prelim_exam_sol_sp07

# prelim_exam_sol_sp07 - ECE PHD PRELIMINARY EXAMINATION...

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ECE PHD PRELIMINARY EXAMINATION SOLUTIONS – SPRING 2007 EXAM

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Problem 7 (Core: EDA-ECE2040) Code Number: ___________ Consider the circuit below. Assume the op-amp is an ideal op-amp. You must write your answers in the line (where a line is given) below to receive any credit. Solution: What is the input resistance / impedance looking into the input terminals of an ideal op-amp? GND 1pF V out V in 4pF __Infinite Resistance ___ For a DC input voltage of 0V applied to the input, the DC output voltage is 2V. What is the cause of the 2V DC output voltage? Charge Stored at the node connected to the 1pF, 4pF and – op amp terminal. For the rest of the problem, assume the statement to be true for a DC input voltage of 0V applied to the input, the DC output voltage is 2V. Solve for the dc gain and the bandpass gain. Dc gain = ______-4__________ (impedance at the – terminal is infinite even with the feedback) (did accept an answer of 0 if justified) Bandpass gain = ___-4_________ s (4pF) V in = s (1pF) ( -V out ); therefore V out / V in = -4. Rigorously, this solution holds upto s=0, therefore gain is -4, but at DC, the gain is undetermined by this method of analysis. A more rigorous analysis even at DC gives a gain of -4. One could argue that in a physical system that charge might not be held infinitely, so the gain at DC might be 0. Of course, a physical op-amp would also not have infinite gain, etc, so not a strong argument. For the circuit below, solve for the dc gain, the bandpass gain, and the corner frequency for the resulting

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prelim_exam_sol_sp07 - ECE PHD PRELIMINARY EXAMINATION...

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