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55:041 Electronic Circuits. The University of Iowa. Fall 2013. Homework Assignment 05 Question 1 (Short Takes). 2 points each unless otherwise noted. 1. Pick the word/ phrase that best completes the following sentence. To obtain a frequency response plot of a circuit in SPICE, one must perform _________ analysis (a) an AC 2. (b) Harmonic Distortion (c) Transient What is the 3-dB bandwidth of the circuit below? (a) (b) (c) (d) 3. (b) Transfer function β 8 kHz 31.83 kHz 15.92 kHz 100 kHz For the current source in the circuit below, πΌπ (π‘) = (0.5)u(π‘) mA, where u(π‘) is the unit step function. What is ππ at π‘ = 22 πs? The capacitor is initially uncharged. (3 points) (a) (b) (c) (d) β 4.5 V β5V β9V β 10 V 1 55:041 Electronic Circuits. The University of Iowa. Fall 2013. 4. What is the 3-dB bandwidth of the amplifier shown below if ππ = 2.5K, ππ = 100K, ππ = 40 mS, and πΆπΏπΏ = 1 nF? (a) (b) (c) (d) 5. What is the 3-dB bandwidth of the amplifier shown below if ππ = 2.5K, ππ = 100K, ππ = 40 mS, and πΆπΏπΏ = πΆπΉ = 1 nF? (3 points) (a) (b) (c) (d) 6. 65.25 kHz 10 kHz 1.59 kHz 10.4 kHz 795.8 Hz 1.59 kHz 5 kHz 4.71 kHz Pick the word/ phrase that best completes the following sentence. The mobility of holes is _____ the mobility of electrons in semiconductor materials. (a) always the same as 7. (b) always greater than (c) always less than Pick the word/ phrase that best completes the following sentence. The diffusion capacitance πΆπ of a forward-biased a pn junction is generally_______ the junction capacitance πΆπ . (a) much larger than (b) much smaller than 2 (c) about the same as 55:041 Electronic Circuits. The University of Iowa. Fall 2013. 8. Pick the word/ phrase that best completes the following sentence. The diffusion capacitance πΆπ of a pn junction is ________when the junction is reverse-biased. (a) negligible (b) large (c) same as a forward-biased diode 9. True or false: a silicon diode is biased so that ππ· = 0.7 at 25 oC. VD changes with 2 mV/ o C, so that at 125 oC, ππ· will be 0.7 + 100×0.002 = 0.9 V. 10. True or false: a diode, forward biased at ID = 1 mA, has a small-signal or incremental resistance ππ of about 260 Ξ©. 11. True or false: the turn-on voltages of Schottky diodes are less than that of Si diodes. However, their reverse leakage/saturation currents are also higher. 12. True or false: The turn-on voltage (ππΎ ) of red LEDs is larger than the turn-on voltage of blue LEDs. 13. Which of the following depicts the correct current direction? Circle one. 14. Consider the small-signal equivalent model for a diode below. A typical value for the series resistance ππ for small silicon diode is (circle one). (π) 20 πΞ©β 20 mΞ© (b) 20 mΞ©β 20 Ξ© 3 (c) 20 Ξ©β 200 Ξ© 55:041 Electronic Circuits. The University of Iowa. Fall 2013. 15. The reverse saturation current for a Si diode is πΌπ = 8 × 10β14 A at room temperature (25 β ). Estimate the value of πΌπ at β5 β. (2 points) 16. In the context of diodes, the term βPIVβ means: (1 point) 17. Consider a linear power supply consisting of a transformer, a full-wave, 4-diode bridge rectifier, smoothing capacitor, and a load current 1.2 A. By what percentage will the ripple voltage increase if the load current increases to 1.5 A? (4 points) a) 100 % 18. (b) 25% (c) Stay the same (d) 50% Consider a linear power supply consisting of a transformer, a full-wave, a bridge rectifier and a smoothing capacitor. Increasing the smoothing capacitor by 50% will (4 points) (a) Reduce ripple voltage by 50% and increase maximum inrush current by 50% (b) Reduce both ripple voltage and maximum inrush current by 50% (c) Reduce ripple voltage by 50% and leave maximum inrush current unaffected (d) Reduce ripple voltage by 50% and increase maximum inrush current by 100% 19. Consider a linear power supply consisting of a transformer, a half-wave, 1-diode rectifier, and a smoothing capacitor. The rectifier diode is now replaced with a bridge (4-diode) rectifier. Neglecting the diodesβ turn-on voltages, the ripple voltage will (1 points) a) b) c) d) 20. Decrease by a factor 4 Decrease by a factor 2 Stay the same Increase by a factor 4 A filtered full-wave rectifier voltage has a smaller ripple than does a half-wave rectifier voltage for the same load resistance and capacitor values because: (a) (b) (a) (b) There is a shorter time between peaks There is a longer time between peaks The larger the ripple, the better the filtering action None of the above 4 55:041 Electronic Circuits. The University of Iowa. Fall 2013. 