6 002 CIRCUITS AND ELECTRONICS The Operational Amplifier Abstraction Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19 Review MOSFET amplifier 3 ports vO output port input vI port VS power port Amplifier abstraction VS vI v O vI vO Function of vI Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19 Review vI vO Function of vI Can use as an abstract building block for more complex circuits of course need to be careful about input and output Today Introduce a more powerful amplifier abstraction and use it to build more complex circuits Reading Chapter 15 from A L Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19 Operational Amplifier Op Amp VS input port power port output port VS More abstract representation vIN vOUT Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19 Circuit model ideal vO i 0 v v v i 0 Av A i e input resistance 0 output resistance A virtually No saturation Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19 Using it 12V VS 12V vO vIN 12V RL VS 12V Demo 12V 10 V vO active region saturation 10 V 12V vIN A 106 but unreliable temp dependent Note possible confusion with MOSFET saturation Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19 Let us build a circuit Circuit noninverting amplifier v v vIN vO R1 R2 Equivalent circuit model i 0 vIN op amp v v vO A v v R1 i 0 R2 Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19 Let us analyze the circuit Find vO in terms of vIN etc vO A v v R2 A vIN vO R1 R2 AR2 vO 1 AvIN R1 R2 AvIN vO AR2 1 R1 R2 What happens when A is very large Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19 Let s see When A is large AvIN AvIN vO AR2 AR2 1 R1 R2 R1 R2 vIN Suppose R1 R2 A 10 6 R1 9 R R2 R R2 gain 10 6 vIN vO 10 6 R 1 9R R 10 6 vIN 1 6 1 10 10 vO vIN 10 Demo Gain determined by resistor ratio insensitive to A temperature fab variations Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19 Why did this happen Insight 5V v vIN 5V v 10V A 6V 6V negative feedback i 0 12V vO 2vIN R vO 2 R e g vIN 5V Suppose I perturb the circuit e g force vO momentarily to 12V somehow Stable point is when v v Key negative feedback portion of output fed to ve input e g Car antilock brakes small corrections Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19 Question How to control a high strung device Antilock brakes is it turning no di s yes release apply Michelin it s all about control c yes no k c a db e e f v v powerful brakes Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19 More op amp insights Observe under negative feedback R1 R2 vIN R1 v 0 v v O A A v v We also know i 0 i 0 yields an easier analysis method under negative feedback Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19 Insightful analysis method under negative feedback v v i 0 i 0 g vO vIN a vIN vIN R1 R2 R2 vO b vIN c vIN R1 f e i 0 vIN d R2 vIN R2 R2 Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19 Question a vIN v vIN b vIN v c vIN vO vO vIN or R1 R2 vO vIN R2 with R1 0 R2 Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19 Why is this circuit useful vIN vO vO vIN Buffer voltage gain 1 input impedance output impedance 0 current gain power gain Cite as Anant Agarwal and Jeffrey Lang course materials for 6 002 Circuits and Electronics Spring 2007 MIT OpenCourseWare http ocw mit edu Massachusetts Institute of Technology Downloaded on DD Month YYYY 6 002 Fall 2000 Lecture 19
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