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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This note was uploaded on 10/24/2009 for the course EE 40 taught by Professor Chang-hasnain during the Fall '07 term at University of California, Berkeley.

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

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