ELE 303

Formal Report

Ispana Tabassum Bishsash

ELE 303: Electronic devices and Applications

Experiment EDA2: “Bipolar Junction Transistor”

FORMAL REPORT

By

Ispana Tabassum Bishsash

Student ID:085393121

Email:ee08u106@elec.qmul.ac.uk1

ELE 303

Formal Report

Ispana Tabassum Bishsash

CONTENTS

ABSTRACT

INTRODUCTION

BACKGROUND THEORY

DESCRIPTION OF THE EXPERIMENT

RESULTS

DISCUSSION

CONCLUSION

REFERENCE

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ABSTRACT:

The aim of the course ELE 303 is to introduce the commonly used semiconductor devices and develop an understanding of their physical basis and application. In lab exercise two we studied the characteristics of bipolar junction transistor common emitter amplifier. This experiment was about testing the characteristics of the BJT in an AC circuit. Firstly the experiment was simulated and then done practically on the circuit board.

The software which is used in the labs is called Electronics Workbench. The Multisim program was used that allows us to design the circuit layout of a vast number of circuits and study their effects through simulation. It was really helpful to provide a good understanding of the electronic circuits and also played an important role in designing its circuit layouts that we were asked to construct.

The hardwares used in the experiment are: Transistor Experiments Board with 2N2222A transistor. Power Supply/signal generator/frequency counter unit Digital multimeter Digital storage oscilloscope Decade resistance box

INTRODUCTION:

An electronic system is any arrangement of electronic components and devices with a defined input and output signal. A system is defined as a set of entities, real or abstract, comprising a whole where each component interacts with or is related to at least one other component and they all serve a common objective. The systems we analysed in our electronics lab 2 were based on BJT’s.

A bipolar (junction) transistor (BJT) is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching

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applications. Bipolar transistors are so named because their operation involves both electrons and holes. Charge flow in a BJT is due to bidirectional diffusion of charge carriers across a junction between two regions of different charge concentrations. This mode of operation is contrasted with unipolar transistors, such as field-effect transistors, in which only one carrier type is involved in charge flow due to drift. By design, most of the BJT collector current is due to the flow of charges injected from a high-concentration emitter into the base where they are minority carriers that diffuse toward the collector, and so BJTs are classified as minority-carrier devices.

A bipolar junction transistor can be connected to form a single stage amplifier in the common emitter configuration shown below using a NPN transistor.

Figure 1:The common emitter amplifier

When this circuit is used as an amplifier, there will be a fall-off in gain at low frequencies due to the input capacitor and to the capacitor CE. There is also a fall-off in gain at high frequencies due to the characteristics of the transistor itself. The variation of gain with frequency will have a characteristic as shown in Figure 2.

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Figure 2:Amplifier frequency response

The frequencies f1 and f2 are known as the upper and lower frequency cut-off frequencies for the amplifier. The bandwidth of the amplifier = f2 - f1 Hz.

Background Theory:The bipolar (point-contact) transistor was invented in December 1947 at the Bell Telephone Laboratories by John Bardeen and Walter Brattain under the direction of William Shockley. The junction version, invented by Shockley in 1948, enjoyed three decades as the device of choice in the design of discrete and integrated circuits. Nowadays, the use of the BJT has declined in favour of CMOS technology in the design of digital integrated circuits.

A transistor is a semiconductor device that uses a small amount of voltage or electrical current to control a larger change in voltage or current. A BJT is type of transistor which has a three-terminal device constructed of doped semiconductor material and may be used in amplifying or switching applications. The three-terminal devices are knows as the collector, emitter and base. Contacts are made to all three regions, the two outer regions are the emitter and collector and the middle region is the base. There are two types of BJT known as PNP and NPN.

The device is called “bipolar” since its operation involves both types of mobile carriers, electrons and holes. In semiconductors, electrons and holes act as charge carriers. The more abundant charge carriers are called majority carriers. In N-type semiconductors they are electrons, while in P-type semiconductors they are holes. The less abundant

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charge carriers are called minority carriers; in N-type semiconductors they are holes, while in P-type semiconductors they are electrons.

A small part of the base-emitter current is carried by the majority carriers and the main current is carried by minority carriers in the base and therefore BJT is classified as minority carrier devices.

A BJT also has a characteristic called the trans-conductance or otherwise known as hFe. This value depends on how the BJT has been doped and remains a constant valuefor a BJT.

The figure below shows PNP and NPN transistors:

Figure 3:NPN and PNP transistors

NPN Transistors:

NPN is one of the two types of bipolar transistors, in which the letters "N" and "P" refer to the majority charge carriers inside the different regions of the transistor. Most bipolar

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transistors used today are NPN, because electron mobility is higher than hole mobility in semiconductors, allowing greater currents and faster operation.

NPN transistors consist of a layer of P-doped semiconductor (the "base") between two N-doped layers. A small current entering the base in common-emitter mode is amplified in the collector output. In other terms, an NPN transistor is "on" when its base is pulled high relative to the emitter.

The arrow in the NPN transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode.

One mnemonic device for identifying the symbol for the NPN transistor is "not pointing in, or 'not pointing, no' "

Figure 4:NPN transistor

PNP Transistor:

The other type of BJT is the PNP with the letters "P" and "N" referring to the

majority charge carriers inside the different regions of the transistor. PNP transistors

consist of a layer of N-doped semiconductor between two layers of P-doped material. A

small current leaving the base in common-emitter mode is amplified in the collector

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output. In other terms, a PNP transistor is "on" when its base is pulled low relative to the

emitter.

The arrow in the PNP transistor symbol is on the emitter leg and points in the direction

of the conventional current flow when the device is in forward active mode.

One mnemonic device for identifying the symbol for the PNP transistor is "pointing

in proudly, or 'pointing in - pah'."

Fig 5: PNP transistor

Description of the Experiment:

Multisim Simulation:

a) As shown in the figure below I drew the schematic in multisim giving it the name bjtamplifier.ms10. I put the input voltage to be 50mVrms,DC offset=0 and frequency=4kHz.

b) I doubled checked the oscilloscope to see its display. I chose suitable timebase and channel settings and ran the simulation. I measured the gain at 4KHz and stopped the simulation.

c) To measure the frequency response of the circuit I selected Simulate > Analyses > AC Analysis. I set the test range from 10 Hz to 1 GHz, the sweep type to decade vertical scale to linear. On the Output tab I selected the output signal that corresponds to my circuit . I clicked on ‘Simulate’ to get the Grapher output. I checked that the axes are labelled correctly and printed it out for the records. I checked the gain 2alternative methods. The 1st method was by calculating the inputand output by looking at the waveform. The input was 2.8 * 50mV = 140mV. The output was 4.4 * 1V = 4.4V thus gain is op/ip = 4.4/0.14 = 31.428

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d) Then I performed the transient analysis by selecting Simulate > Analyses > Transient Analysis. I set start time to zero, the end time to 10ms (0.01s) and the time step to 10μs (1e-005). This is done under ‘More options’ where I ticked ‘set initial time step and specified the value. I set the Output tab as in section c (above). Again, printed out the Grapher result.

e) I repeated b, c and d without the emitter capacitor (C3 in fig. 1) to show what affect this component has.

f) I took all printouts for my records. Yet again I calculated the gain by two alternative methods. The 1st method was by calculating the input and output by looking at the waveform. The input was 2.8 * 50mV = 140mV

The output was 1.9 * 50mV = 95mV thus gain is op/ip = 95/140 = 0.678, which shows the gain has decreased rapidly.

THE EXPERIMENT:

The ELE303 Lab2 board is connected as shown in the above figure. However, the output resistor is not included on the board. For this, decade resistance box set to 1Kl is used.

Bias condition check

Before an input signal is applied the bias circuit is checked for the transistor bymeasuring and noting the d.c. value of VCE. Use the DMM and touch the test leads against the two 470 ohm resistors i.e. the bottom of the collector resistor and the top of the emitter resistor. The value of VCE should be around 6V. Also VBE in is measured and noted in a similar fashion. Setting up the instruments and measuring gain The signal generator is connected between the input and ground. The output of the signal generator is set at 4 kHz (use the frequency counter to monitor the output frequency) and around 100 mV peak to peak (i.e. the same as that used in the simulation. Note: The 20dB attenuation setting on

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the signal generator has to be used by pulling out the voltage control knob.). Channel A of the oscilloscope is connected to the input signal and channel B across the load resistor. The output voltage is measured and noted and the gain of the circuit is calculated under these conditions.

Effect of load variation

The value of RL is varied between 500 _ and 2 K_ and the voltage gain (vout/vin) is plotted at 4 kHz against the total load resistance RLeff (RLeff = RL in parallel with RC). The results are plotted onto a graph paper. Also the phase shift between input and output voltages are observed and noted.

RL(ohms) Vi(mV) Vo(V) Gain(Vo/Vi) Phase shift (degrees)

500 134 2.12 15.82 86.4700 134 2.32 17.31 87.2900 134 2.52 18.81 87.81100 134 2.56 19.10 88.51300 134 2.68 20 89.21500 134 2.76 20.597 90.121700 134 2.84 21.19 92.31900 134 2.88 21.49 03.62000 134 2.96 22.09 100.8

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As can be seen from the graph,in this configuration,RL increases and so is the gain.

The graph shows that RL increases and so is the phase between input and output signal, which means that there is no more phase between input and output signal.

Frequency response

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The input and output voltages are measured for frequencies from 10 Hz to 1MHz with RL set at 1 k _ and hence the voltage gain is plotted against frequency over this frequency range on 5-cycle loglinear graph paper.

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The log linear graph shows frequency on the x axis and gain on the y axis.Effect of CE

The emitter capacitor is removed from the circuit and the frequency response plot is repeated.

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DISCUSSION:

In this experiment we observed the behaviour of voltage amplifier/common emitter amplifier circuit.In this circuit the common amplifier is the node of the transistor which is both the input and the output. The base node acts as input while the collector node acts as output. As you can see from the diagram of common emitter amplifier in my method, that the base is of the transistor is being shared between the load and the signal source.

Common-emitter amplifiers generally have very high gain , which we know can vary from transistor to transistor because of there different configurations. As it is very sensitive to the temperature and bias current , so that the gain is very gets very unpredictable. Now let concentrate on the experiment we performed, as it is clear from the diagram of circuit that we have three capacitors in the circuit. One is at collector side; one is at base side, while the last one is on emitter side. The two capacitors which are associated to the base and collector are called D.C

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blocking capacitors. These capacitors play vital role in order to remove any kind of distortion in resulting signal. This is done by blocking any current to flow towards the signal source. Because if the current flows towards the signal source, there will be lots of noise in the input signal and hence the output will be inaccurate.

As we know the gain of the common emitter amplifier is:

From the circuit, I observed that when there is a capacitor connected to the emitter side of the transistor and other case without the capacitor. Now let see the circuitry with the presence of the capacitor C3. The input voltage coming in the base is a potential divider between R2 and R1, while the base-emitter voltage is 0.7. So the bas voltage of the transistor is:

When the signal is injected in the base it goes through the emitter and automatically it takes the route which involves capacitor. So that signal goes to the ground straight away. This means there is high voltage on the emitter to collector path which gives us a high gain at the output. This is because the output is quite as same as the input voltage, because it is not been used by the resistor R4.

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I worked out the mid-frequency voltage gain by using cut off frequency concept.Which is the frequency when the original amount of gain deviates within 3 db or0.707. And these two points were found to be f2 and f1.

CONCLUSION:

From the experiment we learned a lot about the bipolar transistors and their characteristics,its effect on load variation,response to change in frequency and the effects of adding a capacitor to the circuit.

An increase in load resistance increases the overall voltage gain and also the phase between the input and output signal. If the experiment is carried on with a capacitor in the circuit, increasing the frequency increases the voltage gain initially. In this experiment upto the frequency of 1kHz the voltage gain increases. But it is also seen that doubling this frequency causes the gain to fall.On the other hand if the capacitor is removed, the gain increases but not as much as it did with the capacitor.Also the gain falls slightly when the frequency is increased further than 800Hz.

Due to the bipolar nature of the bipolar junction transistors they can be used for many different applications. They can be used in analogue or digital circuits. The BJT remains a device that excels in some applications, such as discrete circuit

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design, due to the very wide selection of BJT types available, and because of its high transconductance and output resistance compared to MOSFETs. The BJT is also the choice for demanding analog circuits, especially for very-high-frequency applications, such as radio-frequency circuits for wireless systems. Bipolar transistors can be combined with MOSFETs in an integrated circuit by using a BiCMOS process to create innovative circuits that take advantage of the application strengths of both types of transistor.

REFERENCES:

[1] ELE 303 lecture notes and lab sheets

[2] Semiconductor Devices: Basic principles by J. Singh J

[3] http://en.wikipedia.org

[4] http://encyclobeamia.solarbotics.net/articles/bip_junct_trans.

Html

[5] http://www.answers.com/topic/operational-amplifier

[6] http://users.tpg.com.au/ldbutler/TransisVoltAmp.htm

[7] http://www.allaboutcircuits.com/vol_3/chpt_4/7.html

[8] http://www.freepatentsonline.com/4021746.html

[9] http://en.wikipedia.org/wiki/Voltage_bias

[10] http://books.google.co.uk/books?hl=en&id=tisR3uZr1IgC&dq=

Electronic+devices+and+Applications&printsec=frontcover&source=

web&ots=bA-8jKbyFe&sig=1yyhJiIgRBXkiHb-qwh2YzLp7cQ&sa=

X&oi=book_result&resnum=3&ct=result#PPR7,M1

[11] http://en.wikipedia.org/wiki/Bipolar_junction_transistor#NPN

[12] http://en.wikipedia.org/wiki/Operational_amplifier

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