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Berkeley ELENG 105 - Single Stage BJT Amplifiers: Common Emitter

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UNIVERSITY OF CALIFORNIA AT BERKELEYCollege of EngineeringDepartment of Electrical Engineering and Computer SciencesEE105 Lab Exp er imentsPrelab 4: Single Stage BJT Amplifiers: CommonEmitterName:Lab Section:−+VIN−vin+RCVCC+vout−Figure 1: Common emitter amplifier1. Let’s analyze this common emitter amplifier! See Figure 1. Let VCC= 5 V, VT= 26 mV, IS=26.03 fA, VA= 90.7 V, RC= 1 kΩ, and β = 270.(a) For this transistor draw a graph (found on the next page) of ICvs. VCEfor VBE= 600 mV,620 mV, 640 mV, 660 mV, 680 mV, and 700 mV. Take car e drawing this graph as it will beused in answering some of the following questions. Assume the boundary between saturation andforward active occurs at VCE= 400 mV. You can us e a piecewise linear model for this graph.(b) On this graph, draw the load line for RCin this circuit.(c) Using the intersec tion points, draw a graph (also found on the next page) of VOU Tvs. VIN. Whatis this graph useful for?(d) What is the gain for VIN= 650 mV? What is the region of operation of the transistor?2. Now that we’ve analyzed the large-signal properties of the amplifier, let’s analyze some of its small-signal properties. Ass ume VIN= 650 mV for the rest of the prelab.(a) Draw the small signal model of the amplifier (answer space provided on next page ).1203691215IC(mA)0 1 2 3 4 5VCE(V)012345VOU T(V)600 610 620 630 640 650 660 670 680 690 700VIN(mV)3(b) Calculate the following (be sure to include the Early effect):• VOU T• IC• Transconductance gm• Output impedance Rout• Input impedance Rin• Gain Av. Does this gain match your answer to question 1(d)?(c) If we change the bias p oint, which of the above pr operties change?3. Let’s ex plore what happens when we change the bias point by adding a load resistor.(a) Add a load of 5 kΩ to the output of the circuit. Intuitively, how do you think this will affect thegain and why?(b) Draw the new load line on the gra ph you made for question 1(a) (be sure to label which load linegoes with which question).(c) If we keep the same bias (i.e. the same VBEva lue) is the transistor still in forward active mode?(d) What happens to the gain? How about the output voltage swing? Note: The output voltage swingis the maximum and minimum values of vOU Tfor which all transistors stay in forward active.4(e) What is the load resista nce value fo r which the amplifier’s BJT begins to transition b e tween for-ward active mode and saturation


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Berkeley ELENG 105 - Single Stage BJT Amplifiers: Common Emitter

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