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Unformatted text preview: Midterm Examination Electronic Circuits I  ECSE330A November 14 th 2002, 10:05 AM – 11:25 AM Professor David Plant Pertinent Information: 1) This is a closedbook examination, no notes permitted. 2) The examination consists of 3 problems; 35 total possible points. 3) Only the Faculty Standard Calculator is permitted. Question 1 Consider the following circuit. For this circuit, V A =2V, I=2mA, R E =25 Ω , Rs=100 Ω and the transistor β is 99. 8 Q 1 R S v in v out I i in 8 R F R E i out a) [2 pts] Draw the smallsignal model, indicating the relevant smallsignal parameters with their numerical values. b) [4 pts] Find the value of the feedback resistor R F such that the smallsignal voltage gain, v out /v in , is exactly 724mV/V. For part (c) assume that R F is 60 Ω . Neglect the Early effect. c) [4 pts] Calculate the current gain i out /i in . a) Using the Tmodel (because output is at emitter, which is not grounded): r e 12.375 Ω r e 12.375 Ω r e 12.375 Ω g m 0.08 A V r o 1 k Ω o b) Our equations involve two unknowns: R F and V X (the interior node voltage). Applying KCL twice, we have two equations involving the voltages and resistances: v s v x Rs g m v x v out v x v out R F IIr e x v x v out R F IIr e v out r o IIR E out out A bit of math footwork gives R F = 25 Ω when Vout/Vs is fixed. The algebra is made easier by treating the parallel combination involving R F and r e as an unknown, say x, which can be solved for, and R F will follow. NOTE : One cannot use β +1 rule to establish an R in : bear in mind that the resistance R F is neither in the emitter nor in the base. I have shown the resistance R F as being in parallel with r e but this is only true insofar as KCL/KVL is concerned… R F is not actually in the emitter. As such, the current coming from the voltage source is *not* an actual base current. Similarly, current through R F r...
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 Spring '09

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