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Download Lab 4 Common Base Characteristics of a BJT Transistor
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Lab 4 Common Base Characteristics of a BJT Transistor Purpose In this lab, transistor current-voltage (I-V) characteristics of Common Base (CB) circuit will be investigated. The characteristics will be derived using current and voltage measurements. Material and Equipment NI ELVIS External Multimeter Regulated Power Supply 2N3904 Assorted Resistors(500 , 1 k ) Theory The bipolar junction transistor(BJT) can be modeled as a current controlled current source. The circuit symbol and the pin out for the actual device can be seen in Figure 4-1. Figure 4-1: The bipolar junction transistor. The Common Base (CB) (Figure 4-2) circuit characteristics constitute a family of static characteristics plots of collector current versus collector-base voltage for several values of emitter current. Figure:4-2 CB configuration circuit using an npn transistor 12 Fig. 4-3: Transistor I-V Characteristics for Common Base Circuit The curves in Fig. 4-3 can be used to calculate the large signal current gain αDC and the small signal current gain, αAC. These values are in general calculated for a given bias point ICQ, VCBQ using the following equations: αDC = ICQ / IEQ αAC = | ICQ - ICQ’ |/| IEQ- IEQ’| Procedure : Transistor Common-Base Collector Characteristics a) Connect the circuit shown in Figure 4-4 below. Use Elvis Variable Power Supply +, (VPS+) for VCC and External Negative Power Supply for VEE. For current measurement, use the External Multimeter provided. Figure 4-4 Measurement Set-up 13 b) Vary Vcc from 0 to +10V in steps of 1V and measure (record) the collector current and the voltage across collector and base . c) Repeat the procedure for different values of IE (vary IE from 0-10mA in steps of 2mA). d) Now, fix emitter current IE at 2mA. Remove Variable Supply+ source and Replace your Vcc source with a NI Elvis Supply - (a negative power supply). By using a negative source, we are going to slightly forward bias B-C junction. (We want to obtain the whole I-V characteristics as shown in Fig. 4-3.) e) Change Negative power supply from 0 to -1 V in steps of 100 mV. By using negative supply, we are slowly forward biasing the C-B junction. You should still be in forward active region until the forward bias exceeds 200-300 mV as indicated in Fig. 4-3. Later current should fall to zero. f) Repeat the procedure e) for different values of IE (vary IE from 2-10mA in steps of 2mA). g) Finally Plot the V-I characteristics with IC on Y-axis and VCB on X-axis using Excel. h) Calculate DC and AC from the curves. Questions for the Lab Report Explain the reasons where Common Base configuration could be useful? 14