ECE 363 FINAL (F16) NAME: 6 problems for 100 pts Problem #1: Fuel Pump Controller (18 pts) You are asked to design a high-side switch for a remotely operated fuel pump. You decide to use the IRF9520 power MOSFET to drive the pump, which can draw up to 2A. The electronic control unit (ECU) produces a 2.5V logic signal for Vin. Your ECE training tells you that a logic level shifter is a good idea. You are deciding between two possible circuits to interface the ECU with the pump. +12V +12V +12V +12V 10kΩ Q2 IRF9520 10kΩ Q2 IRF9520 Vin Q1 2N7000 Pump Vin RB Q1 2N3904 Pump (Circuit #1) (Circuit #2) (a) Which circuit is the best choice? Use the appropriate data sheet parameters to explain why your chosen circuit works and why the other circuit does not. (b) If you chose Circuit #1, compute the minimum value of Q1 s drain current. If you chose Circuit #2, compute the appropriate 5% standard resistor for RB. (c) What is the largest thermal resistance (e.g. C/W) heat sink that should be used with Q2? Assume T A = 25 C and CS = 0.5 C/W. ECE 363: Design of Electronic Circuits Final Nov 21, 2016 1
extra sheet for work) ECE 363: Design of Electronic Circuits Final Nov 21, 2016 2
Problem #2: Active Filter (18 pts) Suppose you need to generate a square wave with 50% duty cycle, adjustable frequency, and zero DC offset. The square wave amplitude must be +/- 2.5V with a frequency from 1 khz up to 25 khz. One approach is shown in the figure. A microcontroller produces a 0-to-5V signal, where a high-pass filter blocks the DC component. Vin R t C R Vout t (a) Use the Golden Rules to show that: / /, where 1 2 (b) Suppose R = 10 kohm. Choose the smallest standard 10% capacitor value so that the filter gain is higher than -0.2 db at 1 khz. (c) Suppose you are using the ua741c op amp. Sketch both V IN and V OUT for a 25 khz square wave over an 80 us time interval. Label important features! Keep in mind that op amp speed limitations may cause distortion. ECE 363: Design of Electronic Circuits Final Nov 21, 2016 3
(extra sheet for work) ECE 363: Design of Electronic Circuits Final Nov 21, 2016 4
Problem #3: Power Amplifier (18 pts) You are asked to design a single-supply DC power amplifier with a voltage gain of 20 db. The amplifier must deliver up to 10W into a 10 ohm load. You consider the circuit in the figure to the right: The design constraints are the following: Vin +Vcc Q1 You must choose between the following: TIP31 (power npn) Vout TIP110 (Darlington npn) 11kΩ 100kΩ RLOAD The op amp has a max output current of 20 ma. (a) Choose the appropriate transistor for Q1. Explain your reasoning using appropriate data sheet parameters. (b) You must choose between Vcc = 9, 12, 15, or 18V. Show all calculations! (c) Is a heat sink necessary for Q1? If so, you must choose between 5 C/W, 10 C/W, 15 C/W, and 20 C/W. Assume T A = 25 C and CS = 0.5 C/W. Show all work! ECE 363: Design of Electronic Circuits Final Nov 21, 2016 5
(extra sheet for work) ECE 363: Design of Electronic Circuits Final Nov 21, 2016 6
Problem #4: Differential Amplifier (18 pts) Consider an application where you need to maintain temperature near 25 C. You therefore need a way to measure small changes in temperature (e.g. few C). You really like BJT differential amplifiers, so you consider the configuration shown in the figure. The temperature sensor is the AD590 chip: This device produces a CURRENT given by: I SENSOR = 1 ua/ K. For example, I SENSOR = 283.2 ua when the temperature is 10 C = 283.2 K. +6V AD590 I_SENSOR 10.0kΩ Q1 +6V Q2 RC Vout 3V +6V The amplifier should have Z IN 100 kohm 4.3kΩ Assume all transistors have = 100, V BE = 0.7V, and V CE(SAT) = 0.2V. Q3 a) What is the smallest 5% resistor value that is acceptable for R E? RE b) You must choose between R C = 33 kohm and 68 kohm. Keep in mind that you want to maximize the measurable temperature swing (e.g. above and below 25 C). You must also show why the other value is not a good choice. Show all work! c) Compute the highest and lowest temperatures (in C) that can be measured by your amplifier. Assume your CMRR is high enough that A CM can be ignored. Show all work! ECE 363: Design of Electronic Circuits Final Nov 21, 2016 7
(extra sheet for work) ECE 363: Design of Electronic Circuits Final Nov 21, 2016 8
Problem #5: Negative Feedback (14 pts) You are asked to design a battery-powered amplifier with a nominal closed-loop gain of +46 db. The amplifier s open-loop gain is A O at full-battery and drops to A O /4 by the end of the battery life. The closed-loop gain must not drop by more than 5% during the life-span of the battery. (a) What value of A O is acceptable for your design? Show all work. (b) Suppose you only have the LM358 single-supply op amp in your electronics kit. Based on the data sheet, is the LM358 appropriate for this application? Explain your reasoning. HINT: Keep in mind that your answer to part (a) is the smallest acceptable value of A O. NOTE: Large gain values are often specified in units of V/mV. For example, A O = 2,000 is equal to 2 V/mV. (c) Using the minimum value for A O in the LM358 data sheet, compute the proper value of. Specify out to 5 decimal places (e.g. = 0.00783). (d) Assuming A O drops to A O /4 by the end of the battery life, compute the expected percentage drop in closedloop gain. ECE 363: Design of Electronic Circuits Final Nov 21, 2016 9
(extra sheet for work) ECE 363: Design of Electronic Circuits Final Nov 21, 2016 10
Problem #6: Amplifier Stability (14 pts) A customer asks you to fix a non-inverting amplifier. When you test the amplifier with a step input, the output is a step function with growing oscillations that eventually saturate the amplifier, as shown to the right. Your ECE training tells you to suspect an unstable amplifier! Consulting the op amp data sheet shows an open-loop DC gain of 31,600 and poles at 50 khz and 50 MHz. a) Sketch the Bode plots for the magnitude and phase of the open-loop gain from 5 Hz out to 500 MHz. Label important features! b) On your Bode plot for A, sketch 1/ and explain why the amplifier is not stable. You do NOT need to compute the exact phase margin (e.g. just use your plots)! c) Perform frequency compensation by determining the new dominant pole f P that produces a 45 phase margin. ECE 363: Design of Electronic Circuits Final Nov 21, 2016 11
(extra sheet for work) ECE 363: Design of Electronic Circuits Final Nov 21, 2016 12
ECE 363: Design of Electronic Circuits Final Nov 21, 2016 13