transistors 1

Bipolar Junction Transistor CharacteristicsEELE101 Laboratory

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FVCC EELE 101 Transistor Characteristics 1

1 Objective

In this lab we will measure the basic characteristics of an NPN transistor and explore how it operates.

1 Build basic test circuit

2 Load and run LabVIEW VI for transistor curve measurement and Base current input.

3 Download data from LabVIEW and plot in Excel

2 Theory

The importance of transistors in electronics is hard to overstate. While nearly all of us depend on mil-lions,even billions of transistors everyday, few people actually understand how they work. This laboratoryexercise is an introduction to how transistors work. In summary, transistors are current amplifiers. Theyamplify the small controlling current that is input to the base. The idea is to control the flow of currentfrom the collector to the emitter. They can be used in a variety of configurations, but most often theyare used to switch things on and off. TTL stands for transistor-transistor logic where transistors areused in the simple switch mode, creating the ubiquitous 1’s and 0’s used in our digital world. There aretwo types of BJT transistors, NPN and PNP. This discussion will concentrate on the explanation forhow an NPN transistor works.

The story of transistors begins with the concept of a semiconductor. Semiconductors are actuallymade of a base material like silicon which is a poor conductor. But this substance can be doped withan alien element to either supply an excess of electrons (N-type) or and excess of protons (P-type). Tomake an N-type semiconductor, silicon with atomic number 14 having 4 valence electrons, is doped withphosphorus, atomic number 15 having 5 valence electrons. These excess charges can become chargecarriers. ( In the case of the P-type semiconductor, an element with 3 electrons in the valence is addedto the silicon.) So an insulator can be changed into a conductor by doping. The first step in makingan NPN transistor is to heavily dope the semiconductor so that by itself, it would be a conductor. Butinstead of using it like a wire, imagine cutting the N-type semiconductor in half and inserting a P-typesemiconductor in between the halves. We call this the ’base’ of the transistor. The other two ends ofthe transistor are labeled ’collector’ and ’emitter’. Figure 1 illustrates the basic NPN transistor in severalways. The symbol on the left is the schematic representation used for an NPN transistor, the image inthe center is a basic picture showing the P and N materials and the depletion regions established. Theimage on the right is used in the explanation to follow.

With a P-type semiconductor inserted between the N material, two natural depletion regions are es-tablished and current, which would have flowed from collector to emitter, is blocked. Another way to


P-‐type

N-‐type N

P

N

Collector

Base

Emi4er

Natural deple7on regions

Collector

Base

Emi4er

Collector

Base

Emi4er

a b c

Figure 1: NPN Transistor structure and symbol

understand this is to imagine the NPN is two diodes facing in opposite directions. Figure 1 illustratesthe NPN transistor in this opposing diode manor on the far right. As we know from diodes, the arrowindicates the direction for allowed current. In order to get current to flow it is necessary to forwardbias the PN junction. So if the base were positive and the emitter were negative, current would flowfrom base to emitter. You can test a transistor by grounding the emitter and the collector. In this case,illustrated in Figure 2, you can see that current would flow through both junctions. On the other hand,if the collector were more positive than the base, the natural direction of current would be from collectorto base but this would be a reverse-biased PN junction and thus no current flows. This leads to theconcept of forward-reverse DC biasing of a transistor.


P-‐type semiconductor

N-‐type semiconductor

N

P

N

Collector

Base

Emi7er

+ Reduced deple:on region, Base-‐Emi7er Forward Biased

Reduced deple:on region, Base-‐Collector Forward Biased

Current

Figure 2: Testing a Transistor

The transistor is operational when it is in a forward-reverse DC bias. You might think that expres-sion is a logical contradiction but remember that there are 2 PN junctions in transistor. One of thejunctions ( the base-emitter ) is forward biased while the other junction ( the base-collector ) is reversebiased. This single principle is critical to the operation of the transistor. Figure 3 illustrates the circuitneeded to create the forward-reverse bias in an NPN transistor. The transistor is a current amplifier.But the reverse bias on the collector-base junction prevents current from flowing at all. To turn thetransistor on, it is necessary to raise the base voltage, thus its current. This current will flow throughthe base-emitter junction. Additionally, the current through the base provides charge carriers needed forthe collector current to flow. It is this flow of current through the collector, and exiting the emitter thatis useful in circuits. Figure 4 illustrates the transistor in forward-reverse bias for the case when there isno base current, and for when there is a base current. Note that the current from the base appears tobe amplified by the transistor. That is, a small base current turns the transistor on and produces a largecurrent in the collector and consequently the emitter. This laboratory exercise is intended to explorehow a little bit of base current can be amplified by the transistor. These currents are easily captured bysimple mathematics.

There are two current relationships in the transistor. First is the small current amplification from IC ,the collector to IE the emitter. That gain is given in equation 1 and is typically .95 to .99. In practiceit is often simple considered to be 1 such that IC = IE. A much larger gain is found if we look at howmuch collector current is allowed to flow with the small base current. That gain is given in equation 2.


Q12N3904

R2

100!

R1

30000!

V_bb

V_cc

Figure 3: Circuit used to measure characteristics of NPN transistor



N

P

N

Collector

Base

Emi7er

+ Natural deple9on region,

Enhanced deple9on region, Base-‐Collector Reverse Biased Strong E-‐field in the deple9on region

Current

Vbb

+

(a) No base current



N

P

N

Collector

Base

Emi7er

+ Reduced deple:on region, Base-‐emi7er Forward Biased

Enhanced deple:on region, Base-‐Collector Reverse Biased Strong E-‐field in the deple:on region

Current

Vbb

+

(c) Base current on

Figure 4: Illustration of the transistor when it is off and when it is on

It is the βDC , which has values of 20-300, that is referred to when speaking of the gain in a transistor.(On data sheets βDC is often given as hFE) Currents in an NPN transistor flow from the base to theemitter, and from the collector to the emitter. By conservation of charge, the current in the transistor


must follow this simple relationship given in equation 3.

