a solution to midterm

a solution to midterm - M $OLUTIOAJ ’— Physics 123...

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Unformatted text preview: M $OLUTIOAJ ’— Physics 123 Midterm Spring 2008 1 Physics 123: Midterm, Spring 2008 YOUR NAME: Total points: 53; 75 minutes. Please put all your answers on these exam sheets. Remember to show your work. At the end of these papers you’ll find sketchy data sheets for some parts: op amps (’41 1, ’358, CMOS rail-to-rail), single—supply comparator, ’3I7I’337 regulator, power MOSFET, analog switch,’555 oscillator Contents 1 Small Circuits (I4poims, total) 1 1.1 Adjustable Lowpass (6 points) . . . . . , . . . . . . . . . . . . . . . . l 1.2 Single-transistor current source (4 pm) . . . . . . . . , . . . . . . . . . 1 13 Pass Wiggle between differing DC Levels (4 pts) . . . . . . . . . . . . . Op Amp Audio Amplifier (11 points) Impedances (8 points) Fall Detector (25 points) 4.1 Power Supply (13 points, subtotal) . . . . . . . . . . . . . . . . . . . . 4 High-current Version (17 points) . . . . . . . . . . . . . . . . . . . . . . 4 Low-current Version (6 points) The Accelerometer & Fall Detector {12 points, subtotal) . . . . . . . . . 1 Small Circuits (14p0ints, total) 1.1 Adjustable Lowpass (6 points) Please design an RC low-pass filter: make fgdB adjustable in the range lkHz to lOkHz. RSOURCE that drives this filteris 7:) Want“ ’PxF—rbfim 2' l0“ t‘fw ‘l'Lt lmr ll Udall; OK more 10"“ l —_L’_ Wm ":5 C: 2"? ll Feats "‘ A JsIDk-ifik an I 1.2 Single-transistor current source (4 pts) Your circuit should source 10 mA from a +20V supply, into a load returned to ground. The circuit should work up to at least 18V (that is, its “output compliance voltage” should be at least 0 V to +18 V). No op amps, please. ____——T———-—-—1——d +7’Ob (mu per 1.5 3” LOP: Item, émc U -» {y 15 "‘a “F”? =0AOI6 0J0 “I: Physics 123 Midterm Spring 2008 2 1.3 Pass Wiggle between differing DC Levels (4 pts) That title is a bit long-winded. Here’s the task: let signals, at 100Hz and above ass from a source centered on 0V DC to an output centered on 10V DC. You are given a +15V supply with which t-bx‘Ie—bias” the voltage. NO op amps, please. I + D =3 F 1..) {EH-3 _ ml ) , _ I . ZWBWE‘K l 2‘1 30K ~Wv~mv :>C“5.iok.sp wwfiye W p. 31336 m -.— $?SM”‘ .3, \— " Th8 on)?” ($2 maria 00am- RTHE/U 2 10- R6126: 10M 2. Figure 1: DC—shift problem: pass wiggle, from one DC level to another Erfi " (3 R =3 2 Op Amp Audio Amplifier (11 points) Small audio players, like iPods, may not provide large enough output signals to drive high-impedance headphones. Such headphones require a booster amplifier. We’d like yen to design such a device, here. SpecificatiOns: use inverting amp configuration ' : d‘ tbl ' th ' us one-half atthis settin , treall a“booster" to minus three gain ayusa em erangenun ( g 110 y ) *3 m c. I signal frequencies that your circuit is to amplify: above 20Hz ' ________.. I I I 0 power supply: +5V only 0 O I Rgource (out ofiPod or other audio piayer): S 1009 560 \ m E : VOUT from signal source (out of iPod or other audio la er c ‘ =§ l W —-— E RL—QAD (headphones) 2 6009 \ f3“) é fiehter VOUT (to headphones} on zero m 13 :5 (JV! W MU '5 5E 3 I - choose an op amp from those sketched in a brief data sheet at the end of these exampapers (and briefly explain your choice) Physics 123 Midterm Spring 2008 3 3 Impedances (8 points) Please state the impedances at the points and directions indicated. Assume. . . a we are intcrested in the answers a: signal frequencies; a Lransislorfi : 100; RFeetilruk - you can use an adjective if you cannot provide a numerical value. A . a f“ Wu) U EI’V‘ 6— 10k 1 ow ‘ M ' 'J— \J , = «2". a. (0k 5—? <____ § [005‘- Jk v __ Rourhd n 1.. - 12' = R 0 cf W: K41?: = I) z I k A Figure 2: Impedances '. st-DL 010 =3 Rou’r Uehfilw sum-0L OFF '53 P‘DUT '3- L Isoum ) Ron-r: EUR Physics 123 Midterm Spring 2008 4 4 Fall Detector (25 points) Recent laptop computers have used accelerometers to detect whether the computer is falling, and in that event to shut down the hard drive, so as to protect it from the jolt it will get when the fall ends with a bump. We’d like you to design such a drive-shut-down circuit, using a particular accelerometer, Analog Devices’ ADXL330. We’ll ask you to do this in stages. 