Operational Amplifier Fundamentals Unit 8 – Voltage Comparators
Exercise 3 – Square Wave Converter
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to operate a sine wave to square wave
converter. You will verify your results with an oscilloscope.
EXERCISE DISCUSSION
- U1 is configured as an open-loop comparator that uses positive feedback and is referred to as
a Schmitt trigger. - Vi is converted from a sine wave into a square wave. Vo switches between +VSAT and -VSAT
and is in phase with Vi. - Positive feedback voltage (VTH) is provided by hysteresis resistor RH. Two trigger points are
established by the positive feedback. - VTH provides a voltage at the noninverting input of U1. Therefore, the circuit is not zero
based. - Hysteresis, or positive feedback, creates a window, or band, through which Vi must cross
before the comparator switches Vo. - Based on a specific hysteresis level, a switch in Vo occurs at trigger points A and B.
- Vo cannot switch back until Vi crosses a trigger point. Within the window, variations of Vi
do not cause U1 to switch. - The voltage difference between points A and B, labeled +VTT and -VTT, equals the circuit
hysteresis voltage. - An increase of positive feedback increases the width of the hysteresis window.
- A decrease of positive feedback decreases the width of the hysteresis window.
- Hysteresis reduces the sensitivity of a comparator, increasing the noise immunity of the
circuit. - Based on Vi, a zero based comparator (no hysteresis) generates many changes in Vo. The
number of transitions in Vo decreases as hysteresis increases. - Two equations, based on the saturation level of U1, define the circuit trigger voltages
- (+VT and -VT). +VT = +VSAT/(R2/R1)
-VT = -VSAT/(R2/R1) - The trigger voltages are based on the saturation voltage of U1. Therefore, the trigger points
are not symmetrical and vary from op amp to op amp.