IC = αDCIE (1)

IC = βDCIB (2)

IE = IB + IC (3)

In a circuit the transistor is placed as a control component. That is, it is controlling the flow of currentfrom the collector to the emitter (IC ≈ IE). So we would like to know how the current in the collectorchanges as the voltage difference between the collector and emitter change. The transistor will havea characteristic response when a specific base current is applied. The curve produced by mapping thecurrent response to a change in VCE is called the characteristic curve. A set of characteristic curvescan be generated when multiple base currents are input, and this family of curves is the main point ofthe laboratory exercise. Each base current will have its own characteristic curve. Figure 5 shows andexample of theoretical characteristic curves for various different base currents. You will produce thisgraph for a real transistor in this laboratory exercise.

The second exercise in this lab is to plot the load line, shown in Figure 5. The load line showswhat the current in the collector will be as the base current is changed. It also shows what the voltagedrop is between the collector and emitter. To create this line, you can set the collector voltage andthen vary the base current. This is important information as it allows you to know how to account fortransistors in circuits theoretically. (Allows us to use Kirchhoff’s laws on circuits with transistors)

3 Equipment

The basic equipment for this experiment is as follows:

1 Resistor of about 33[kΩ]

2 Resistor of about 100[Ω]

3 NI ELVIS workstation

4 Computer with LabVIEW installed

5 VI: EELE101 Transistor Lab.vi

6 VI: EELE101 Load Line.vi


Figure 5: Sample characteristic curves

7 Jumper wire set

8 Multimeter for measuring resistance

9 External variable power supply

10 3-NPN Transistors, 2N3906

4 Procedure

To capture the characteristic curves of a transistor, you will build the circuit shown in Figure 3 and runthe experiment with a virtual instrument in LabVIEW.

4.1 Initial Experimental Set-up

1. Select and measure the exact resistors you will be using in the circuit. Record these values.

2. On the National Instruments ELVIS board, build the circuit shown in Figure ??. This circuit iscalled the common emitter configuration since the emitter is attached to ground. In this circuitR1 will limit the current into the base. The external power supply should be used for the power


to the base, Vbb, and the variable power supply on the Elvis unit should be used to apply voltageto the collector of the circuit, Vcc.

3. Start LabVIEW on your computer and make sure the ELVIS unit is connected by USB to thecomputer.

4. Load the VI named EELE101 Transistor Lab.vi. The front panel for this VI is shown in Figure 6

5. On the front panel of EELE101 Transistor Lab, the data parameter inputs should be changed tothe values you will use. Here is some guidance on the input.

# of Curves Begin by tracing 3 to 5 curves.

# of Data Points how many points of data will be recorded for each characteristic curve, gen-erally 100 will be plenty.

Start Vcc The voltage to start the Variable voltage applied to the collector. 0 volts is generallyalways used.

End Vcc The last voltage to apply to the collector in a given data run. 4 to 5 [V] will work well.

R1 The value of the resistor connected to the base.

R2 The value of the resistor connected to the collector.

Write Data Leave this off until you are sure you are getting the data you want. You only needto save the data once.

6. Take data by turning up the base voltage as prompted by the VI.

7. Practice taking data until you’re satisfied that the data is exactly what you want.

8. Switch the Write Data to on and take data. You will be prompted to choose a file to save. Createa file the first time you are prompted, then append to this file for all subsequent data.

4.2 Measure Transistors

9. Run the virtual instrument and create 3 to 5 characteristic curves for a 2N3906 transistor.

10. When you are comfortable with the data, turn the write data switch on and record the data to atext file.

11. Repeat the measurement with 2 more transistors. You should end up with characteristic data for3 transistors.

12. Open the text file in Excel and rearrange the data so that you can make multiple plots in Excelthat match the plots shown in the EELE101 Transistor Lab virtual instrument window.


Figure 6: VI: Transistor Curve Tracer


4.3 Load-line Experiment Set-Up

13. Make a small modification to your set-up by switching the variable voltage source on ELVISwith the external variable voltage source. This will make it possible for a virtual instrument toautomatically change the input base voltage and thus take load line data.

14. Open the VI named EELE101 Load Line.VI

15. Set the VCC to approximately 6 [V]

16. Run the VI with the following guidance on inputs.

# of Curves It is sufficient to do just one curve

# of Data Points how many points of data will be recorded for each characteristic curve, gen-erally 100 will be plenty.

Start V base The voltage to start the Variable voltage applied to the collector. 0 volts is generallyalways used.

End V base The last voltage to apply to the collector in a given data run. 4 to 5 [V] will workwell.

R1 The value of the resistor connected to the base.

R2 The value of the resistor connected to the collector.

Write Data Leave this off until you are sure you are getting the data you want. You only needto save the data once.

17. practice taking data, until you have the plot you want. Then repeat the experiment by runningthe VI with the write data switch on. Append the data to the existing data from the characteristiccurves.

4.4 Data Analysis

18. Create an Excel plot from all of the individually saved data. Plot the data so that the load line isoverlaid on the characteristic curves.

5 Questions

1. Briefly describe how you would change the circuit in Figure 3 if you had a PNP transistor ratherthen an NPN.


2. Why do the characteristic curve keep getting shorter as the base voltage increases?

3. In Figure 2 an NPN transistor is tested for current flow. How would you know if you had an NPNor a PNP transistor in this test?

Revision date: July 10, 2013

transistors 1

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