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Chapter 31

Chapter 31. Applications of Op-Amps. Comparators. Op-amp as a Comparator No negative feedback Output saturates with very small + or – input. Comparators. Comparator Non-linear device v out has two discrete values, ± V SAT v out = + V SAT if + input is greater than – input

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Chapter 31

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  1. Chapter 31 Applications of Op-Amps

  2. Comparators • Op-amp as a Comparator • No negative feedback • Output saturates with very small + or – input

  3. Comparators • Comparator • Non-linear device • vout has two discrete values, ±VSAT • vout = +VSAT if + input is greater than – input • vout = –VSAT if – input is greater than + input

  4. Comparators • A comparator circuit: Sine wave in, square wave out

  5. Comparators • Input sine wave • Output square wave Vout = ±VSAT • +VSAT (determined by VCC) when sinusoid is + • –VSAT (determined by VEE) when sinusoid is – VCC + - 741 vout __ _ - __ _ - VEE

  6. Comparators • Compare input waveform to reference • Reference can be ground or dc source • Can compare two waveforms • Specialized comparator IC’s also available • Detects when waveform reaches given level

  7. Comparators • Zero-Crossing Detector

  8. Voltage Summing Amplifier • Circuit

  9. Voltage Summing Amplifier • Inverse sum

  10. Voltage Summing Amplifier • Multiplies each input by

  11. Integrators and Differentiators • In general • Using resistors and capacitors • Integrators • Differentiators ZF - + vin Z1 vout __ _ -

  12. Integrators and Differentiators • Voltage across capacitor • Current through capacitor

  13. Integrators and Differentiators • Op-amp Integrator C R1 - + vin i = 0 i 0 V vout __ _ -

  14. Integrators and Differentiators • Op-amp differentiator • Circuit inherently unstable + - RF Cin i - + vin i vout __ _ -

  15. Integrators and Differentiators • Stable op-amp differentiator

  16. Instrumentation Amplifiers • Op-amp in differential amplifier configuration • Noise suppression • High CMRR • Reasonable gain • IC instrumentation amps

  17. Instrumentation Amplifiers • An op-amp instrumentation amp circuit

  18. Instrumentation Amplifiers • Measurement of very small voltages • Transducer • Converts a physical change into an electrical change

  19. Instrumentation Amplifiers • Strain gage • Converts force into ∆R • ∆R is milliohms • Use bridge circuit

  20. Instrumentation Amplifiers • Strain gage example • Thin metal foil (resistor) on plastic backing • Glued to metal bar • Bar subjected to tension and compression

  21. Instrumentation Amplifiers • Strain gage example • Tension • Resistance of strain gage is R + ∆R • Compression • Resistance of strain gage is R –∆R

  22. Active Filters • Basic filter types • Passive elements, gain < 1 • Low-pass • High-pass • Bandpass • Band reject

  23. Active Filters • With op-amps/active filters • Gain can be ≥ 1 • Filter response closer to ideal

  24. Active Filters • Low-pass (RF= R1) • Add resistor for gain > 1 RF = R1 I = 0 - + 0 V R1 vout vin C __ _ -

  25. Active Filters • High-pass (RF= R1) • Add resistor for gain > 1 RF = R1 I = 0 - + 0 V C vout vin R1 __ _ -

  26. Active Filters • dc gain • Easily achieved • Not used much due to gain-bandwidth product • Example • GBWP = 106, Gain = 10 • Cutoff for filter (HP or LP) only 105

  27. Active Filters • Bandpass • Wideband • Cascade HP and LP active filters • LP must have higher cutoff frequency • HP and LP cutoff frequencies far apart • Narrowband • Can use single op-amp

  28. Active Filters • Narrowband BP circuit C C vin 2R - + R vout R1 __ _ - __ _ -

  29. Active Filters • Active notch filter • Cascade narrowband BP filter • Adder circuit • Result is 1 – (frequency response of BP filter) • Frequency at resonant frequency of BP filter will be eliminated

  30. Voltage Regulation • Voltage regulator • Constant voltage to load • Specified current range • Specified input voltage range • Zener diode regulator • Inefficient • Dissipates power

  31. Voltage Regulation • Types of regulators • Fixed voltage regulator • Variable voltage regulator • Switching regulator • Specialized IC regulators • For different voltages, e.g. +5 V, –5 V, +12 V, –12 V, +15 V, –15 V, etc.

  32. Voltage Regulation • Line Regulation • Small output change with change in input Regulated output Unregulated input Voltage regulator RL __ _ -

  33. Voltage Regulation • Load regulation • Small output voltage change with smaller RL • VNL = no-load voltage (open-circuit load) • VFL = full-load voltage (specified by manufacturer)

  34. Voltage Regulation • Circuit to increase efficiency of Zener regulator with an op-amp Q1 Unregulated input + RD + R1 + - vin vout R2 - - __ _ -

  35. Voltage Regulation • Three-terminal IC regulators • 7800 series, positive voltage • 7900 series, negative voltage

  36. Voltage Regulation • 5 V output, 7805 • 12 V output, 7812 • –5 V output, 7905 • –12 V output, 7912 IN OUT μA7812 + + COM Unregulated input __ _ - Vout=12 V RL - - __ _ -

  37. Voltage Regulation • Ripple • Greatly reduced by IC regulator Vr(in) = input ripple voltage Vr(out) = output ripple voltage

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