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Unformatted text preview: AA320 Aerospace Instrumentation
Prelab Assignment for Lab 5 Week of October 31
Useful Operational Amplifier and Comparator circuits Optional Reading: Review Rizzoni Ch. 8: Operational Amplifiers Section 15.5 Comparator and Timing Circuits Note: The Midterm Exam will be on Wednesday, Nov. 9 during class time. It will cover material through Prelab Assignment 5 and Lab 5. Closed book; one page of notes allowed. Prelab questions: 1. Using the op-amp inverting input: Consider the following circuit ("inverting amplifier"): !
In contrast to the non-inverting amplifier of Lab 4, the signal is applied to V , instead of V +. This does have negative feedback, and V + is now connected to ground (i.e., V + = 0). What is V ? [Remember your op-amp rules.] Suppose Vi n = 0.6 V. What is I1? What is If in the feedback resistor? [Remember the rule: no current goes into V or V +] What is Vout? What is the voltage amplification of this circuit [Av = Vout /Vin]? Give a general expression for Av in terms of Rf and R1. Suppose we add a second input Vin,2 as shown: Using the same reasoning as above, express Vout in terms of Vin, Vin,2, Rf and R1. [This is a "summing amplifier".] (continued)
Page 1 of 2 2. Comparators: Comparators are modified op-amps that do use the high open-loop gain G. Their purpose is to produce an output that tells whether input V + is less than or greater than V . This simple function can be very valuable for level-sensitive circuits, such as thermostats. The symbol looks like an op-amp, but for comparators you usually do not use feedback. The following sketch shows the comparator symbol and the equivalent function (essentially a switch). When V + > V , the switch is "HI", and the output is the positive supply voltage: Vout = Vs . When V + < V , the switch is "LO", and the output is the negative supply voltage: Vout = V . Suppose we have a triangle wave input to V + in the above circuit, and that V is connected to a DC voltage, which we can vary. + Sketch the time behavior of Vout, when V = 0 V. Repeat for V = +4 V, -4 V, and +10 V. Draw these sketches below the triangle wave sketch for reference. If the frequency of the triangle wave is 1 kHz, and V is set to +4 V, what is the duration of the HI pulses on Vout? NOTE: The above applies to "active output" comparators, like the TS861 comparator we will be using in Labs 5 and 6. Some comparators require a "pull-up" resistor on Vout to make the output go HI when V + > V . Page 2 of 2 ...
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- Fall '09