Describe Schmitt trigger.

Schmitt Trigger

A Schmitt trigger is a comparator circuit with hysteresis, or positive feedback. This circuit exhibits two distinct switching thresholds for its input, one for when the input is rising and one for when the input is falling. This behavior is useful in applications where a clean and stable switching action is required, such as in digital logic circuits and oscillators.

Operation

The Schmitt trigger consists of two basic components:

1. A comparator circuit that compares the input signal to a reference voltage.
2. A feedback loop that provides positive feedback to the comparator, causing it to switch states more abruptly.

When the input signal is below the reference voltage, the comparator's output is low. This causes the feedback loop to pull the input signal even lower, creating a stable low state.

When the input signal rises above the reference voltage, the comparator's output goes high. This causes the feedback loop to pull the input signal even higher, creating a stable high state.

The difference between the two switching thresholds is called hysteresis. Hysteresis is what gives the Schmitt trigger its characteristic "snap" action.

Applications

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

• Digital logic circuits: Schmitt triggers can be used to clean up noisy digital signals and to provide a stable switching action.
• Oscillators: Schmitt triggers can be used to generate square waves and other waveforms.
• Level detectors: Schmitt triggers can be used to detect when a signal reaches a certain level.
• Voltage comparators: Schmitt triggers can be used to compare two voltages and to generate an output signal based on the comparison.

Analysis

The following equations describe the operation of a Schmitt trigger:

• Upper switching threshold:

$$V_{UT} = V_{REF} + V_{H}$$

• Lower switching threshold:

$$V_{LT} = V_{REF} - V_{H}$$

where:

• $$V_{UT}$$ is the upper switching threshold.
• $$V_{LT}$$ is the lower switching threshold.
• $$V_{REF}$$ is the reference voltage.
• $$V_{H}$$ is the hysteresis voltage.

Design

The design of a Schmitt trigger is based on the following considerations:

• The reference voltage must be chosen so that the input signal crosses both the upper and lower switching thresholds.
• The hysteresis voltage must be large enough to prevent the circuit from oscillating or chattering.
• The feedback loop must be designed to provide sufficient positive feedback to cause the circuit to switch states abruptly.

Conclusion

The Schmitt trigger is a versatile circuit that can be used in a wide variety of applications. Its unique properties make it ideal for applications where a clean and stable switching action is required.