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ECE 2006 Chapter 5: Operational Amplifiers

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Differential Amplifier Not Practical Prior to IC Fabrication 2 Inputs, Output is A v *(V 1 - V 2 )

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Common Mode Rejection Differential Amps won’t Amplify Signals that are Common to both Inputs

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Operational Amplifier Three Stages: –Differential Amplifier (Input Stage) –Voltage Amplifier (High Gain) –Output Stage (low output impedance)

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Op Amp Characteristics High Input Impedance ( 1 M-Ohm) Low Output Impedance (100 Ohms) High Voltage Gain (100,000) 2 Differential Inputs –Inverting Input (V-) –Non-Inverting Input (V+)

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Equivalent Circuit Model Differential Input Voltage determines Output Voltage

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Ideal Op Amp Input Impedance is Infinite (No input Current) Voltage Gain is Infinite Inverting and Non-inverting inputs at equal potential

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Feedback Open Loop Voltage Gain is so High that External Feedback is always used Output Signal is Fed Back to an Input Terminal

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Analysis of Ideal Op Amp Circuits Set V- = V+ and I- = I+ = 0 Write Nodal Equation(s)

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Basic Op Amp Circuits Voltage Follower: Impedance Matching or Isolation Unity Gain: V out = V in

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Inverting Amplifier V out = -(R 2 /R 1 )*V in

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Non-Inverting Amplifier V out = V in *(R 1 +R 2 )/R 1

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Summing Amplifier V out = -(V 1 *R f /R 1 + V 2 *R f /R 2 + V 3 *Rf/R 3 )

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Difference Amplifier V out = (V 2 -V 1 )*R 2 /R 1

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Analysis of the Voltage Follower Provides “Current Isolation” – No Power Drawn from Input Circuit

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References Heathkit, Electronic Circuits, EB-6104A, 2002 Alexander, Fundamentals of Circuit Analysis – 2 nd Edition, McGraw-Hill, 2004

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