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8 . Wave-shaping Circuits Design

8 . Wave-shaping Circuits Design. 8.1 Schmitt Trigger Design 8.2 Diode Clipping Circuits 8.3 Diode Clamping Circuits 8.4 Wave shaping circuits design 8.5 Frequency effect on the clampers . Comparator Circuits. Comparator IC = Two inputs of the IC are compared to each other as below:

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8 . Wave-shaping Circuits Design

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  1. 8. Wave-shaping Circuits Design 8.1 Schmitt Trigger Design 8.2 Diode Clipping Circuits 8.3 Diode Clamping Circuits 8.4 Wave shaping circuits design 8.5 Frequency effect on the clampers

  2. Comparator Circuits • Comparator IC = Two inputs of the IC are compared to each other as below: • When “V+” is more positive than “V-” Output Vo will go high to +VCC • ( = “V-” is more negative than “V+” ) • When “V-” is more positive than “V+” Output Vo will go low to –VCC • ( = “V+” is more negative than “V-” ) 8.1 Schmitt Trigger Circuit Design Output is binary either “+10V” or “-10V”

  3. When Reference is “0V” (Zero-crossing Comparator) Reference Input • When “V+” is more positive than “0” Output Vo will go high to +VCC • When “V+” is more negative than “0” Output Vo will go low to –VCC • When “V-” is more positive than “0” Output Vo will go low to -VCC • When “V-” is more negative than “0” Output Vo will go high to +VCC Input Output is digital either “+10” or “-10” Output is digital either “+10” or “-10” Reference

  4. Zero-crossing Comparator can be used for non-sinusoidal waveforms. It can be used to detect whether the signal has overshot a given level and what duration it has overshot, etc.

  5. When “V-” is more positive than Vref “-4” Output Vo will go low to -VCC • When “V-” is more negative than Vref “-4” Output Vo will go high to +VCC Output is digital When Reference is “Non-0V” (Reference-crossing Comparator)

  6. Reference-crossing Comparator can be used for non-sinusoidal waveforms. It can be used to detect whether the signal has overshot a given level and what duration it has overshot, etc.

  7. Analysis of Schmitt trigger If input voltage is higher than UTL vo will become VLO If input voltage is lower than LTL vo will become VHI

  8. Example: Find HTL and LTL of the given Schmitt Trigger circuit. Take ein = ±5V, VHI=+5V and VLO=-5V, VREF=3V. Find Hysterisis and Sketch the output waveform.

  9. ein Noise +5V vo -5V Example: An inverting Schmitt trigger has a sine wave input ein of amplitude 5V. If output level is to be VHI=+5V and VLO=-5V respectively, design R1 and feedback resistor R2 so as to eliminate random switching by a noise level between a Hysterisis from (-0.5) V to +1V. Take VREF=0.6V and R1 + R2=100k.

  10. input voltage output voltage • Diode conducts (becomes short) for voltages above zero • When diode conducts the output voltage = zero • Diode opens for voltages below zero • When diode opens the output voltage is the same as input voltage ( No current, no drop at “R” ) Diode Positive Clipper 8.2 Diode Clipping Circuits Positive peak is clipped to 0

  11. Diode conducts for input voltages below zero • When diode conducts the output voltage = zero • Diode opens for input voltages above zero • When diode opens the output voltage is the same as input voltage ( No current, no drop at “R” ) Diode Negative Clipper Negative peak is clipped to 0

  12. Diode conducts for input voltages above VB • When diode conducts the output voltage = VB • Diode opens for input voltages below VB • When diode opens the output voltage is the same as input voltage ( No current, no drop at “R” ) Diode Positive Clipper with positive bias voltage Positive peak is clipped to VB

  13. Diode conducts for input voltages below VB • When diode conducts the output voltage = VB • Diode opens for input voltages above VB • When diode opens the output voltage is the same as input voltage ( No current, no drop at “R” ) Diode Negative Clipper with positive bias voltage Negative peak is clipped to VB

  14. Diode conducts for input voltages below (-VB) • When diode conducts the output voltage = (-VB) • Diode opens for input voltages above (-VB) • When diode opens the output voltage is the same as input voltage ( No current, no drop at “R” ) Diode Negative Clipper with negative bias voltage Negative peak is clipped to -VB

  15. Diode conducts for input voltages above (-VB) • When diode conducts the output voltage = (-VB) • Diode opens for input voltages below (-VB) • When diode opens the output voltage is the same as input voltage ( No current, no drop at “R” ) Diode Positive Clipper with negative bias voltage Positive peak is clipped to -VB

