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EUT 1040. Lecture 4: Operational Amplifiers. Differential Amplifier. Not Practical Prior to IC Fabrication 2 Inputs, Output is A v *(V 1 - V 2 ). Common Mode Rejection. Differential Amps won’t Amplify Signals that are Common to both Inputs. Operational Amplifier. Three Stages:

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Eut 1040 l.jpg

EUT 1040

Lecture 4:

Operational Amplifiers


Differential amplifier l.jpg
Differential Amplifier

  • Not Practical Prior to IC Fabrication

  • 2 Inputs, Output is Av*(V1 - V2)


Common mode rejection l.jpg
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 (A = 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, I- = I+ = 0)

  • Voltage Gain is Infinite (A = Infinity)

  • Inverting and Non-inverting inputs at equal potential (V+ = V- )


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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: Vout = Vin


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Inverting Amplifier

  • Vout = -(R2/R1)*Vin


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Non-Inverting Amplifier

  • Vout = Vin*(R1+R2)/R1


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Summing Amplifier

  • Vout = -(V1*Rf/R1 + V2*Rf/R2 + V3*Rf/R3)


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Difference Amplifier

  • Vout = (V2-V1)*R2/R1


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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 – 2nd Edition, McGraw-Hill, 2004