UNIVERSITY OF CALIFORNIA AT BERKELEY College of Engineering Department of Electrical Engineering and Computer Sciences EE105 Lab Experiments Prelab 4 Single Stage BJT Amplifiers Common Emitter Name Lab Section VCC RC vout vin VIN Figure 1 Common emitter amplifier 1 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 k and 270 a For this transistor draw a graph found on the next page of IC vs VCE for VBE 600 mV 620 mV 640 mV 660 mV 680 mV and 700 mV Take care drawing this graph as it will be used in answering some of the following questions Assume the boundary between saturation and forward active occurs at VCE 400 mV You can use a piecewise linear model for this graph b On this graph draw the load line for RC in this circuit c Using the intersection points draw a graph also found on the next page of VOUT vs VIN What is 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 smallsignal properties Assume VIN 650 mV for the rest of the prelab a Draw the small signal model of the amplifier answer space provided on next page 1 2 15 12 IC mA 9 6 3 0 0 1 2 3 4 5 VCE V 5 VOU T V 4 3 2 1 0 600 610 620 630 640 650 660 VIN mV 670 680 690 700 3 b Calculate the following be sure to include the Early effect VOUT 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 point which of the above properties change 3 Let s explore what happens when we change the bias point by adding a load resistor a Add a load of 5 k to the output of the circuit Intuitively how do you think this will affect the gain and why b Draw the new load line on the graph you made for question 1 a be sure to label which load line goes with which question c If we keep the same bias i e the same VBE value is the transistor still in forward active mode d What happens to the gain How about the output voltage swing Note The output voltage swing is the maximum and minimum values of vOUT for which all transistors stay in forward active 4 e What is the load resistance value for which the amplifier s BJT begins to transition between forward active mode and saturation mode
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