ECE2210 Final given: Fall 12

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1 ECE Final given: Fall (5 pts) a) Find and draw the Thévenin equivalent of the circuit shown The load resistor is R L b) Find and draw the Norton equivalent of the same circuit c) Find the load current using your Thévenin equivalent circuit I S 90 ma 30 Ω Ω R L 90 Ω 0 Ω (5 pts) a) Find the s-type transfer function of the circuit shown Consider I in as the input and as the "output" You MUST show work to get credit Simplify your expression for H(s) so that the denominator is a simple polynomial with no coefficient before the highest-order s term in the denominator I in C L "output" H( s ) b) How many zeroes does this transfer function have? If it has or more, express them (probably in terms of,,, L and C) c) How many poles does this transfer function have? 3 ( pts) A generator produces 300-V, 0-Hz power It is connected through an extension cord to a single load which consumes kw with a 0% lagging power factor The total resistance of the wires in the extension cord is R line The system efficiency is 9% Source end: 300-V wires: R line Efficiency: 9% = P out Load end: kw, 0% pf, lagging P in a) Find the complex power (P and Q) provided by the source b) Find the current flowing in each wire of the extension cord c) What is the value of the line resistance? R line d) What is the line voltage at the load? Just magnitude ( pts) The transformer shown in the circuit below is ideal It is rated at 0/55 V, 0 VA, 0 Hz Find the following: R 3 Ω a) V b) Is this transformer operating within its ratings? Show your evidence V S V 7 Ω ECE Final given: Fall p

2 ECE Final given: Fall p 5 ( pts) The same input signal (at right) is connected to several op-amp circuits a) Sketch the output waveform for this circuit Clearly label important voltage levels on the output If I can't easily make out what your peak values are, I'll assume you don't know Don't forget to show inversions kω kω v S ( t) (Volts) v S 0 kω V v oa ( t) V 0 b) Devise an op-amp circuit which will output the waveform shown below given the input waveform shown at right Choose the power supplies and use whatever passive parts you need output v ob ( t) (Volts) You may not use any other batteries, input signals, or power supplies beyond the two that power the op amp ECE Final given: Fall p

3 ECE Final given: Fall p3 V CC V (3 pts) A couple of transistors are used to control the current flow through an inductive load The switch has been closed, as shown, for a long a) You measure the voltage at each collector (referenced to ground) as shown on the drawing Find the power dissipated by transistor Q Q 5 V Inductive load L L 0 mh kω b) Find the β of transistor Q R L Ω c) Find the β of transistor Q 0 Ω V d) Find the minimum β for transistor Q to be in saturation Q β min You replace Q with a different transistor so that now: β 00 Use this from now on e) Find the new load current ( ) assuming transistor Q is in the active region f) Check the assumption that Q is in the active region and recaculate if necessary g) The diode in this circuit conducts a significant current: (circle one) A) never C) whenever the switch is closed E) when the switch opens B) when the switch closes D) always F) whenever the switch is open h) What is the maximum diode current you expect when the switch is cycled (Answer 0 if it never conducts) 7 ( pts) Find the values below Show your work Note: feel free to show work & a) V Is a) V Is answers right on the schematic b) I V c) P S V S V I S ma V Is kω c) P S 500 Ω V V I V kω b) I V ECE Final given: Fall p3

4 ECE Final given: Fall p LED v o Use constant-voltage-drop models for the diodes and LEDs on this exam ( pts) A voltage waveform (dotted line) is applied to the circuit shown Accurately draw the output waveform (v o ) you expect to see Label important s and voltage levels Ω LED v in Ω v in ( volts) 0 0 ms If you're not specific about your s and voltages, I'll assume you don't know! 0 Ω 9 (30 pts) Assume that diodes D, D 3 and D DO conduct I D D Assume that diode D does NOT conduct D a) Stick with these assumptions even if your answers come out absurd Find the following and anything else you need in order to check the assumptions: I R3 I D3 V S V D 3 V D Ω I R3 0 Ω V D I D I D D I D3 LED R 0 Ω I D ECE Final given: Fall p More on next page ===>

5 ECE Final given: Fall p5 b) Based on the numbers above, was the assumption about D (circle one) How do you know? (Specifically show a value which is or is not within a correct range) c) Was the assumption about D (circle one) d) Was the assumption about D 3 e) Was the assumption about D f) Based on your answers to parts b), c), d) & e), Circle one: Justify your answer (circle one) i) The real I D < I D ii) The real I D = I D iii) The real I D > I D Do you want your grade and scores posted on the Internet? If your answer is yes, then provide some sort of alias: Answers otherwise, leave blank The grades will be posted on line in pdf form in alphabetical order under the alias that you provide here I will not post grades under your real name It will show the homework, lab, and exam scores of everyone who answers here a) R Th 0 Ω b) a) V Th V I N 0 ma c) 0 ma R N R N 0 Ω L C s s R C L L C b) 0 c) 3 a) 5 09 j kva b) 53 A c) 90 Ω d) 9 V a) 3 V b) 9 A 5 a) b) one possible solution a) 3 W b) 39 c) 59 kω V kω 9 V 3 V v ob ( t) d) 973 e) A f) 95 A g) E h) 95 A 7 a) 3 V b) ma c) 0 mw 9 a) 0 ma 75 ma 0 V 35 ma 75 ma b) yes I D = 75 ma > 0 c) no V D = 0 V > 07V d) no I D3 = 75 ma < 0 e) yes I D = 35 ma > 0 f) i The additional current from D 3 doesn't really flow ECE Final given: Fall p5