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Last Updated: 2023-11-10 18:41 1 EECS 16A Designing Information Devices and Systems I Fall 2023 Homework 11 This homework is due November 17, 2023, at 23:00. Self-grades are due November 20, 2023, at 23:00. Submission Format Your homework submission should consist of one file. hw11.pdf : A single PDF file that contains all of your answers (any handwritten answers should be scanned). Submit the file to the appropriate assignment on Gradescope. 1. Reading Assignment For this homework, please review Notes 18.5-18.6, Notes 19.5-19.6, and Note 20 (Op-Amp Current Source and Circuit Design). (a) Summarize the 4-step design process outlined in the notes, and describe why each step is useful in practice. UCB EECS 16A, Fall 2023, Homework 11, All Rights Reserved. This may not be publicly shared without explicit permission. 1
Last Updated: 2023-11-10 18:41 2 2. Op-Amp in Negative Feedback (Contributors: Adhyyan Narang, Ava Tan, Aviral Pandey, Deepshika Dhanasekar, Lam Nguyen, Panos Zarkos, Titan Yuan, Vijay Govindarajan, Urmita Sikder, Sunash Sharma) In this question, we analyze op-amp circuits in which the op-amp has a finite gain. Consider the circuit shown in Figure 1. Note that this circuit is in negative feeback. This circuit is known as a non-inverting amplifier circuit. In parts (a) - (e), we will assume the op-amp in this circuit is ideal (i.e., gain is infinite), and then we will consider the case of finite gain in parts (f) - (h). For all parts, assume V SS = V DD . + V DD V SS + v s R 2 I R 2 R 1 i L R + v out + v x u + u Figure 1: Non-inverting amplifier circuit (a) Consider the circuit shown in Figure 1. What is u + u ? (b) Find v x as a function of v out . (c) What is I R 2 , i.e. the current flowing through R 2 as a function of v s ? Hint: Find the current through R 1 first. (d) Find v out as a function of v s . (e) What is the current i L through the load resistor R ? Give your answer in terms of v out . (f) We will now examine what happens when the op-amp gain is not . To understand what happens in this case, we will consider the equivalent circuit for an op-amp with gain A , shown in Figure 2. We will use this equivalent circuit to analyze the non-inverting amplifier circuit in Figure 1. First draw an equivalent circuit for Figure 1, by replacing the ideal op-amp in the non-inverting amplifier in Figure 1 with the op-amp model shown in Figure 2. You do not need to include V DD and V SS in your equivalent circuit. Then, using this setup, calculate v out and v x in terms of A , v s , R 1 , R 2 and R . Assuming A is finite, is the magnitude of v x larger or smaller than the magnitude of v s ? Do these values depend on R ? Hint: Note that the first golden rule still applies, i.e. the currents through the input terminals are zero. UCB EECS 16A, Fall 2023, Homework 11, All Rights Reserved. This may not be publicly shared without explicit permission. 2
Last Updated: 2023-11-10 18:41 3 u + + v in u u out + Av in + v out V SS V DD Figure 2: Internal op-amp model (g) Using your solution to the previous part, calculate the limits of v out and v x as A . You should get the same answer as in part (d) for v out . (h) [OPTIONAL, CHALLENGE] Now you want to make a non-inverting amplifier circuit whose gain is nominally G nom = v out v s = 1 + R 2 R 1 = 4. However, G nom can only be achieved only if the op-amp is ideal, i.e, if its internal gain A . But, as with most considerations in the physical world, we must account for nonidealities! In reality, because you will be working with an op-amp with finite gain A , your designed circuit gain may come close to but will never quite reach G nom as a result of the real op-amp’s finite internal gain A . Suppose you would like your real op-amp circuit to have a minimum error of 1% (i.e, a minimum circuit gain of 3 . 96, i.e. v out v s 3 . 96). Remember that only if your op-amp were ideal, you would have a nominal circuit gain of G nom = v out v s = 1 + R 2 R 1 = 4. What is the minimum required value of A , called A min , to achieve that specification? Hint: Use your expression of v out in part (f) to find an expression for G min = v out v s when A ̸→ . 3. Basic Amplifier Building Blocks (Contributors: Ava Tan, Aviral Pandey, Lam Nguyen, Michael Kellman, Titan Yuan, Vijay Govindarajan, Urmita Sikder) The following amplifier stages are used often in many circuits and are well known as (a) the non-inverting amplifier and (b) the inverting amplifier. UCB EECS 16A, Fall 2023, Homework 11, All Rights Reserved. This may not be publicly shared without explicit permission. 3
