CTU: EE 375 – Electronics 1: Lab 3: BJT Amplifier 1 Colorado Technical University EE 375 – Electronics 1 Lab 3: BJT Amplifier March 2010 L. Schwappach and C. Fresch ABSTRACT: This lab report was completed as a course requirement to obtain full course credit in EE375, Electronics 1 atColorado Technical University. This lab report examines the Bipolar Junction Transistors Amplifiers. This device is used in manyapplications such as amplifying and switching. This lab will focus on the gain, input/output resistance and the frequency bandwidthusing a common emitter circuit. In particular how to achieve a gain of 7 with the selected load resistance of 10kohm and how to obtainthe Re, Rc,R1,and R2. If you have any questions or concerns in regards to this laboratory assignment, this laboratory report, the process used indesigning the indicated circuitry, or the final conclusions and recommendations derived, please send an email toLSchwappach@yahoo.com or firstname.lastname@example.org. All computer drawn figures and pictures used in this report are of original andauthentic content. The authors authorize the use of any and all content included in this report for academic use. collector, and so BJTs are classified as minority-carrier devices. I. INTRODUCTION The proportion of electrons able to cross the base and reach the collector is a measure of the BJT efficiency. The T HE Bipolar-Junction-Transistors-Amplifiers are active devices used in many applications such as heavy doping of the emitter region and light doping of the base region cause many more electrons to be injected from theswitching and amplifying and etc. The DC biasing determinesthe operating point of the device and its performance emitter into the base than holes to be injected from the basecharacteristics. The BJT transistor structure contains three into the emitter. The common-emitter current gain isregions, the emitter, base and collector. The objective of this represented by β; it is approximately the ratio of the DClab is to design and gain a understanding of how an amplifier collector current to the DC base current in forward-activecan be built by using a BJT. region. It is typically greater than 100 for small-signal transistors but can be smaller in transistors designed for high- power applications. Another important parameter is the II. OBJECTIVES common-base current gain, α. The common-base current gain is approximately the gain of current from emitter to collector The objective of this lab is to design and gain an in the forward-active region. This ratio usually has a valueunderstanding of the physical structure, operation, and close to unity; between 0.98 and 0.998. Alpha and beta arecharacteristics of the bipolar junction transistors (BJT). In more precisely related by the following identities (NPNparticular how to determine the gain, input resistance, output transistor):resistance, and the frequency bandwidth of the amplifier bysimulation, hand calculations, actual measurement. The lastobjective is to recognize the discrepancies between the threeand why they may differ from each-other. III. DIODE THEORY A bipolar (junction) transistor (BJT) is a three-terminal electronic device constructed of doped semiconductormaterial and may be used in amplifying or switchingapplications. Bipolar transistors are so named because theiroperation involves both electrons and holes. Charge flow in aBJT is due to bidirectional diffusion of charge carriers across a IV. DESIGN APPROACHES/TRADE-OFFSjunction between two regions of different charge The performance of this lab will depend on how wellconcentrations. This mode of operation is contrasted with the circuit is developed. If the circuit is developed correctlyunipolar transistors, such as field-effect transistors, in which the results should be similar to the simulation results that wereonly one carrier type is involved in charge flow due to drift. obtained by PSpice and the hand calculations (analysis). TheBy design, most of the BJT collector current is due to the flow performance of the lab also depends on how well theof charges injected from a high-concentration emitter into the equipment is calibrated and accurate the components tolerancebase where they are minority carriers that diffuse toward the is.
