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MALVINO & BATES

MALVINO & BATES. ELECTRONIC PRINCIPLES. SEVENTH EDITION. Chapter. 22. Nonlinear Op-Amp Circuits. Topics Covered in Chapter 22. Comparators with zero reference Comparators with non-zero references Comparators with hysteresis Window comparator The integrator Waveform conversion.

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MALVINO & BATES

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  1. MALVINO & BATES ELECTRONIC PRINCIPLES SEVENTH EDITION

  2. Chapter 22 Nonlinear Op-Amp Circuits

  3. Topics Covered in Chapter 22 • Comparators with zero reference • Comparators with non-zero references • Comparators with hysteresis • Window comparator • The integrator • Waveform conversion

  4. Topics Covered in Chapter 22(Continued) • Waveform generation • Another triangular generator • Active diode circuits • The differentiator • Class D amplifier

  5. Comparators with zero reference • Called zero-crossing detectors • Diode clamps used to protect comparators against large inputs • Usually interface their outputs with digital circuits

  6. Diode clamps for protection +Vsat -Vsat Inverting comparator +VCC AVOL @ 100,000 vin vout -VEE vin vout

  7. Inverting comparator with input bias compensation +VCC RB vin vout -VEE RB The trip point of a zero-crossing detector should be near zero volts.

  8. +VZ + 0.7 -VZ - 0.7 Bounded output zero-crossing detector +VCC . . vin vout -VEE vin vout

  9. Comparators with non-zero references • Sometimes called limit detectors • Op amps may be used • IC comparators are optimized by removing the internal compensating capacitor which increases switching speed

  10. R2 Vref = (−VEE) R1+R2 Non-inverting threshold detector +VCC vin vout R2 . -VEE R1 vin Vref +Vsat vout -Vsat

  11. IC comparators • Op amps have speed limitations due to the internal compensation capacitor. • Comparator ICs are optimized for non-linear operation so the capacitor is not needed. • Comparator ICs often have open-collector outputs to make interfacing more flexible.

  12. Using a pullup resistor with an open-collector output stage +V pullup resistor . vout IC comparator When the transistor is off, the resistor pulls vout up to +V.

  13. Comparators with hysteresis • Noise is any unwanted signal not related to the input • Noise can cause false triggering • Positive feedback creates hysteresis • Hysteresis prevents false triggers and speeds up switching

  14. R1 B = R1+R2 UTP LTP The Schmitt trigger uses positive feedback +VCC vin . vout UTP = BVsat -VEE R1 . LTP = -BVsat R2 H = 2BVsat vin +Vsat vout -Vsat

  15. UTP LTP vin +Vsat vout -Vsat Comparing a zero-crossing detector to a Schmitt trigger when both have noisy input signals. vin +Vsat vout -Vsat

  16. Window comparator • Called a double-endedlimit detector • Detects when the input voltage is between two limits • Uses two comparators with two different trip points

  17. Window comparator operation +VCC vout LTP +Vsat D1 . . . vin -VEE vout LTP HTP vin +VCC RL D2 Output is low when input is in window HTP -VEE

  18. Integrator • Useful for converting rectangular pulses into linear ramps • Earliest part of exponential charge is used • Output ramps are almost perfect • Used to create time bases for oscilloscopes

  19. T -T V = vin RC V Integrator circuit 10R . R C . vin vin vout 0 0 vout

  20. Waveform conversion • A Schmitt trigger can be used to convert a sine wave to a rectangular wave • An integrator can convert a square wave to a triangular wave • The duty cycle of a limit detector can be controlled with an adjuster resistor

  21. VP vout(pp) = 2fRC Rectangular to triangular converter 10R . R C . vin +VP vout vin -VP vout

  22. R2 Vref = VCC R1+R2 Vref Triangle to pulse converter vin vout R2 . +VCC R1 vin vout

  23. Waveform generation • Oscillators require no external input and still generate an output signal • Oscillators work because of positive feedback • A relaxation oscillator uses a charging capacitor • A triangular waveform is produced by cascading a relaxation oscillator and an integrator

  24. UTP LTP 1+B 1-B T = 2RCln Relaxation oscillator R . vout C . vC R2 R1 vout

  25. Triangular waveform generator R5 C2 . R3 . . R4 . C1 vout . R2 R1

  26. R1 R2 UTP = Vsat R2 f = 4R1 R3 C Another triangular waveform generator R2 R4 . . . R3 .. C . vout R1 Schmitt trigger and cascaded integrator H = 2UTP Vout(pp) = H

  27. Active half-wave rectifier . vin RL VK VK(CL) = AVOL An op amp with negative feedback allows the rectification of signals with amplitudes less than the diode knee voltage.

  28. Active peak detector with reset . . RLC > 10T . vin C RL Reset

  29. Active positive clamper . . 0 vin RL 0 An op amp with negative feedback allows the clamping of signals with amplitudes less than the diode knee voltage.

  30. Differentiator • When a square wave drives an RCdifferentiator, the output is a series of narrow positive and negative voltage spikes • Using an op amp will improve the differentiator and provide a low output impedance

  31. Differentiator circuit and operation R . C vin . vout 0.01R to 0.1R

  32. Class D amplifier • Uses output transistors as switches • These transistors are alternately driven into saturation and cutoff by the output of a comparator • Provides very high circuit efficiencies • Popular in portable equipment

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