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Last Updated: 2023-11-10 18:41 4 + v s + v o R 2 R 1 R 1 + v s + v o R 2 (a) (b) (a) Label the input terminals of the op-amp with (+) and ( ) signs in Figure (a), so that it is in negative feedback. Then derive the voltage gain ( G = v o v s ) of the non-inverting amplifier in Figure (a) using the Golden Rules. Why do you think this circuit is called a non-inverting amplifier? (b) Label the input terminals of the op-amp with (+) and ( ) signs in Figure (b), so that it is in negative feedback. Then derive the voltage gain ( G = v o v s ) of the inverting amplifier using the Golden Rules. Can you explain why this circuit is called an inverting amplifier? 4. Putting on the Pressure: Build Your Own InstantPot (Contributors: Craig Schindler, Ava Tan, Wahid Rahman, Urmita Sikder) Prof. Ranade had a great experience with her automatic pressure cooker, so she was inspired to try and build her own. She’s enlisting your help! The design of the pressure cooker uses a pressure sensor and a heating element. Whenever the pressure is below a set target value, an electronic circuit turns on the heating element. Pressure Sensor Resistance The first step is designing a pressure sensor. The figure below shows your design. As pressure p c is applied, the flexible membrane stretches. (a) You attach a resistor layer R p with resistivity ρ = 0.1 m, width W , length L , and thickness t to the pressure sensor membrane, as illustrated in the figure below. When the pressure p c = 0Pa (i.e. there is no applied pressure), W = 1mm, L = L 0 = 1cm, t = 100µm = 100 × 10 6 m. R p 0 is the value of R p when there is no applied pressure. Calculate R p 0 . Note that direction of current flow in the resistor is from A to B as marked in the diagram. UCB EECS 16A, Fall 2023, Homework 11, All Rights Reserved. This may not be publicly shared without explicit permission. 4
Last Updated: 2023-11-10 18:41 5 (b) When pressure is applied, the length of the resistor L changes from L 0 and is a function of applied pressure p c , and is given by L = L 0 + β p c , where L 0 is the nominal length of the resistor with no pressure applied, and β is a constant with units m / Pa. As a result of the length change, the value of resistance R p also changes from its nominal value R p 0 (the value of R p with no pressure applied). Derive an expression for R p as a function of resistivity ρ , width W , thickness t , nominal length L 0 , constant β , and applied pressure p c , when pressure is applied. Note: The width and thickness of the resistor will also change with applied pressure. However, we ignore this to keep the math simple. (c) Pressure Sensor Circuit Design For this sub-part and the following sub-parts, we will use a new model for pressure-sensitive resistance R p . Assume that the resistance R p is a function of applied pressure p c according to the relationship R p = R o × p c p ref , where R o = 1k , and p ref = 100kPa. To complete our sensor circuit, we would like to generate a voltage V p that is a function of the pressure p c . Complete the circuit below so that the output voltage V p depends on the pressure p c as: V p = V o × p c p ref , where V o = 1V . Restrictions on your pressure sensor circuit design are as follows: You may add at most one ideal voltage source and one additional resistor to the circuit, but you must calculate their values and mark them in the diagram. Mark the positive and negative inputs of the operational amplifier with “+” and “-” symbols, respectively, in the boxes provided. Assume op-amp supply voltages V DD and V SS = V DD are already provided. You may assume that the operational amplifier is ideal. UCB EECS 16A, Fall 2023, Homework 11, All Rights Reserved. This may not be publicly shared without explicit permission. 5
Last Updated: 2023-11-10 18:41 6 + R p + V p (d) Resistive Heating Element To heat the pressure cooker, you use a heating element with resistance R heat . Calculate the value of R heat such that the power dissipated is P heat = 1000W with V heat = 100V applied across the heating element. (e) Pressure Regulation You are finally ready to complete the design of your pressure cooker. Using all of the circuit elements below, make a circuit that will turn the heater on (i.e. will cause a current to flow through R heat ) when the pressure is less than 500 kPa, and off (i.e. will cause no current to flow through R heat ) when the pressure is greater than 500 kPa. The elements are: • A voltage source V s = 10V in series with a resistance of 500 . • A voltage source V p = V o × p c p ref , with V o = 1V and p ref = 100kPa. (This is a voltage source whose voltage is a function of pressure p c , unrelated to any previous parts of the question.) • A comparator that controls switch S 0 . The switch is normally opened (i.e. an open circuit between nodes V a and V b ), and is closed only when V 1 > V 2 (i.e. a short circuit between nodes V a and V b ). • The heater supply ( V heat = 100V). • The heater resistor R heat . • One additional resistor R extra that can have any value . • You may assume you have access to a ground node. • Assume comparator supply voltages V DD and V SS = V DD are already provided. UCB EECS 16A, Fall 2023, Homework 11, All Rights Reserved. This may not be publicly shared without explicit permission. 6
