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
• The Op-amp comparator
compares one analog voltage
level with another analog voltage
level, or some preset reference
voltage, VREF 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 ±Vsat , it can be said that it is a type of Analog to
Digital converter.

6. BASIC COMPARATOR (cont.)
• Sometimes, due to excessive
input voltage Vin, OP-AMP may
damage.
• To prevent it, we use two diodes,
as shown in figure. Because of
these diodes D1 and D2, 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 Vin may be a slow changing waveform, that is, a low
frequency signal.
• Therefore, it will take Vin more time to cross 0V; therefore, Vo 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 Vo may fluctuate between two saturation
voltages +Vsat and -Vsat, detecting zero reference crossings for noise voltages as well
as Vin.
• Both of these problems can be cured with the use of regenerative or positive feedback
that causes the output Vo 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 Vin triggers (changes the state of) the output
Vo every time it exceeds certaon voltage levels called the upper
threshold voltage Vut and lower threshold voltage Vlt, 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 R1 - R2 where the voltage across R1 is fed back to the
non-inverting input. The voltage across R1 is a variable
reference threshold voltage that depends on the value and
polarity of the output voltage Vo. When Vo = +Vsat, the voltage
across R1 is called the upper threshold voltage, Vut. The input
voltage must be slightly more positive than Vut in order to cause
the output Vo to switch from +Vsat to -Vsat.

13. SCHMITT TRIGGER (cont.)
• As long as Vin<Vut, Vo is at +Vsat. Using voltage-divider rule,
• And when Vin is slighly more negative than Vlt in order to cause
Vo to switch from +Vsat to -Vsat. In other words, for Vin values
greater than Vlt, Vo is at -Vsat.
)(
21
1
satut V
RR
R
V 



14. SCHMITT TRIGGER (cont.)
• Vlt 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.
)(
21
1
satlt V
RR
R
V 



15. SCHMITT TRIGGER (cont.)
)]([
21
1
VsatVsat
RR
RV
VVV
hy
ltuthy



• The hysteresis voltage is, of
course, equal to the
difference between Vut and
Vlt. 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.

17. THANK YOU