Bipolar Junction Transistor Characteristics EELE101 Laboratory

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FVCC EELE 101

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Transistor Characteristics

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Objective

In this lab we will measure the basic characteristics of an NPN transistor and explore how it operates. 1 2 3

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Build basic test circuit Load and run LabVIEW VI for transistor curve measurement and Base current input. Download data from LabVIEW and plot in Excel

Theory

The importance of transistors in electronics is hard to overstate. While nearly all of us depend on millions,even billions of transistors everyday, few people actually understand how they work. This laboratory exercise is an introduction to how transistors work. In summary, transistors are current amplifiers. They amplify the small controlling current that is input to the base. The idea is to control the flow of current from the collector to the emitter. They can be used in a variety of configurations, but most often they are used to switch things on and off. TTL stands for transistor-transistor logic where transistors are used in the simple switch mode, creating the ubiquitous 1’s and 0’s used in our digital world. There are two types of BJT transistors, NPN and PNP. This discussion will concentrate on the explanation for how an NPN transistor works. The story of transistors begins with the concept of a semiconductor. Semiconductors are actually made of a base material like silicon which is a poor conductor. But this substance can be doped with an alien element to either supply an excess of electrons (N-type) or and excess of protons (P-type). To make an N-type semiconductor, silicon with atomic number 14 having 4 valence electrons, is doped with phosphorus, atomic number 15 having 5 valence electrons. These excess charges can become charge carriers. ( In the case of the P-type semiconductor, an element with 3 electrons in the valence is added to the silicon.) So an insulator can be changed into a conductor by doping. The first step in making an NPN transistor is to heavily dope the semiconductor so that by itself, it would be a conductor. But instead of using it like a wire, imagine cutting the N-type semiconductor in half and inserting a P-type semiconductor in between the halves. We call this the ’base’ of the transistor. The other two ends of the transistor are labeled ’collector’ and ’emitter’. Figure 1 illustrates the basic NPN transistor in several ways. The symbol on the left is the schematic representation used for an NPN transistor, the image in the center is a basic picture showing the P and N materials and the depletion regions established. The image 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 established and current, which would have flowed from collector to emitter, is blocked. Another way to

FVCC EELE 101

Transistor Characteristics

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Collector   Collector  

Base  

N  

Base  

Emi4er  

N-­‐type  

Natural  deple7on   regions  

P  

Collector  

P-­‐type   Base  

Emi4er  

N  

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 illustrates the NPN transistor in this opposing diode manor on the far right. As we know from diodes, the arrow indicates the direction for allowed current. In order to get current to flow it is necessary to forward bias the PN junction. So if the base were positive and the emitter were negative, current would flow from 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 collector to base but this would be a reverse-biased PN junction and thus no current flows. This leads to the concept of forward-reverse DC biasing of a transistor.

FVCC EELE 101

Transistor Characteristics

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N-­‐type  semiconductor   Collector  

P-­‐type  semiconductor   Current  

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

Base   +  

P  

N  

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

Emi7er  

Figure 2: Testing a Transistor

The transistor is operational when it is in a forward-reverse DC bias. You might think that expression is a logical contradiction but remember that there are 2 PN junctions in transistor. One of the junctions ( the base-emitter ) is forward biased while the other junction ( the base-collector ) is reverse biased. This single principle is critical to the operation of the transistor. Figure 3 illustrates the circuit needed 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 the transistor on, it is necessary to raise the base voltage, thus its current. This current will flow through the base-emitter junction. Additionally, the current through the base provides charge carriers needed for the collector current to flow. It is this flow of current through the collector, and exiting the emitter that is useful in circuits. Figure 4 illustrates the transistor in forward-reverse bias for the case when there is no base current, and for when there is a base current. Note that the current from the base appears to be amplified by the transistor. That is, a small base current turns the transistor on and produces a large current in the collector and consequently the emitter. This laboratory exercise is intended to explore how a little bit of base current can be amplified by the transistor. These currents are easily captured by simple 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 practice it is often simple considered to be 1 such that IC = IE . A much larger gain is found if we look at how much collector current is allowed to flow with the small base current. That gain is given in equation 2.