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Last Updated: 2023-11-10 18:41 7 + V s = 10V 500 + V p + V 2 V 1 S 0 V a V b + V heat R heat R extra (i) Since you are looking to compare the change in voltage associated with a change in pressure, you decide to assign the variable voltage source V p as one of the inputs to your comparator. What is the value of the variable voltage source V p for p c = 500kPa? (ii) For your comparator inputs, you also need to generate a reference voltage which can be used to compare against the value of the variable voltage source which you calculated above. Combine the voltage source V s = 10V with an associated resistance of 500 with an an additional resistor R extra to generate a reference voltage equal to the voltage V p you calculated above. What would you choose the value of R extra to be? (iii) Label the circuit elements in the schematic below with the circuit elements presented above and your calculated R extra value to turn the heater on when the pressure is less than 500 kPa. 5. Island Karaoke Machine (Contributors: Aviral Pandey, Lam Nguyen, Pangiotis Zarkos, Sashank Krishnamurthy, Titan Yuan, Urmita Sikder, Vijay Govindarajan) Learning Goal: The objective of this problem is design a circuit that calculates the difference between two signals and amplifies the result. You’re stuck on a desert island and everyone is bored out of their minds. Fortunately, you have your EECS16A lab kit with op-amps, wires, resistors, and your handy breadboard. You decide to build a karaoke machine. You recover one speaker from the crash remains and use your iPhone as your source. You know that many songs put instruments on either the “left" or the “right" channel, but the vocals are usually present on both channels with equal strength. In our case, the vocals are present on both left and right channels, but the instruments are only present on the right channel, i.e. v left = v vocals v right = v vocals + v instrument , UCB EECS 16A, Fall 2023, Homework 11, All Rights Reserved. This may not be publicly shared without explicit permission. 7
Last Updated: 2023-11-10 18:41 8 where the voltage source v vocals can have values anywhere in the range of ± 120mV and v instrument can have values anywhere in the range of ± 50mV. What is the goal of a karaoke machine? The ultimate goal is to remove the vocals from the audio output. We’re going to do this by first building a circuit that takes the left and right audio outputs of the smartphone and then calculates its difference . Let’s see what happens. The equivalent circuit model of the iPhone audio jack and speaker is shown in Figure 3. We model the audio signals and jack as v left and v right with equivalent source resistance of the left/right audio channels of R left = R right = 3 . The speaker has an equivalent resistance of R speaker = 4 . R left + v left R right + v right iPhone R speaker Figure 3: Audio jack and speaker of an iPhone. (a) One of your island survivors suggests the circuit in Figure 4 to do this. Find the expression for the voltage across the speaker R speaker as a function of v vocals and v instruments . Does the voltage across the speaker depend on v vocals ? In other words, what do you think the islanders will hear – vocals, instruments, or both? + v left R left R speaker R right + v right Figure 4: Circuit for part (a). UCB EECS 16A, Fall 2023, Homework 11, All Rights Reserved. This may not be publicly shared without explicit permission. 8
Last Updated: 2023-11-10 18:41 9 (b) We need to boost the sound level to get the party going. To this end, we want a range of ± 2V across the speaker. Design a circuit by completing the Figure 5 below that takes in { v left , R left } and { v right , R right } combos as inputs and outputs an amplified version of v instrument across R speaker . Consider all op-amps to be ideal for this problem. + v left + v right R left R right R speaker Figure 5: Circuit for part (b). Hint 1: We need to get an output voltage with the range of ± 2V . The input voltage v instrument can have values anywhere in the range of ± 50mV . What gain is needed from the op-amp based amplifier circuits? Hint 2: Use two op-amps in the non-inverting configuration. The non-ideal voltage source { v left , R le ft } must be the input to one non-inverting amplifier and the non-ideal voltage source { v right , R right } must be the input to the other non-inverting amplifier. Hint 3: Connect the speaker R speaker across the outputs of those two op-amps. (c) The trouble with the approach in part (b) is that two op-amps are required. Let’s say you only have one op-amp with you. What would you do? One night in your dreams, you have an inspiration. Why not combine an inverting and non-inverting amplifier into one, as shown in Figure 6! + R 1 + v 1 R 3 + v 2 R 2 R 4 + v o Figure 6: The new amplifier for part (c). UCB EECS 16A, Fall 2023, Homework 11, All Rights Reserved. This may not be publicly shared without explicit permission. 9
