Operational Amplifier Fundamentals Unit 8 – Voltage Comparators
Zener diode CR1 provides a feedback path to limit the voltage swing of VO, thus preventing U1
saturation.
If VI equals 0V, VD equals 0V. VO is not 0V because of the characteristics of CR1 when it
"sees" insufficient current.
If VI is positive, VO equals -VZ (CR1's zener voltage), but VD equals 0V.
If VI is negative, VO equals +VF (CR1's forward diode drop), but VD equals 0V.
This circuit uses the noninverting input side of U1 as the circuit input. If Vi is positive, Vo is
positive; if Vi is negative, Vo is negative.
Positive feedback through resistor R2 generates hysteresis. Hysteresis prevents false triggering of
U1 around the zero crossing points of Vi by increasing the width of the threshold voltage