June 23, 2013
June 23, 2013
June 26, 2013
Division Experimentation & Lab-Oriented Studies
23.1024.1 - 23.1024.15
Real-World vs. Ideal Op-Amps: Developing Student Insight into Finite Gain-Bandwidth Limitations and CompensationAbstractIn learning circuit design using operational amplifiers, most EE students aretaught about inverting and non-inverting amplifier design, summing amplifierdesign, and simple filter design (first and second order), all assuming the use of anideal operational amplifier. While this is a great place to start a design discussion,students can be left with the lingering impression that a µA741 op-amp (whichworked fine for anything they needed to accomplish in their carefully scriptedlabs from sophomore or junior year) is a generic op-amp that will do anythingthey will ever need to do!This paper presents some successful design and compensation techniques fromone laboratory in a junior-level Linear Circuits class at the U.S. Coast GuardAcademy. In this lab, students are asked to design two Sallen-Key second orderlow pass sections, using the µA741 op-amp, in order to meet two specificresonant frequency ( f 0 ) and quality factor ( Q ) specifications. They typicallyachieve the first design specification (design #1, for f 0 = 7.23 KHz and Q = 2 ) onthe first try. However they typically fall short of the resonant frequency in theirsecond design specification (design #2, f 0 = 72.3 KHz and Q = 2 ) by 20% ormore. This creates a teachable moment, as students reflect on their success indesign #1, and their failure to meet specifications in design #2. We then remindstudents that they are dealing with a real-world µA741 op-amp, whose Gain-Bandwidth Product is approximately 1MHz, and they learn the theory thatpredicts the impact of that limitation.Recognizing that the real-world op-amp itself can be modeled as a first ordertransfer function, we present graphical techniques that can be used early in thedesign process to “pre-compensate” for gain-bandwidth product limitations.Using these techniques, students are then able to meet specifications for design #2with their µA741 op-amp. Finally, in the last part of this lab students use a moreexpensive (LM318) op-amp, with 15MHz gain-bandwidth product, to illustratethat minimal (if any) compensation is required to meet design specification #2.This paper presents typical measurement results, along with informal studentfeedback that suggests to us that this exercise does reinforce student learning withrespect to real-world characteristics of op-amps.
Emami, T., & Hartnett, R. J. (2013, June), Real-World vs. Ideal Op-Amps: Developing Student Insight into Finite Gain-Bandwidth Limitations and Compensation Paper presented at 2013 ASEE Annual Conference & Exposition, Atlanta, Georgia. https://peer.asee.org/22409
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