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Table of Contents

Acknowledgement:...............................................................................2

Abstract.................................................................................................3

Objective...............................................................................................3

Bipolar Junction Transistor (BJT)........................................................4

States of transistor:............................................................................6

Active region:............................................................................6

Cutoff region..............................................................................6

Saturation region........................................................................6

β & α..................................................................................................6

Common Base Configuration............................................................8

Circuit.................................................................................................10

Calculation..........................................................................................10

DC Analysis..................................................................................10

AC analysis:..................................................................................12

Hybrid-pi Model...........................................................................13

Discussion and conclusion..................................................................16

References:.........................................................................................19

Appendix:...........................................................................................20

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Acknowledgement: I would like to thank my lecturers, Mr, Karunanithi, for the unwavering support

and advices on doing this assignment and valuable time he has dedicated to check

each step of completion of it.

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Abstract

The aim of this assignment is to design and construct an audio amplifier by using a NPN - BJT transistor. In addition, I have briefly explained Bipolar Junction Transistor, their types, application and functions. Moreover, I have attached the data sheet of the chosen transistor in appendix section. Furthermore, I have done both calculations DC and AC analysis for this audio amplifier. Also ,I have designed the circuit in Multisim software.

.

ObjectiveThe objective of this assignment is to design and construct a low cost audio amplification

circuit for use in the budget segment of high fidelity (hifi) sets.

The audio amplifier specifications are as follows:

Input: 200 mVp-p

Output: 20 Vp-p

Phase offset: 0 (non-inverting)

Methodology:

1. Calculate the voltage gain (Av) based from the specification.

2. Design the circuit using AC and DC analysis by using nominal standard resistance values.

3. Ensure that the BJT operates at forward active mode.

4. Verify the design circuit by using Multisim.

5. Construct the circuit in the lab to calculate the voltage gain and to display both input and output waveforms.

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Bipolar Junction Transistor (BJT)

A transistor is a semiconductor device which is made up of three layers. There are two types of BJT transistors: PNP and NPN. PNP is made up of two p-type (positively charge) and one n-type which is in between these two p-type material. For NPN is same theory but the type of layers are vice versa. Since transistor is a three-layer semiconductor material, it also has three terminals. Each terminal is connected to each layer, the middle layer is called Base, and the side layer are called Emitter and Collector (as it is shown in figure 1).

Figure 1: NPN & PNP transistor (fourier.eng.hmc.edu)

From figure above, it is obvious that the sandwiched layer Base is doped between the other outer two layers. The outer layers have widths much greater than the sandwiched layer. Again as it is obvious in each transistor two p-n junctions exist. Furthermore, one of these p-n junctions always operates in forward bias and the other junction operates in reversed bias which depends the voltage is applying on which side.

Figure 2: pnp and npn transistors and their p-n junctions

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Figure3: npn and pnp transistor and direction of current (electronics-tutorials.ws)

In figure above, in npn transistor, the positive side of VCC is connected to the Collector of transistor which is n-type material. As in n-type material the number of free electrons are more than p-type material, these electrons become attracted by the VCC, due to this the depletion region between the p-n junction of collector and base increases. On the other side, the negative side of VEE is connect the Emitter of transistor, in this case negative voltage repels the free electrons in n-type layer, and because of this the depletion region between emitter and base decreases, so the current flow through easily. Same theory is true for pnp transistor.

Moreover, as it is shown in figure 3, in pnp transistor the emitter current is addition of collector current and base current. However in pnp type, the emitter current splits into the collector current and base current.

I E=IC+ I B

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States of transistor:

Figure 4: transistor states (electronic devices and circuit theory)

Active region: In active region, the base –emitter junction is forward biased while the base-collector region is reversed biased. This state is used for amplification function.

Cutoff regionIn this region, both base-emitter junction and base-collector junction are reversed biased.

Saturation regionIn saturation region, both base-emitter junction and base-collector junction are forward bias. Saturation region and cutoff region are used for switching function.

β & αIn DC mode the level of IC and IE due the majority carriers between the two junctions

are related by a quantity called alpha. (Electronic devices and circuit theory)

α=IC

IE

In DC mode the levels of IC and IB is defined by a quantity:

β=IC

I B

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Figure 5: levels of IC and IB

Therefore, as it is obvious from figure 5, range of IB current is in micro ampere, therefore in ideal transistor IC will be equal to IB. Moreover, the value of alpha is always less than one:

I E=IC+ I B

IE is the sum of IB and IC, which clearly shows that IE is always greater than IC.

Figure 6: Changes in beta by collector current (allenhollister.com)

β

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Transistors are mostly used for either amplifying or switching. In operation, transistor as I mentioned before has three states such as: active region, cut off region and saturation region. Normally the switching function of transistor happens in saturation region and cut off region. However the amplification happens in active region of transistor. There are different transistor configurations which each of them has its own features. Such as common emitter, common emitter with resistor, common base, and common collector.