FVCC EELE 101

Transistor Characteristics

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R2 100! R1

Q1 2N3904

30000!

V_cc

V_bb

+  

+  

Figure 3: Circuit used to measure characteristics of NPN transistor

N-­‐type  semiconductor  

N-­‐type  semiconductor  

P-­‐type  semiconductor  

Collector  

P-­‐type  semiconductor  

Collector  

Current   N   Base   +  

Current   N  

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

Base  

P  

+  

Vbb  

Natural  deple9on  region,  

N  

Emi7er  

(a) No base current

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

P  

Vbb   N  

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

Emi7er  

(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 hF E ) Currents in an NPN transistor flow from the base to the emitter, and from the collector to the emitter. By conservation of charge, the current in the transistor

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Transistor Characteristics

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must follow this simple relationship given in equation 3. IC = αDC IE

(1)

IC = βDC IB

(2)

IE = IB + IC

(3)

In a circuit the transistor is placed as a control component. That is, it is controlling the flow of current from the collector to the emitter (IC ≈ IE ). So we would like to know how the current in the collector changes as the voltage difference between the collector and emitter change. The transistor will have a characteristic response when a specific base current is applied. The curve produced by mapping the current response to a change in VCE is called the characteristic curve. A set of characteristic curves can be generated when multiple base currents are input, and this family of curves is the main point of the laboratory exercise. Each base current will have its own characteristic curve. Figure 5 shows and example of theoretical characteristic curves for various different base currents. You will produce this graph 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 shows what the current in the collector will be as the base current is changed. It also shows what the voltage drop is between the collector and emitter. To create this line, you can set the collector voltage and then vary the base current. This is important information as it allows you to know how to account for transistors in circuits theoretically. (Allows us to use Kirchhoff’s laws on circuits with transistors)

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Equipment

The basic equipment for this experiment is as follows: 1 2 3 4 5 6

Resistor of about 33[kΩ] Resistor of about 100[Ω] NI ELVIS workstation Computer with LabVIEW installed VI: EELE101 Transistor Lab.vi VI: EELE101 Load Line.vi

FVCC EELE 101

Transistor Characteristics

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Figure 5: Sample characteristic curves

7 8 9 10

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Jumper wire set Multimeter for measuring resistance External variable power supply 3-NPN Transistors, 2N3906

Procedure

To capture the characteristic curves of a transistor, you will build the circuit shown in Figure 3 and run the 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 is called the common emitter configuration since the emitter is attached to ground. In this circuit R1 will limit the current into the base. The external power supply should be used for the power

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Transistor Characteristics

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to the base, Vbb , and the variable power supply on the Elvis unit should be used to apply voltage to the collector of the circuit, Vcc . 3. Start LabVIEW on your computer and make sure the ELVIS unit is connected by USB to the computer. 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 to the 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, generally 100 will be plenty. Start Vcc The voltage to start the Variable voltage applied to the collector. 0 volts is generally always 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 need to 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. Create a 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 a text file. 11. Repeat the measurement with 2 more transistors. You should end up with characteristic data for 3 transistors. 12. Open the text file in Excel and rearrange the data so that you can make multiple plots in Excel that match the plots shown in the EELE101 Transistor Lab virtual instrument window.

FVCC EELE 101

Transistor Characteristics

Figure 6: VI: Transistor Curve Tracer

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FVCC EELE 101

4.3

Transistor Characteristics

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Load-line Experiment Set-Up

13. Make a small modification to your set-up by switching the variable voltage source on ELVIS with the external variable voltage source. This will make it possible for a virtual instrument to automatically 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, generally 100 will be plenty. Start V base The voltage to start the Variable voltage applied to the collector. 0 volts is generally always used. End V base 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 need to save the data once. 17. practice taking data, until you have the plot you want. Then repeat the experiment by running the VI with the write data switch on. Append the data to the existing data from the characteristic curves.

4.4

Data Analysis

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

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

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Transistor Characteristics

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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 NPN or a PNP transistor in this test?

Revision date: July 10, 2013