CTU: EE 375 – Electronics 1: Lab 3: BJT Amplifier 2 This is not a very cost effective lab except for the VI. CIRCUIT SCHEMATICSdevelopment and time it took to construct the lab components. The circuit schematics below were built in PSpiceBut to save money for a lab project, whether it’s the testing or and allowed our team to analyze the circuit digitally beforedeveloping phase of a new design, depending on what the performing the physical build.schematic is, a circuit can be reduced, if done correctly. Figure 1: BJT LT-Spice diagram showing Common Emitter Circuit for EE-375 Lab #3 V. HAND CALCULATIONS The hand calculations used for this lab (See figure 1)can be found below. Equations: -Rc||Rl/Re = Av Ri = R1||R2||Rib Rib = Rpi + (1+Beta)Re Rpi = Beta(Vt)/Icq AV = 7, choose Re = 1kohms 7 = Rc||RL/Re 7 = Rc||10k/Rc + 10k 7Rc + 70k = 10RcK Rc = 3Rc = 70 Rc = 23.3Kohms Choose R2 = 15kohms 10 – Icq(34) -1 = (Icq –0.1)(7.6744) 9 – 34Icq = 7.67(Icq – 0.1) 9 – 34Icq = 7.67Icq – 0.767 9 = 41.674Icq – 0.767 9.767 = 41.674Icq Icq = 0.234mA VII. COMPONENT LIST Icq + Icq/Beta = Ie The following is a list of components that were used in Ie =0.234+( 0.234/200) Ie = 0.23517 constructing the BJT amplifier from the giving specs in lab Ie (Vbe) = Vb Vb = (0.23517)(0.65) #3. Component values were selected by the professor. Vb = 0.885 A digital multimeter for measuring circuit 15k(10)/R1 + 15K = 0.885 voltages, resistor resistances, and capacitor 150k = 0.885R1 + 13.275 capacitance. 136.725 = 0.885R1 A oscilloscope for viewing the input and output R1 = 154.49kohms waveforms of the circuit. A power supply capable of producing Vcc = 10V Ri = R1||R2||Rib A Pulse Generator capable of delivering input Rib = Rpi + (1 + Beta)Re voltage of (100mV) and a signal at about 2Khz. Rib = 22.1 + (201)(1k) Rib = 223.1 A 2N3904 Transistor, V(BE) = .65V, Vt = 0.026V Beta (B) = 200 Ri = 154.49||15||223.1 5 resistors RL = 10kohms, Rc = 23.2kohms, Ri = 12.88kohms (raised to 33kohms), Re = 1kohms, R1 = 154.49kohms (raised to 160kohms) and R2 = 15kohms. Two capacitors C1 and C2 = 0.1uF Bread board with wires. NOTE: Resistors can normally provide around +/- 5%-25% difference between actual and designed values while Capacitors generally provide around
CTU: EE 375 – Electronics 1: Lab 3: BJT Amplifier 3 20%-50% difference between actual and designed Figure 3 is a graph of a Bode line. This is showing the FL values. You can add resisters in series as (R1+R2) (Low Frequency Range and High Frequency Range) to closer approximate required resistance values and you can add Capacitors in parallel as (C1+C2) Figure 4: BJT Circuit Schematic LT-Spice Simulation to closely approximate required capacitance. DC Bias Results VIII. PSPICE SIMULATION RESULTS The P-Spice simulation results below confirmed ourcircuit schematics and allowed our team to confirm the circuitdigitally before performing the physical build. Figure 2: BJT Amplifier P-Spice Simulation Results Voutput and Vinput Figure 5: BJT Amplifier Circuit Schematic Actual Experimental Results from Figure 1.Key: Green line = Vout, Purple line = VinScale: Vout ( Y – Axis) Range: -800mV to 800mV (400mVincrements) Vin (X – Axis) Range: 1.000s to 1.0045s (0.005sincrements)PSpice results show that when Vin ranges from -100mV to100mV. It is showing that Vin is 200mV peak-peak.Vout = Pspice results show that Vout ranges from 667.466Vto -694.957V. Vout = 1.362mV. Which produces a gain of6.81. The graph above is the actual experimental results. It shows that the Vpp voltage/200mv is appx the gain (Av). Rc and R1 were raised to increased to achieve a close gain of 7. Figure 3: BJT Amplifier Circuit Schematic P-Spice Simulation Results from Figure 1.
CTU: EE 375 – Electronics 1: Lab 3: BJT Amplifier 4 IX. EXPERIMENTAL DATAThe above diagram is the experimental data.The change in the Vpp voltage alters between 1.35V and 1.38depending on what resistor values were used. The gain isapproximately 7, according to this result Vpp/.2mV =1.35/0.2. X. ANALYSIS/DATA COMPARISON The analysis/PSpice/Experimental data results wereall accurate, but the results differed between the three. Thereasons that the results were different is because theexperimental results have equipment calibrations, componenttolerances, and actual measures values from the components.The PSpice results is a close estimate of what the resultsshowed actually be. For example figure 1 shows a graph ofwhat the output showed look like. And the actual resultsverifies that to be true. The analysis is a good estimate of whatthe PSpice results should be. If the PSpice results match theanalysis results, then it’s time to work on the actual lab. Allthree were not in total, agreement however, the results wereclose to each other, and can be proved by applying the PSpiceResults from Figure 1 to the experimental results. XI. CONCLUSIONSThe DC Bias values that were calculated by hand are similarto that of the P-Spice results. The results from the differentmethods proves that the P-Spice and hand calculation arecorrect. The measured characteristics closely matched thosein the specifications. A 2N3004 Transistor was used , arequirement of a gain of 7 was closely achieved because weobtained a gain of 6.89 from the actual experiment results. Theinput resistance that had to be at least 12K was achievedbecause the input resistance that we resulted in was12.9Kohms. The signal was undistorted which is arequirement for the lab. There was no clipping, or saturation.Look at figure 5. This BJT Amplifier lab may be improved to achievea closer gain of 7 by altering Rc. Depending on the otherresistor values, whether or not to increase or decrease theresistance value. XII. ATTACHMENTS All figures above follow. REFERENCES D. A. Neamen, “Microelectronics: circuit analysis and design - 3rd ed.” McGraw-Hill, New York, NY, 2007. pp. 1-107.