Characteristic Common Base Common Emitter Common Collector

Input impedance Low Medium High

Output impedance Very High High Low

Phase Angle 0o 180o 0o

Voltage Gain High Medium Low

Current Gain Low Medium High

Power Gain Low Very High Medium

Table 1: transistors configurations (electronics-tutorials.ws)

Common Base Configuration

In table 1, we can see that the voltage gain for only common base is high and there is no phase shift. So it exactly matches the requirements of the objective. Before we go for calculation part there are a few points that I should mention.

Figure below is an example of common base configuration

Figure 6: An example of common base configuration circuitry

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In this configuration, base-emitter junction is in forward biased due to the negative voltage applied on emitter terminal. On the other hand, base-collector junction is in reversed biased due to positive voltage applied on collector terminal (positive voltage source attracts the free electrons on n-type layer of npn transistor). Moreover, we can say that across base-emitter junction the resistance is small and on the base-collector the resistance is higher due to reverse biased. On the other hand, collector current has almost same value as emitter current, so produced current across emitter terminal due to the small resistance will be a high value and as I mentioned collector current is almost same as emitter current. Then, the product of collector current and collector resistance will result a high voltage gain. Moreover, the voltage gain depends on the amount of DC power source on the input signal. Also it depends on the internal resistance between base and emitter. It varies with different levels of current through the emitter. (Allaboutcircuits.com). The resistance between emitter and base terminal is called rπ and the voltage across this resistor is Vπ. Later on in hybrid-pi model, I have shown this resistance and voltage.

Furthermore, in AC analysis there are some factors which are important such as Earlier voltage (vA), Trans conductance (gm), Input resistor (rπ), Output resistor (ro).

Early voltage (early effect)

Collector voltage has some effect on collector current – it increases slightly with increases in voltage. This phenomenon is called the “Early Effect” and is modeled as a linear increase in total current with increases in VCE (seas.upenn.edu). Moreover, at the begging the depletion region in layers between base and collector, the width of depletion region between these two layers increases by a greater reverse biased due to voltage across collector and base. Meanwhile, the width of sandwich layer decreases.

Normally Earlier voltage is given in data sheet of transistors. However if it is not given it can be get by two methods:

First one is drawing the IC-VCE Graph and extending the knee point of the base current until all these extended line intersect the VCE axis in negative side.

Figure 7: Earlier voltage graph (physics.csbsju.edu) Second one is, using the following formula (seas.upenn.edu)

I C=I S eV BE

V T (1+V CE

V A)

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Circuit

Figure 8: common base circuit designed in Multisim

In above design the base of transistor is grounded the emitter terminal is connected to AC power source with the value of .0707v (RMS value) in series with a capacitor, and a resistor in series with a DC power source. The collector terminal is connected to a resistor in series with a DC power source and a capacitor which is parallel with the resistor and the DC power source. The output voltage is shown as Vo.

Calculation

DC analysis can be used to design a circuit, by assuming some nominal value of components, we can design the circuit which is suitable for our requirements. Furthermore, I have done the DC and AC analysis of above circuitry to show and prove that the AV is 100.

DC AnalysisIn DC analysis, capacitor become open circuit, as it is shown in figure below:

Figure 9: Circuit under DC analysis

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KVL at BE loop:

V 3−I E RE−V BE=0

4−I E (50 k )−0.7=0

I E=3.710 k

I E=0.37mA

β:

Minimum: 100

Maximum: 300

And measured by DMM: 157

I have chosen 157 which is near by the average value of Beta.

I C=( ββ+1 ) I E

I C=( 157158 )× 0.37

I C=0.3676mA

KVL at CE loop:

V 3−I E RE−V CE−IC RC+V 1=0

4− (0.37 ×10 k )−V CE−0.3676 ×30+20=0

V CE=9.272v

IB:

I C=β × I B

I B=0.3676

157

I B=2.3414μA

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AC analysis: In AC analysis capacitors and DC power sources become short circuit.

Figure 10: Circuit under AC analysis

Output resistance due to early voltage:

I C=I S eV BE

V T (1+V CE

V A)

In order to find IS(saturation current) , I have assumed VCE is equal to zero (saturation region VCE is equal to zero) Therefore ICS:

I CS=V 3+V 2

(( ββ+1 )RE+RC)

I CS=24

(( 157158 )10+30)

I CS=0.601mA

Therefore:

V A=V CE

IC

I S ×eV BE

V T

−1

V A=9.272

0.3676

0.601 ×e0.7

0.026

−1

V A=9.272

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Therefore for output resistance:

rO=V A

I CQ

rO= 9.2720.3676

rO=25.223KΩ

Therefore for internal resistor:

r π=β ×V T

ICQ

r π=157 × 0.026

0.3676

r π=11.104KΩ

Therefore for trans conductance:

gm=ICQ

V T

gm=0.36760.026

gm=14.231

Hybrid-pi Model

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Figure 11: Hybrid-pi model

KCL at output node:

( gm × vπ )+(vo ×( 1rC

+ 1ro ))=0

vo=(−gm × vπ )

( 1rC

+ 1r o )

Therefore, in order to find the voltage amplification, I need find the relation between Vπ and VS from the hybrid-pi model.

In addition, by doing KVL at BE loop:

Noting that VS and voltage across the emitter resistor are in parallel and voltage in parallel is same. Therefore the total voltage of VS and VRE:

V total=V S ×V ℜ

V S+V ℜ

Since VS = VRE

V total=V S ×V S

2 V S

+

_

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V total=V S

2

Therefore:

V π+V total=0

V π+V S

2=0

V π=−V S

2

Therefore by substituting Vπ in equation derivate by KCL at output node:

vo=( gm × vS )

2( 1rC

+ 1ro )

AV =gm

2( 1rC

+ 1ro )

Since

gm= 7.108

VS= 0.2vp-p

rc = 30kΩ

ro = 33.304 kΩ

Therefore:

AV = 14.231

2( 130

+ 125.223 )

AV =97.5

Output wave input wave

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Figure 12: wave form of input and output taken from Multisim, divisions are highlighted

Discussion and conclusion I have done the practical circuitry and tested, I have set the input voltage to 0.2 voltage by Function Generator, also this output was checked with oscilloscope, it was showing sinusoidal signal with a pick to pick voltage of 0.2mV. Furthermore in practical I have used a 2N2222A transistor, two 330µF capacitors, one 10kΩ, and a30 kΩ resistors. Also I have used two DC power source for emitter and collector terminals. Moreover, I have followed the same circuitry which I have done the calculation for it. In addition I have measured the input and output by a digital oscilloscope, I have taken some snap shot of the input and output waves. The amplification was almost 100, the output was almost nearby 20. In below I show the input and output voltage by an oscilloscope which works with a software on computer so I have taken some snap shot of signal:

Input voltage:

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Figure 12: input signal taken from Digital storage oscilloscope

Each division is 100mV. In below I have shown its specification which the oscilloscope provide:

Figure 13: input voltage’s specification

Output voltage:

Figure 13: Output voltage

Each division is set to 5V , therefore the pick to pick of signal is in four divisions, which will be 20V.

Output voltage specification:

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Figure 14: output voltage specification

At the end, the problem which I faced was when I measured the output voltage by DMM, the voltage was varying in range of 5 to 7, and also the waves which were showing in oscilloscope was clipping while I was measuring the voltage.

In conclusion, by this assignment I have learnt more about common base configuration and its feature and important factors that have effect on it. Common base configuration has no phase shift and high voltage gain which is suitable to use for audio amplifier. Furthermore, I have understood the operation of common base configuration. The forward bias on emitter- base junction and reverse bias on collector-base junction due to the DC power sources across them. Moreover, I have learnt more the effect of important factors in AC analysis and amplification such as early voltage, internal resistance trans conductance.

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Practical circuit successfully done

Digital storage oscilloscope to take measure and snap shots were taken of

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References:

Floyd, T.L. 2006, Digital Fundamentals, 9th edition, Pearson Prentice Hall, Singapore.

Wang.R,. 2009,. Bipolar Junction Transistor [online],.Available from: http://fourier.eng.hmc.edu/e84/lectures/ch4/node3.html [Accessed on 9th of April]

Anonymous,. Common Base Amplifier[online],. Available from: http://www.allaboutcircuits.com/vol_3/chpt_4/7.html [Accessed on 9th of April]

Doolittle.A,. Bipolar Junction Transistor [online],. Available from: http://users.ece.gatech.edu/~alan/ECE3040/Lectures/Lecture20BJT%20Small%20Signal%20Model.pdf [accessed on 9th of April]

Storr.W,. 2010,. Bipolar Transistor [online]., available from: http://www.electronics-tutorials.ws/transistor/tran_1.html [Accessed on 10th of April]

Hollister.A,. 2009., Physics, SPICE Parameters & Transistor Models [online]. Available from: http://www.allenhollister.com/allen/files/physics.pdf [accessed on 10th of April]

Anonymous., NPN Characteristics Curves[online],. Available from: http://www.physics.csbsju.edu/trace/NPN.CC.html [Accessed on 10th of April]

Kenneth R. Laker,. 2008. Early Effect and BJT Biasing[online],. Available from: http://www.seas.upenn.edu/~ese319/Lecture_Notes/Lec_4_BJTBias1_08.pdf [Accessed on 12th of April]

Appendix:

DATA SHEET

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