ELTRNX1 – ELETRONIC DEVICES AND CIRCUITS NAME: DATE PERFORMED: GROUP NO.: INSTRUCTOR: LABORATORY WORK NO. 4 BJT CHARACTERISTICS INTRODUCTION The transistor is a two junction, three terminal semiconductor device which has three regions namely the emitter region, the base region, and the collector region. There are two types of transistors. An npn transistor has an n type emitter, a p type base and an n type collector while a pnp transistor has a p type emitter, an n type base and a p type collector. The emitter is heavily doped, base region is thin and lightly doped and collector is moderately doped and is the largest. The current conduction in transistors takes place due to both charge carriers- that is electrons and holes and hence they are named Bipolar Junction Transistors (BJT). Depending upon the biasing of the two junctions, emitter-base (EB) junction and collector-base(CB) the transistor is said to be in one of the four modes of operation. The transistor is used as a switch in Cut-off (OFF) and Saturation (ON) regions and as an amplifier in Active region. Reverse Active mode is rarely used (it is used as input stage in TTL gates in digital circuits). The transistor can be considered as a two port network. Three configurations are possible depending upon which terminal acts as input port, output port, and the common terminal. They are common base, common emitter, and common collector configuration. In this experiment we will consider common emitter configuration in which ELTRNX1 – ELETRONIC DEVICES AND CIRCUITS the input is applied between base and emitter, and the output taken at collector with respect to emitter. This is the most popular configuration used in both switches and amplifiers. Operation of transistor in active mode: We consider here the active mode of operation, by forward biasing the base-emitter junction and reverse biasing the base-collector junction as shown in Fig 3. Electrons diffuse from the emitter into the base and holes diffuse from the base into the emitter. The electrons diffused into based region become minority carriers in base. Since the base region is lightly doped and thin, very few electrons will recombine with the holes in the base region and contribute to base current and majority of the electrons arrive at the base-collector depletion region and are swept through the depletion layer due to the electric field. These electrons contribute to the collector current. The direction of motion of electrons is shown by the arrows in red colour in Fig 3. CE input characteristics: The input characteristics are obtained as family of IB -VBE curves at constant VCE. Since the base emitter junction is forward biased, the IB -VBE (Fig.4) characteristics resemble that of a forward biased junction diode. The increase in VCE causes increase in reverse bias to C-B junction. This causes the depletion region to widen and penetrate into the base region more reducing effective base width. This results in less base current to flow and hence increase in VCE causes the characteristics to shift to the right as shown in Fig. 5. The reciprocal of ELTRNX1 – ELETRONIC DEVICES AND CIRCUITS the slope of the linear part of the characteristic gives the dynamic input resistance of the transistor. Output Characteristics These characteristics are obtained as family of IC-VCE at different values of IB. At small values of VCE, the collector voltage is less than that of base causing CB junction to get forward biased. This causes the transistor to enter saturation region where both the junctions are forward biased. For a given base bias, increase in VCE reduces the forward bias and eventually reverse bias the CB junction. This now results in narrowing the base width and thereby reducing base current. This makes the collector current to slightly increase at higher values of VCE causing the characteristics to exhibit some slope. This is Early effect. The collector current is given by ELTRNX1 – ELETRONIC DEVICES AND CIRCUITS where, VA is Early voltage. One can estimate Early voltage by extending the linear portion of Ic curves in the second quadrant. All these curves are found to meet at VA. See Fig. 6. One can find output conductance from the slope of the linear portion of the characteristic curves and also find β= to be defined at a given value of VCE. I. LEARNING OUTCOME After completing this lab activity, the student will be able to: 1. to graph legibly the collector characteristics of a transistor using experimental methods. 