier