DEPARTMENT OF BIOMEDICAL ENGINEERINGFACULTY OF ENGINEERING

UNIVERSITY MALAYA

Lab 9 : Transistor

Objective1. To learn about the parameters of the transistor.2. To determine the characteristics of the transistor.

IntroductionTransistor is a semiconductor which has abilities to amplify, control and generate signals. It is a device with three terminals/legs, which are base(B), collector(C), and emitter(E). There are two types of transistors, which are bipolar junction transistors(BJTs) and field-effect transistors(FETs). The bipolar junction transistors are divides into NPN and PNP type. The letters are referred to the type of the semiconductor used to make each layer. N are the element from Group 15 in the Periodic Table which are electrons rich group while the P are the element from Group 13 in the Periodic element which are electrons deficient group. The most common materials used for making the layers of transistors are silicon, gallium-arsenide, and germanium, into which the impurities had been introduced by a process called “doping process”. The N-type transistor are the device which having excess of electrons while for P-type transistor, it is having excess of holes of deficiency of electrons.

There is a fundamental fact within the transistor. For a proper operational transistor, its input is normally forward biased while its output is reversed biased. The transistors are usually found embedded in the Integrated Circuit (IC) as it can carry out various functions.

The figure shows the symbol of the transistor.

Equipments1. 2 Power supplies2. 2 Digital Multimeter (DMM)

Materials1. 1 BC141(T039) Transistor2. 1 47kΩ resistor3. 1 1.2kΩ resistor4. Connecting wires/Jumpers

Procedure

Figure 9.1 Transistor Test Circuit1. The circuit is connected as shown in the Figure 9.1. The power supplies VCC and

VBB should be at zero before turning on the power supply.2. The power supply VCC is turned on but the VBB remains off. The VCC are gradually

being increased its output until VCE reads 3V. The values of IC are being taken are recorded. The voltage being measured is VCE and not VCC.

3. The values of VCC are continually being increased until VCE reads values of 5V, 10V, 15V and 20V. In every case, the values of the IC are being taken and recorded.

4. The power supply of VCC is turned to zero while VBB are being turned on. Its values are increased until the values of IB read 10μA.

5. The values of VCC are being adjusted so the VCE reads 3V, 5V, 10V, 15V, and 20V. In every case, the values of IC are measured in the same time the IB should be kept constant at 10μA.

6. Before repeat step 5, the IB is increased to 20μA.7. The values of IB are now increased to 30μA, 40μA, 50μA, 60μA, and 70μA. For

every values of IB, the step 5 is repeated as to measure the values of IB.8. The results obtained are recorded into Table 1. A graph of IC versus VCE is plotted

on a graph paper.9. By referring the graph that had been drawn, a “load line” are drawn in the graph

by select an appropriate point at the center of the graph and then draw a straight line from VCC=20v, pass through the selected center point and the line are extended to the x-axis. The transistor parameter defining this condition are:a) IBQ for the quiescent base currentb) ICQ for the quiescent collector currentc) VCEQ for the quiescent collector-emitter voltageThese values are read and recorded in the Table 2. These parameters are important as it enable the calculation and set the transistor biasing necessary for linear operation.

47kΩ

1.2kΩ

10. Alpha(α) and Beta(β) are two parameters that used to defining the DC current gain of the transistor for the common-emitter configurations. From the collector characteristics curves, Beta can be determined at the operating point by :

β=IC/IB

11. To calculate the amplifier performance of the transistor, the transistor h-performance is arbitrarily defined values. They are derived from the collector and base characteristics curve. The parameters are : a) hie - transistor input impedance(Ω)b) hoe – transistor output impedance(Ω)c) hfe – dynamic current gaind) hre – reverse voltage ratio

12. For design purpose, the transistor manufacture normally will provide approximately values. For example,a) hie≈800 to 1200(Ω)b) hoe≈2.5X10-4c) hfe≈50 to 100d) hre≈25 to 40 μA/V

13. From the graph, the differences in values of IC, ΔIC and IB, ΔIB. The value of the hfe

can be calculated. The calculation will be shown and the date will be recorded into the Table 3. The following formula can be used to calculate hfe.

hfe=ΔIC/ΔIB

ResultsTable 1

IB VCE Transistor(mA) IB VCE Transistor(mA)0 3 0.00 40μA 3 5.88

5 0.00 5 5.9810 0.00 10 6.2415 0.00 15 6.4920 0.00 20 6.75

10μA 3 1.13 50μA 3 7.565 1.14 5 7.70

10 1.17 10 8.0515 1.19 15 8.4320 1.22 20 8.83

20μA 3 2.75 60μA 3 9.045 2.79 5 9.23

10 2.87 10 9.7115 2.94 15 10.2020 3.03 20 10.74

30μA 3 4.41 70μA 3 10.535 4.46 5 10.75

10 4.61 10 11.3015 4.77 15 11.9320 4.94 20 12.30

Table 2IBQ(μA) ICQ(mA) VCEQ (V) β

40 6.2 10 155The values of IBQ, ICQ, and VCEQ can be obtained from the graph.The value of B are calculated by the following formula,

β = IC/IB

= 6.2401000

= 6.20.04

= 155Table 3

hfe 160To calculate the value of hfe, the following formula are used,

hfe=ΔIC/ΔIB

ΔIC = 6.20 – 4.60 = 1.60ΔIB = 40 – 30 = 10

hfe = 1.60101000

= 160Graph 1 IC versus VCE

Discussion

For a NPN transistor, the most common circuit configuration is the Common Emitter Amplifier. To understanding the basic operation of a bipolar transistor, we can take a look into VCE-IC curves. This curve is derived from a common-emitter transistor and the characteristics of the common-emitter output of the transistor can be determined. A common-emitter transistor is when the emitter is the common terminal for the input and output. In order to analysis the transistor circuits, the input current, IB and the output voltage, VCE of the transistor are considered as the independent variables. Biasing is very important in amplifier process as it able to establish the correct operating point of a transistor amplifier ready to receive signals. Therefore, it can reduce the distortion of the output signal.

A “DC load line” can be drawn in the curve to show all possible operating points of the transistor from “fully ON” to “fully OFF”. The quiescent operating point and

IC versus VCE

the Q-point(Quiescent point) can be found from the curve. The most important factor to be taken into consideration is the effect of VCE to the IC when VCE is larger than 1.0V. From the curve, we can see that IC is mainly unaffected by changes in VCE. For the common-emitter configuration, we can use this equation, IE=IB + IC.

Based on the graph, we can see that IC is increase rapidly to a saturation level for a constant value of IB and in the same time, the VCE increase from zero. There is a small

amount of collector current flows although IB is equal to zero.

The output characteristics curve of the transistor can be divided into 3 regions.Which are the active region, cut-off region and the saturation region.

The active region is the region where the collector is reverse biased and the emitter is forward biased. The collector current, IC is very sensitive due to the changes of the IB. only a slight change is able to produces a very large change in β and the collector current are changed substantially.

The Cut-off region is the region when IE=0 and IC=ICO. The emitter junction has to

be in a slightly reverse biased condition. For example, 0.1V for germanium and 0V for silicon

.The last region will be the saturation region. Saturation region is the bottom part

of the curve. The voltage is fallen almost to zero. In this configuration, the collector is still in reverse biased as the saturation entered.

Below showing the formula of IC and VCC when VCE and IC are zero respectively.

The value of β is obtained by dividing the IC with IB. β is defined as the parameter of the transistor common-emitter configuration. The value of β can be determined by getting the values from the curves. β is also known as hfe. They have no unit as there are the ratio of the IC and IB.

As we compare the curve with the theoretical graph, we can see that there are deviations. This might because of some errors such as random errors, zero errors and also parallax error. Besides errors, the deviation might be caused by the leakage of the current in the circuit. Therefore, precaution must be taken in order to obtaine accurate data in the experiment. One of it is the electrical elements that used during the experiment had to be double checked as to determine whether the elements are in good conditions to be used in the experiment. Besides that, the digital multimeter plays a very important role in this experiment. We must use the digital multimeter in the correct ways as it is very sensitive and easily fused. To prevent leakage of current, jumpers or connection wires should be avoid using too much as it will create more resistance in the circuit. By the way, the elements should be connect to the breadboard correctly as if connect wrongly then the circuit might will burned. The power supply should be turned off if not in used, and the VCC should be turn to zero before increase the VBB. This is to ensure that data obtained is more accurate.

ConclusionThe parameters of the transistors are learned and the characteristics are determined. Thus, the objectives of the experiment are achieved. From the parameters and the data obtained, we can conclude that transistor is a device that can used for amplification or act as a switch in the electric circuit.

References

1. C.K. Alexander, Matthew N. O. Sadiku, (2007). Fundamental of Electric Circuits 3rd Edition. The McGrow Hill Companies.

2. BC141(T039) Transistor Data Sheet. 1997. SGS-Thomson Microelectronics.3. http://mediatoget.blogspot.com/2011/09/common-emitter-configuration-of.html 4. http://ecelab.com/vce-ic-curves.htm 5. http://www.electronics-tutorials.ws/transistor/tran_2.html