  16. Diode1 (Left) conducts for input voltages above VB • When diode conducts the output voltage = VB • Diode2 (Right) conducts for input voltages below -VB • When diode conducts the output voltage = -VB Between VB and –VB no diode conducts & no current, no drop at “R” Output voltage = Input voltage Diode Double Clipper with bias voltage Positive peak is clipped to VB Negative peak is clipped to –VB

  17. 1.Average level Moves down to –1V 2.All negative peak below zero will be clipped to zero When VS > +1 Diode negative Series-Bias Negative Clipper -1 2V(p) When VS < +1

  18. 2 1 1.Average level is lifted to +1V 2.All negative peak below zero will be clipped to zero When VS < -1 When VS > -1 Diode positiveSeries-Bias Negative Clipper 1 2V(p)

  19. Diode attenuator

  20. negative peak clamp to zero Diode is used only to charge the capacitor Diode NegativeClamper 8.3 Diode Clamping Circuits

  21. positive peak clamp to zero Diode is used only to charge the capacitor Diode PositiveClamper

  22. negative peak clamp to +VB Diode is used only to charge the capacitor Diode Biased NegativeClamper

  23. positive peak clamp to +VB Diode is used only to charge the capacitor Diode Biased PositiveClamper

  24. Design of R 8.4 Wave shaping circuits design While designing the clipper, tolerance is given 10% at the output amplitude if RL = 10 R This condition should be satisfied at highest resistance seen from RL ( in this case, when the diode open at negative peaks ) When the diode short at positive peaks, resistance seen by RL is already zero which requires no design of R

  25. Design of C While designing the clamper, tolerance is given 10% capacitor discharge rate at the output amplitude if T = ( 1/f ) = 0.1 RL C or RL C = 10 T = 10/f This condition should be satisfied at highest resistance seen from RL ( in this case, when the diode open at input negative cycles )

  26. f f Capacitor discharge High frequency-OK Low frequency-not OK For high signal frequencies T < RC/10, Capacitor discharge very slow = capacitor charged voltage is nearly constant (Requirement for clamping) When signal frequencies become low, T > RC/10, Capacitor discharge follows the signal path = capacitor is short (becomes clipping circuit) 8.5 Frequency effect on the clampers Capacitor discharge should be very slow (RC high) to make capacitor voltage = VP

  27. Summary of Design Equations Wave Shaping Circuits Clipper Design RL = 10 R Attenuator Design R1 orR2<< 0.1 RL Clamper Design RL C = 10 T = 10/f If RC is not > 10T capacitor becomes coupling. Circuit will behave as a clipper.

  28. clamper clipper Design example Design all components in the following wave shaping circuit if the frequency of the input wave is 1kHz. and the load resistance RL = 100k • When the diode of the clipper opens, resistance seen by RL is R • Then R = 10k (= RL / 10 ) • The lowest resistance seen by C is R • Then C=10T/R=10/fR=10/(103x104)=10-6F=1 mF

  29. 0.09 kW 0.1R C R 10kHz 1 mF 0.9 kW example (a) Draw the clamping circuit where the positive peak is clamped to 0V. Input voltage is 20V(pp) sine wave having a frequency of 10kHz. If C = 1 mF with a series resistor of 0.1R , find the value of R (connected across the output terminals) required for clamping. Draw the input and output waveforms. Clamping circuit design is when RC > 10T RT C = 1.1Rx1x10-6 > 10 (1/f) = 10/10x103 or 1.1R > 1/103-6 > 103 R = 0.9 kW

  30. 0.09 kW 0.1R C R 5kHz 1 mF 0.9 kW (b) If the input into the above clamping circuit is changed to 5 kHz , 20V(pp) sine wave. Draw the input and output waveforms. Clamping circuit design is when RC > 10T RC = 103 x10-6 = 10–3 sec , 10 T = 10/5x103 = 2x10-3 RC is not > 10T , it cannot clamp , capacitor discharge will follow the signal path , making capacitor like a short. Circuit will become a clipper.

  31. 5 kHz , 20V(pp) 0.001mF 1k A clamping circuit is used with a 5 kHz , 20V(pp) sine wave. The time constant of the circuit is changed by using R = 1kW , C = 0.001 mF. Draw the input and output waveforms. What kind of wave shaping circuit is it? Clamping circuit design is when RC > 10T RC = 103 x10-9 = 10–6 sec , 10 T = 10/5x103 = 2x10-3 RC is not > 10T , it cannot clamp , capacitor discharge will follow the signal path , making capacitor like a short. Circuit will become a clipper.

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