Describe Schmitt trigger.

Schmitt Trigger

A Schmitt trigger is a comparator with hysteresis, meaning that it has two different threshold levels for switching. This is in contrast to a standard comparator, which has only one threshold level. The hysteresis of a Schmitt trigger allows it to be used in applications where the input signal is noisy or unstable, as it will not switch back and forth between its two states as easily as a standard comparator.

Operation

A Schmitt trigger is typically implemented using an operational amplifier (op-amp). The op-amp is configured as a comparator, with the input signal applied to the non-inverting input and a reference voltage applied to the inverting input. The output of the op-amp is then fed back to the inverting input through a resistor.

The feedback resistor creates a positive feedback loop, which causes the Schmitt trigger to have two different threshold levels. When the input signal is below the lower threshold level, the output of the op-amp is low. This causes the feedback resistor to pull the inverting input down, which in turn causes the output of the op-amp to go even lower. This positive feedback loop continues until the output of the op-amp reaches the lower threshold level.

At this point, the feedback resistor is no longer able to pull the inverting input down any further. This causes the output of the op-amp to start to rise. The positive feedback loop then causes the output of the op-amp to rise even further, until it reaches the upper threshold level.

When the input signal is above the upper threshold level, the output of the op-amp is high. This causes the feedback resistor to pull the inverting input up, which in turn causes the output of the op-amp to go even higher. This positive feedback loop continues until the output of the op-amp reaches the upper threshold level.

At this point, the feedback resistor is no longer able to pull the inverting input up any further. This causes the output of the op-amp to start to fall. The positive feedback loop then causes the output of the op-amp to fall even further, until it reaches the lower threshold level.

Applications

Schmitt triggers are used in a wide variety of applications, including:

• Noise filtering: Schmitt triggers can be used to filter out noise from a signal. The hysteresis of the Schmitt trigger prevents the output from switching back and forth between its two states as easily as a standard comparator, which makes it more resistant to noise.
• Debouncing: Schmitt triggers can be used to debounce a switch. When a switch is pressed, it can bounce back and forth between its open and closed states several times before it finally settles into one state. A Schmitt trigger can be used to prevent these bounces from causing the output of the circuit to change.
• Level shifting: Schmitt triggers can be used to level shift a signal. The upper and lower threshold levels of the Schmitt trigger can be set to the desired output levels.
• Oscillators: Schmitt triggers can be used to create oscillators. The positive feedback loop of the Schmitt trigger can be used to create a sustained oscillation.

Advantages and Disadvantages

Advantages:

• Noise immunity: Schmitt triggers are more immune to noise than standard comparators.
• Debouncing: Schmitt triggers can be used to debounce switches.
• Level shifting: Schmitt triggers can be used to level shift signals.
• Oscillators: Schmitt triggers can be used to create oscillators.

Disadvantages:

• Slower response time: Schmitt triggers have a slower response time than standard comparators.
• Hysteresis: The hysteresis of a Schmitt trigger can make it difficult to control the output of the circuit.

Conclusion

Schmitt triggers are versatile devices that can be used in a wide variety of applications. Their hysteresis makes them more immune to noise and more resistant to bouncing than standard comparators. However, their slower response time and the need to control the hysteresis can make them more difficult to use in some applications.