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Last Updated: 2023-11-10 18:41 10 If we set v 2 = 0V, what is the output v o in terms of v 1 ? (This is the inverting path.) (d) Consider the circuit in Figure 6 again. If we set v 1 = 0V, what is the output v o in terms of v 2 ? (This is the non-inverting path.) (e) Now, determine v o in terms of v 1 and v 2 by superposing your results from the last two parts. Choose values for R 1 , R 2 , R 3 and R 4 , such that the speaker has v o = ± 2V across it for v 2 v 1 = ± 50mV. 6. Wireless Communication With An LED (Contributors: Aviral Pandey, Lam Nguyen, Pangiotis Zarkos, Sashank Krishnamurthy, Titan Yuan, Urmita Sikder) Learning Goal: This problem is designed to find the response of a complex circuit by breaking it into smaller circuit blocks and analyzing each block individually. In this question, we are going to analyze the system shown in the figure below. It shows a circuit that can be used as a wireless communication system using visible light (or infrared, very similar to remote controls). + V in R D 1 Transmitter Receiver + + + O 1 O 2 O 3 D 2 I s V DD R 1 i 1 V 0 V 1 i 2 R 2 V 2 R 3 i 3 V 3 v out + V ref The element D 1 in the transmitter is a light-emitting diode (LED). An LED is an element that emits light and whose brightness is controlled by the current flowing through it. You can recall controlling the light emitted by an LED in various labs during the course. In our circuit, the current through the LED, hence its brightness, can be controlled by choosing the applied voltage V in and the value of the resistor R . The light from the element D 1 in the transmitter is received by the element D 2 in the receiver. In this circuit, the LED D 1 is used as a means for transmitting information with light , and the element D 2 is used as a light receiver to see if anything was transmitted . In the receiver, the element labeled as D 2 behaves like a "reverse biased" solar cell , which means that when it receives light, it generates a current. We will denote the current generated by the solar cell by I S . Remark: Non-idealities, such as background light, affect the performance of a system that does light mea- surements. In this question, we will assume ideal conditions, that is, there is no source of light around except for the LED. UCB EECS 16A, Fall 2023, Homework 11, All Rights Reserved. This may not be publicly shared without explicit permission. 10
Last Updated: 2023-11-10 18:41 11 In our system, we define two states for the transmitter: (i) the transmitter is sending something when they turn on the LED and (ii) the transmitter is not sending anything when they turn off the LED . On the receiver side, the goal is to convert the current I S generated by the solar cell into a voltage and amplify it , so that we can read the output voltage V out to see if the transmitter was sending something or not. The circuit implements this operation through a series of op-amps and a comparator. It might look look complicated at first glance, but we can analyze it one section at a time. (a) Currents i 1 , i 2 and i 3 are labeled on the diagram. Assuming op-amps are ideal and the Golden Rules hold, is I S = i 1 ? i 1 = i 2 ? i 2 = i 3 ? Treat the solar cell as an ideal current source supplying I s . (b) Use the Golden Rules to find V 0 , V 1 , V 2 and V 3 in terms of I S , R 1 , R 2 and R 3 . Hint: Solve for V 0 first, then use V 0 to find V 1 . Afterwards, find V 2 and V 3 in a similar manner. (c) Now, assume that the transmitter has chosen the values of V in and R to control the intensity of light emitted by LED, such that when the transmitter is sending something , I S is equal to 0 . 1A and when the transmitter is not sending anything , I S is equal to 0A. The following figure shows a visual example of how this current I S might look like as time changes. 0 . 1 Time I S [ A ] For the receiver, suppose V ref = 2V , R 1 = 10 , R 2 = 1000 , and the supply voltages of the com- parator are V DD = 5V and V SS = 5V. Pick a value of R 3 such that V out is V DD when the transmitter is sending something (i.e. I S = 0 . 1A ) and V SS when the transmitter is not sending anything (i.e. I S = 0 ) . 7. Homework Process and Study Group Who did you work with on this homework? List names and student ID’s. (In case you met people at home- work party or in office hours, you can also just describe the group.) How did you work on this homework? If you worked in your study group, explain what role each student played for the meetings this week. UCB EECS 16A, Fall 2023, Homework 11, All Rights Reserved. This may not be publicly shared without explicit permission. 11