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EE40_Fall08_Lecture23-26

EE40_Fall08_Lecture23-26 - Slide 1 EE40 Fall 08 Connie...

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Unformatted text preview: Slide 1 EE40 Fall 08 Connie Chang-Hasnain EE40 Lecture 23-26 Connie Chang-Hasnain October 22, 24, 29, 31 Reading: Chap. 14 Slide 2 EE40 Fall 08 Connie Chang-Hasnain The Operational Amplifier • The operational amplifier (“ op amp ”) is a basic building block used in analog circuits. – Its behavior is modeled using a dependent source. – When combined with resistors, capacitors, and inductors, it can perform various useful functions: • amplification/scaling of an input signal • sign changing (inversion) of an input signal • addition of multiple input signals • subtraction of one input signal from another • integration (over time) of an input signal • differentiation (with respect to time) of an input signal • analog filtering • nonlinear functions like exponential, log, sqrt, etc – Isolate input from output; allow cascading Slide 3 EE40 Fall 08 Connie Chang-Hasnain Op Amp Terminals • 3 signal terminals: 2 inputs and 1 output • IC op amps have 2 additional terminals for DC power supplies • Common-mode signal= (v 1 +v 2 )/2 • Differential signal = v 1-v 2- + V + V – Inverting input v 2 Non-inverting input v 1 positive power supply negative power supply v output Slide 4 EE40 Fall 08 Connie Chang-Hasnain Model for Internal Operation • A is differential gain or open loop gain • Ideal op amp – Common mode gain = 0 i o A R R → ∞ → ∞ = v 1 v 2 R i + – R o v o A ( v 1 – v 2 ) i 1 i 2 i o + _ • Circuit Model ( ) 1 2 1 2 1 2 , 2 ( ) , cm d o cm cm d d o cm v v v v v v v A v A v Since v A v v A + = =- = + =- = Slide 5 EE40 Fall 08 Connie Chang-Hasnain Model and Feedback • Negative feedback – connecting the output port to the negative input (port 2) • Positive feedback – connecting the output port to the positive input (port 1) • Input impedance: R looking into the input terminals • Output impedance: Impedance in series with the output terminals v 1 v 2 R i + – R o v o A ( v 1 – v 2 ) i 1 i 2 i o + _ • Circuit Model Slide 6 EE40 Fall 08 Connie Chang-Hasnain Summing-Point Constraint • Check if under negative feedback – Small v i result in large v o – Output v o is connected to the inverting input to reduce v i – Resulting in v i =0 • Summing-point constraint – v 1 = v 2 – i 1 = i 2 =0 • Virtual short circuit – Not only voltage drop is 0 (which is short circuit), input current is 0 – This is different from short circuit, hence called “virtual” short circuit. Slide 7 EE40 Fall 08 Connie Chang-Hasnain Non-Inverting Amplifier • Ideal voltage amplifier _ + v in +- R 2 R 1 R L v 2 v 1 2 1 2 2 2 2 1 1 2 1 , 2. ( ) ( 0) ( ) o v in in o in in v Closed loop gain A v v v v i i Use KCL At Node v v v i R R v R R A v R v Input impedance i = = = = = =-- = = + = = = → ∞ 2 Since V is independent of R L , Output impedance is 0....
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EE40_Fall08_Lecture23-26 - Slide 1 EE40 Fall 08 Connie...

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