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Berkeley PHYSICS 111 - Lab 7 Op Amps II

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Physics 111 ~ BSC Student Evaluation of Lab Write-UpUniversity of California at Berkeley Physics 111 Laboratory Basic Semiconductor Circuits (BSC) Lab 7 Op Amps II ©2013 Copyrighted by the Regents of the University of California. All rights reserved. References: Sedra & Smith Chapter 2, skim Chapter 10 Hayes & Horowitz Chapter 4 (Student Edition Book) Horowitz & Hill Chapter 4 In this week’s lab you will continue your study of ideal op amp circuits. Circuits to be constructed include a current to voltage converter, a NIC (negative impedance converter), a gyrator, and an os-cillator. The lab ends with the construction of a complex circuit that calculates root mean squares. Before coming to class complete this list of tasks: • Completely read the Lab Write-up • Answer the pre-lab questions utilizing the references and the write-up • Perform any circuit calculations or anything that can be done outside of lab. • Plan out how to perform Lab tasks. Warning – This lab will involve a lot of debugging. This is why you have two weeks to complete the lab. Don’t wait to start just because it looks short. Also plan out your circuit layout because each circuit is a small piece of 7.12 This will allow you to complete this part without much addi-tional work. Pre-lab questions: 1. How does a NIC work? 2. How does the logarithmic converter in 7.6 work? 3. How does the exponentiator (7.8) work? 4. Derive the oscillator frequency of the circuit in 7.3. Draw the two output signals. The Laboratory Staff will not help debug any circuit whose power supplies have not been properly decoupled! Last Revision: August 2013 Page 1 of 14 ©2013 Copyrighted by the Regents of the University of California. All rights reserved.Physics 111 BSC Laboratory Lab 7 Op Amps II Background Almost all the circuits in this lab can be analyzed with the op amp golden rules. Oscillators Oscillators are circuit designed to put out periodic waveforms like sine or square waves. For exam-ple, oscillators are at the heart of our waveform generator, form a basic part of any radio or TV tun-er1, and generate the 33, 66, 90, or 200MHz (etc.) clock signals that characterize and control all computer CPUs. You may have already encountered accidental parasitic oscillations. Many types of deliberate oscilla-tor circuits exist, but the simplest is called a relaxation oscillator.2 In this lab you will build a relax-ation oscillator with an op amp. In practice, relaxation oscillators are usually built with a special purpose chip called a 555. Filters Filter design is one of the most complicated subjects in analog circuit design; the UCB libraries, for example, have ten entire books on the subject. The ideal filter is called a brickwall filter; it has ex-actly unity gain in its pass region, exactly zero gain everywhere else, and does not induce any phase shifts. Unfortunately brick wall filters are impossible to construct.3 Many different filter designs exist, each attempting to optimize different aspects of filter performance. For example, the simple RC low and high pass filters you constructed at the beginning of this course, and other filters con-structed entirely from passive components, suffer from gradual frequency response, unwanted phase shifts, and high output impedance. Better filters can be constructed with op amps. The Chebyshev active filter constructed in this lab is optimized for a sharp fall in its transition, or skirt region. 1 You might wonder why an oscillator is necessary in a tuner. It turns out that it is difficult to build sharp, variable-frequency bandpass filters, but is easy to build variable frequency oscillators. To ex-ploit this difference, tuners use a complicated circuit called a superheterodyne receiver. Superheter-odyne receivers use a tunable bandpass filter stage to crudely select the desired station, followed by a stage that mixes (multiplies) the signal with a signal from a variable frequency oscillator. As the receiver’s tuning knob is adjusted, the oscillator frequency shifts so that its beat frequency with the desired station is always at the same frequency: usually 455kHz for AM, and 10.7MHz for FM. The beat signal is then passed to a sharply tuned, single frequency filter, which rejects all but the de-sired station. The oscillator in the tuner is responsible for the ban on operating radio receivers during airplane takeoffs and landings. The FAA is afraid that the oscillator might accidentally broadcast a signal that would interfere with the cockpit instruments. Computers, CD players, gameboys, etc. are banned because of their clock oscillators. The ban is based more on paranoia than reality. 2 The term relaxation oscillator dates from when these circuits were built with neon bulbs. A capaci-tor would be charged until it reached the neon bulb’s turn-on, or breakdown voltage. The bulb would then avalanche or relax down to its much lower turn-off voltage, discharging the capacitor, after which the process repeats. 3 Actually, brickwall filters can be constructed digitally at the expense of a long time delay between the input and output signals. This delay allows a digital filter to use information from future times to calculate the response at the current time. Use of this future information allows the filter to be perfect. Analog filters, however, must be causal; as they cannot anticipate the future, their response can only depend on past information, and they can never be perfect. Last Revision: August 2013 Page 2 of 14 ©2013 Copyrighted by the Regents of the University of California. All rights reserved.Physics 111 BSC Laboratory Lab 7 Op Amps II In the lab (A) NICs and Gyrators A NIC, shown below, is a negative impedance converter. Looking into Vin, the NIC appears to have an impedance −Z to ground. In other words, the circuit inverts it internal impedance Z to −Z. 7.1 Construct the circuit below. 2.21k2.21k47k47kOffsetAdderCurrent MeterVoltage MeterABLF356 Use 1%, 2.21k precision resistors, available from the laboratory staff. First connect the meters to point A. Vary the offset adder voltage, and prove that the 47k resistor does indeed behave appropri-ately. (This purpose of this step is to check your meter polarities.) Next connect the meters to point B. Vary the offset adder voltage and measure the current. Prove that the NIC appears to be a −47k resistor. A gyrator is a circuit that converts an impedance of Z to


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