21. The PIV across a nonconducting diode in a bridge rectifier equals approximately: (a) half the peak (b) 2 × the peak value of the transformer secondary voltage. (c) the peak (d) 4 × the peak Problem 2 An engineer designs a power supply that consists of a transformer, a full-wave, 4diode bridge rectifier and a smoothing capacitor. She designed the supply to operate in the U.S. where the power line (mains) frequency and voltage is 60 Hz and 120 V respectively. The ripple voltage at full load is 20 mV. Estimate the ripple voltage when the unmodified supply is used in regions of Japan where the corresponding values are 50 Hz and 100 V respectively. Assume that the equivalent load resistance stays the same. (5 points) 5 55:041 Electronic Circuits. The University of Iowa. Fall 2013. Problem 3 An engineer designs a power supply that consists of a transformer, a full-wave, 4diode bridge rectifier and a smoothing capacitor. The nominal load current is 1.2 A. By what percentage will the ripple voltage increase/decrease if the supply is used with an actual load of 1.5 A? Assume the transformer and rectifier diodes are capable of handling the increase in load current. (5 points) 6 55:041 Electronic Circuits. The University of Iowa. Fall 2013. Question 4 Consider a battery-operated consumer electronics device that uses an embedded microcontroller that will accept a 2.7β5.5 V power. The device can also be powered from a power supply that plugs into a mains outlet. Consumersβ expectation is that the switchover between battery- and mains power is transparent. That is, the instant a user inserts the power supply connector, the device switches to the power supply, and the instant the user unplugs the device, it switched to battery power. Draw a block diagram/schematic that shows how to implement this functionality using diode(s), linear regulator(s), battery, etc. Explain how the circuit works. Provide as much details as you can. For example, specify the type of diodes, indicate voltages on the diagram, and include critical capacitors, and so on. You can assume that unregulated 9 V dc power is available. (8 points) 7 55:041 Electronic Circuits. The University of Iowa. Fall 2013. Question 5 Determine the resistance that the 1K load resistance π π πΏπΏ sees. (8 points) π π πΏπΏ Hint: Use the open circuit voltage, short circuit current technique. 8 55:041 Electronic Circuits. The University of Iowa. Fall 2013. Question 6 What is the voltage ππ in the circuit below? Assume an ideal op-amp. (6 points) 9 55:041 Electronic Circuits. The University of Iowa. Fall 2013. Question 7 Find the expression for ππ for the differential amplifier circuit below. Assume an ideal op-amp. (6 points) Question 8 Using nodal analysis, find πΌπ in the circuit below as follows. Write a KCL equation at the node that connects the 4K, 2K, and other 4K resistor. Use the convention that currents flow away from the node. (6 points) 10 55:041 Electronic Circuits. The University of Iowa. Fall 2013. Question 9 (Principles) Design an RC circuit to delay a 1-kHz sine wave voltage by 62.5 πs. Provide a schematic that shows where the input and output connections are, and specify values for π π and πΆ. (8 points) 11 55:041 Electronic Circuits. The University of Iowa. Fall 2013. Question 10 (Varactor) For the circuit shown, πππ = 5 V and = 10K . The varactor characteristics are shown in the graph. What is the bandwidth of the circuit? (5 points) 12 55:041 Electronic Circuits. The University of Iowa. Fall 2013. Question 11 For the circuit shown, πππ = 5 V, π π = 5K, π£πΎ = 0.6 V. The input is π£π = 0.1 sin(ππ‘) V. (8 points) Draw a small-signal ac model and provide numerical values for the model parameters. Then determine and expression for the time-varying component of the output voltage π£π . Next determine an expression for the complete (timevarying and dc) solution. Provide your answer to 3 significant digits. 13 55:041 Electronic Circuits. The University of Iowa. Fall 2013. Question 12 Determine the time-constant for the circuit shown. π π = 10 Ξ© and πΆ = 0.01 πF. Start by calculating the resistance the capacitor sees. That is, the Thevenin resistance π π ππ» of the controlled source and resistor. (6 points) 14