EXPERIMENT 5 CURRENT AND VOLTAGE CHARACTERISTICS OF BJT

Transcription

1 EXPERIMENT 5 CURRENT AND VOLTAGE CHARACTERISTICS OF BJT 1. OBJECTIVES 1.1 To practice how to test NPN and PNP transistors using multimeter. 1.2 To demonstrate the relationship between collector current (I C ) and base current (I B ). 1.3 To provide the opportunity for plotting the output characteristic curves for a transistor using measured component values. 2. INTRODUCTION Basically, a bipolar (BJT) transistor can be represented internally by two diode or p-n junctions. Consequently, a multimeter can be used to check each diode junction, as was done in Experiment 2. If we know the three terminals, then it is possible to determine if a given transistor is NPN or PNP bipolar transistor. Figure 5.1: Schematic diagram of circuit The ratio of the dc collector current (I C ) to the dc base current (I B ) is the dc beta ( ) which is the dc current gain of transistor is β = I I C B (6.1) The voltage across R 1 can be calculated by Ohm s Law where I B is dc base current. V 1 = I B R 1 (6.2) 36

2 3. COMPONENT AND EQUIPMENT 3.1 2N3904 NPN transistor 3.2 2N3906 PNP transistor 3.3 Multimeter 3.4 Breadboard kω resistor 3.2 2N3904 NPN transistor 3.3 DC power supply (2 units) 3.4 Multimeter (2 units) 3.5 Breadboard 4. PROCEDURE 4.1 Measuring Voltage across 2N3904 NPN transistor junction: The schematic diagram and diode junction represented are shown in Figure 5.2. Figure 5.2: Pin configuration of 2N3904 transistor Take your multimeter and select a low-resistance meter range Connect the meter s positive lead to the transistor s base lead, with the meter s negative lead connected to the transistor s emitter lead. (NOTE: You have forward biased the transistor s base-emitter diodes junction.) Record the display reading in Table Now, reverse the meter s leads so that the positive lead is connected to the emitter and the negative lead is connected to the base Record the display reading in Table Then, connect the meter s positive lead to the base and the negative lead to the transistor s collector lead Record your result in Table After that, reverse the meter s leads so that the positive lead is connected to the collector and the negative lead is connected to the base Record the result in Table Now connect the meter s positive lead to the collector the negative lead to the transistor s emitter lead Note and record this value in Table

3 Then, reverse the meter s leads so that the positive lead is connected to the emitter and the negative lead is connected to the collector Record this value intable 5.1. (NOTE: If transistor diode junction was forward biased, you should have obtained a value between 0.5 and 0.8 and if the transistor diode junction was reverse biased, the transistor would be in an open-circuit condition). 4.2 Measuring Voltage across 2N3906 PNP transistor junction: The schematic diagram and diode junction represented are shown in Figure 5.3. Figure 5.3: Pin configurations of 2N3906 transistor Take your multimeter and select a low-resistance meter range Now, connect the meter s positive lead to the transistor s base lead and the negative lead to the transistor s emitter lead Record your result in Table Then, reverse the meter s leads so that the positive lead is connected to the emitter and negative lead is connected to the base Note the meter reading, and record the result in Table Then connect the meter s positive lead to the base and the negative lead to the transistor s collector lead Record your result in Table After that, reverse the meter s leads so that the positive lead is connected to the collector and the negative lead is connected to the base Record the result in Table Now, connect the meter s positive lead to the collector and the negative lead to the transistor s emitter lead Note the meter reading, and record this result in Table After that, reverse the meter s leads so that the positive lead is connected to the emitter and the negative lead is connected to the collector Note the meter reading, and record this result in Table Compare the result of Table 5.1 and Table 5.2.Write your observations. 38

4 4.1 Measure and record the actual resistance of your 100 kω resistor. I C V 1 B C V CE 100 kω E V CC V BB Figure Construct the circuit shown in Figure 5.4. Both supply voltages should initially be set to 0 V DC. 4.3 Calculate the value of V 1 that is required to generate a current of 5 µa through R Adjust V BB to obtain the value of V 1 calculated in Step Adjust V CC so that V CE is 0.5 V DC. 4.6 Measure and record the value of I C in appropriate space in Table Increase V CC to provide a V CE of 1 V DC. Measure and record the corresponding value of I C in Table Repeat step 4.7 for each of the values of V CE listed in Table After completing the measurements in step 4.8 for I B = 5 µa, return V CE to 0 V DC. Calculate the value of V 1 that is required to generate a current of 10 µa through R 1. Adjust V BB to provide this value of V Repeat steps 4.5 through 8 for I B = 10 µa Repeat steps 4.5 through 4.10 until Table 5.3 is complete Using the data recorded in Table 5.3, plot the characteristic collector curves for the 2N3904 in graph paper. 39

5 DMT 121/3 ELECTRONIC DEVICES ASSESSMENT FORM NAME: MATRIX NO: COURSE: EXPERIMENT NO: RESULTS: Table 5.1: 2N3904 NPN Transistor Step Number Meter Leads Base Emitter Emitter Base Base Collector Collector Base Collector Emitter Emitter Collector Result Table 5.2: 2N3906 PNP Transistor Step Number Meter Leads Base Emitter Emitter Base Base Collector Collector Base Collector Emitter Emitter Collector Result 40

6 (1) For Step 4.1 : The value of R 1 : Ω (2) For Step 4.3 : The value of V 1 : V Table 5.3 V CE (V dc ) I B (µa)

7 CALCULATION 42

8 DISCUSSION 2.6V 0.7V 2.6V 0.7V (a) (b) (c) (d) Figure 6.5 Again, a student used Digital Multimeter (DMM) to test the transistor as shown in Figure 6.6. From the testing, what type of the BJT transistor? Using the characteristic curves, predict the values of I C for each of the I B and V CE combinations listed below. I C = I C = when I B = 25 µa and V CE = 10 Vdc when I B = 35 µa and V CE = 8 Vdc CONCLUSION 43