2. to correctly measure the AC and DC voltages in a common-emitter amplifier. II. EQUIPMENT/MATERIALS/RESOURCES Desktop PC or Laptop with Windows OS Simulator (MULTISIM) CANVAS (LMS) III. PROCEDURES and DATA and RESULTS Part A : The Collector Characteristics (BJT) Construct the circuit of Fig. 4.1. Vary the 1M potentiometer to set IB = 10 A as in Table 4.1. ELTRNX1 – ELETRONIC DEVICES AND CIRCUITS 20V DC Supply - VRC + A B RC=1k B RB=330k 1M C Base Collector A 10k C Emitter IB Fig. 4.1 1. Set the VCE to 2V by varying the 10k potentiometer as required by the first line of Table 4.1. 2. Record the VRC and VBE values in Table 4.1. 3. Vary the 10 k potentiometer to increase VCE from 2V to the values appearing in Table 4.1. (Note: IB should be maintained at 10 A for the range of VCE levels.) 4. Record VRC and VBE values for each of the measured VCE values. Use the mV range for VBE. 5. Repeat step 2 through 5 for all values of IB indicated in Table 4.1. 6. Compute the values of IC (from IC = VRC/RC) and IE (from IE = IB+IC). Use measured resistor value for RC. 7. Using the data of Table 4.1, plot the collector characteristics of the transistor on a graph paper. (Plot IC versus VCE for the various values of IB. Choose an appropriate scale for IC and label each IB curve). Part B : Common-Emitter DC Bias 1. Measure all resistor values (R1, R2, RC and RE) from circuit in Fig. 4.2 using DMM. 2. Calculate DC Bias values (VB, VE, VC and IE) and record them. 3. Calculate AC dynamic resistance, re. 4. Construct circuit as of Fig. 4.2 and set VCC = 10 V. 5. Measure the DC bias values (VB, VE, VC and IE) and record them. 6. Calculate IE using values obtained in Step 5. 7. Calculate re using the value of IE from Step 6. ELTRNX1 – ELETRONIC DEVICES AND CIRCUITS 8. Compare value of re obtained both from Step 3 & 7. DC Supply To Oscilloscope Or DMM Fig. 4.2 Part A ELTRNX1 – ELETRONIC DEVICES AND CIRCUITS Table 4.1 IB VCE VRC IC VBE IE (A) (V) (V) (mA) (V) (mA) meas meas (calc) meas (calc) 2 4 10 6 8 2 4 30 6 8 2 50 4 6 8 Graph IC versus VCE for each value of IB. Part B ELTRNX1 – ELETRONIC DEVICES AND CIRCUITS 1. R1 (measured) = ____________, R2 (measured) = ___________, RC (measured) = ___________, RE (measured) = ___________ 2. VB (calculated) = ___________, VE (calculated) = ___________ VC (calculated) = ___________, IE (calculated) = ___________ 3. re (calculated) = ___________, 5. VB (measured) = ___________, VE (measured) = ___________ VC (measured) = ___________, 6. IE (calculated) using measured values of VE and RE = __________ I E VE / RE 7. re (measured) = ____________, using IE from Step 6. IV. Personal Reflection and Conclusion ELTRNX1 – ELETRONIC DEVICES AND CIRCUITS V. QUESTIONS 1. Define the conditions for cut-off, forward-active mode, and saturation mode for a NPN bipolar transistor. 2. Describe how an NPN transistor can be used to switch an LED diode on and off. RATING SHEET Criterion Data collection 4-5 2-3.9 0-1.9 Follows and Follows the Does not follow the interprets the procedure and asks procedure and asks procedure to collect questions to collect questions to collect data data data Data Presentation All data can be Some data are The data cannot be (x2) easily understood hardly understood understood and and interpreted and interpreted interpreted Data accuracy Answers to Some answers to Most of the answers questions for the the data gathered to the data gathered data gathered are are inaccurate are inaccurate accurate Conclusion/Personal Conclusion/Personal Conclusion/Personal Conclusion/Personal Reflection Reflection answered Reflection answered Reflection answered the objectives the some of the not related to the completely objectives objectives Answers to Answers to Some answers to Most answers to Questions questions are questions are questions are not correct correct correct References: Boylestad, R. L., & Nashelsky, L. (2013). Electronic devices and circuit theory. Pearson. Score