4.1 Power Supply (13 points, subtotal) This IC, like many recent parts, runs on a low voltage, 3V in this case. We’d like you to provide this supply in two versions. High-current Version (7 points) If you anticipate adding several 3V parts, some of which will demand almost IOOmA, you’d need a conventional voltage regulator. Show how to use an LM317 adjustable regulator to provide a 3V regulated supply, from an available 15V supply. Make the supply fine—tunable, to 3V plus or minus 10%. .___._.—. “‘-——-0£;V ,' +DHAU =0-6v 72-: lid “in, :I'zs‘u : ( F Liar" I :50 7'43“ R “firm a ’u 1" .‘i all! ’3 I‘fl'os—U i @ -. 0! IT m H *5” "U z: .. 0.5 /,'_%"=a.ma M U 2:230.» Th ‘P'Mfi v *fii’m": am c.730le "*5" 1 “V rm Low-current Version (6 points) If the accelerometer is to be the only part powered from 3V, or if just a few low-current parts are to work with it, you don’t need to resort to an ordinary IC regulator. Show h0w to use an op amp and whatever other circuitry you need to provide a modest supply current of under about lOmA. This time, start with a regulated 5V supply, and hold the 3V supply voltage constant (variation less than about 1%) despite variations in loading that might be as large as lOmA. We want the circuitry that generates this homebrew 3V source to use little power, itself (drawing less than 1111A). Note that you should expect that the output voltage from this home-made power supply will be “decoupled” with caps of perhaps 0.01 nF to ground. Make sure that such decoupling does not destabilize your circuit. Physics 123 Midterm Spring 2008 5 4.2. The Accelerometer & Fall Detector (12 points, subtotal) The accelerometer can be used to detect a fall by sensing the sudden disappearance of the usual 1G acceleration caused by gravity. (Inverting or radically tilting the accelerometer will have the same sort of effect; that’s OK, in this design: we’re willing to treat such events as cause for shutting down the hard drive.) The accelerometer puts out a voltage that is quiescent at the midpoint of its supply—1.5V—with 0G acceleration. A 1G downward acceleration causes VOUT to fall by 0.3V, to 1.2V. So, the accelerometer’s VOUT might look like the image below, just before and just after it was dropped: Vow? J‘- 05 Info ‘15 ""m ‘ "a .. resi— Fw M” mt- HIP Figure 3: hypothetical waveform from accelerometer showing elfect of a fall Your design task: We’d like you to design a circuit that will shutoff power to the disk drive during t ’ fall; that is, when the acceleration due to gravity has shrunk from its usual value of -1G to about 1/3 of its I a ma] value, to abou When this occurs, your circuit should shutoff power to the disk drive, by ceasin; o sink current from its great d pin 1 Make sure that your circuit switches decisively, rather than ch ering. The disk drive includes a otor, and thus is inductive. Here is a reminder of specifications: o power supply: only +SV and the +3V derived f In this by your circuit of part 4.1 o accelerometer supply voltage: +3V - oration: 1/2 accelerometer supply voltage: +1.5V - accelerometeroutput voltage . !_ . .. o accelerometer sensitivi nt an +7 ‘7 My 3.:- ClOS'er ‘lo 00 NJ“ *) Figure 4: disk drive as load and your circuit to switch it off in case of fall l l l 1 i i threshold‘ ’23 MW mdtrerarOBIex -, March 17, 08) End Exam: Sketchy Data Sheet follows 1This is a slightly unconventional design; more u al would be to shut off power sourced from the pOSitive supply, but we’d like to keep your task simpler and more familiar. lg Show l: I SW7: “Va—.3 amwfigfi ...
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a solution to midterm - M $OLUTIOAJ ’— Physics 123...

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