Comparator, Zero Crossing Detector and schmitt trigger using opamp

Comparator, Zero Crossing Detector and Schmitt Trigger using LM741

1. AEC Project Prepared by Divyanshu Rai (1804297) Vinayak Malviya (1804349) ETC-05

2. Contents... Introduction Basic Comparator Zero Crossing Detector Schmitt Trigger Comparator Characteristics

3. Introduction The amplifier, filter, and oscillator applications illustrate a fair sampling of typical OP-AMP uses. However, OP-AMPs are used in many other circuits where they are employed under specific names. Such circuits include comparators, detectors, limiters, and digital interface devices. In this presentation, we will discuss comparators, limiters, detectors and converter using general purpose OP-AMP.

4. BASIC COMPARATOR T he Op-amp comparator compares one analog voltage level with another analog voltage level, or some preset reference voltage, V REF and produces an output signal based on this voltage comparison. In other words, the op-amp voltage comparator compares the magnitudes of two voltage inputs and determines which is the largest of the two.

5. BASIC COMPARATOR (cont.) As comparator's output voltage changes and varies between ±V sat , it can be said that it is a type of Analog to Digital converter.

6. BASIC COMPARATOR (cont.) Sometimes, due to excessive input voltage V in , OP-AMP may damage. To prevent it, we use two diodes, as shown in figure. Because of these diodes D 1 and D 2 , the diffrential input voltage Vid of the OP-AMP is clamped to either +0.7V or -0.7V; hence they are called Clamp Diodes.

7. ZERO-CROSSING DETECTOR An immediate application of the comparator is the Zero-Crossing Detector or Sine wave to Square wave converter. We can use both, inverting and non-inverting comparators for Zero-Crossing Detector. The difference will come in the output waveform (as shown in figure in next slide).

8. ZERO-CROSSING DETECTOR (cont.) Circuit diagram of Zero Crossing detector with inverting configuration Output waveform of the adjacent Zero Crossing Detector circuit

9. ZERO-CROSSING DETECTOR (cont.) In some applications, the input V in may be a slow changing waveform, that is, a low frequency signal. Therefore, it will take V in more time to cross 0V; therefore, V o may not switch quickly from one saturation levelto another. On the other hand, because of the noise at the OP-AMP's input terminals, the output V o may fluctuate between two saturation voltages +V sat and -V sat , detecting zero reference crossings for noise voltages as well as V in . Both of these problems can be cured with the use of regenerative or positive feedback that causes the output V o to change faster and eliminate any false output transitions due to noise signals at the input.

10. SCHMITT TRIGGER Inverting comparator with positive feedback is known as the Schmitt Trigger or Squaring Circuit. It converts irregular-shaped waveforms to a square wave pulse. The input voltage V in triggers (changes the state of) the output V o every time it exceeds certaon voltage levels called the upper threshold voltage V ut and lower threshold voltage V lt , as shown in figure.

11. SCHMITT TRIGGER (cont.) Circuit diagram of Schmitt Trigger Output waveform of the adjacent Schmitt Trigger circuit

12. SCHMITT TRIGGER (cont.) These threshold voltages are obtained by using the voltage divider R 1 - R 2 where the voltage across R 1 is fed back to the non-inverting input. The voltage across R 1 is a variable reference threshold voltage that depends on the value and polarity of the output voltage V o . When V o = +V sat , the voltage across R 1 is called the upper threshold voltage, V ut . The input voltage must be slightly more positive than V ut in order to cause the output V o to switch from +V sat to -V sat .

13. SCHMITT TRIGGER (cont.) As long as V in <V ut , V o is at +V sat . Using voltage-divider rule, And when V in is slighly more negative than V lt in order to cause V o to switch from +V sat to -V sat . In other words, for V in values greater than V lt , V o is at -V sat .

14. SCHMITT TRIGGER (cont.) V lt is given by following equation, When the input is a triangular wave, the output of the Schmitt trigger is a square wave, whereas if the input is a sawtooth wave, the output is a pulse waveform.

15. SCHMITT TRIGGER (cont.) The hysteresis voltage is, of course, equal to the difference between V ut and V lt . Therefore,

16. SCHMITT TRIGGER'S APPLICATIONS Schmitt triggers are typically used in open loop configurations for noise immunity and closed loop configurations to implement function generators. Analog to digital conversion:   The Schmitt trigger is effectively a one bit analog to digital converter. When the signal reaches a given level it switches from its low to high state. Level detection .    Line reception .

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