Schmitt Trigger: Circuits, Working, and Applications

Introduction to Schmitt Triggers

A Schmitt trigger is a comparator circuit that incorporates positive feedback to prevent noise and provide hysteresis. It is used to convert analog input signals into clean digital output signals. The Schmitt trigger is named after American scientist Otto H. Schmitt who invented it in 1934.

The key features of a Schmitt trigger are:

• It has two threshold voltages – an upper threshold voltage (VTH) and a lower threshold voltage (VTL)
• The output switches cleanly between low and high states depending on whether the input is above VTH or below VTL
• The hysteresis between VTH and VTL prevents output chatter due to noisy input signals near the threshold
• It can be implemented using transistors, op-amps, or dedicated Schmitt trigger ICs

How a Schmitt Trigger Works

Inverting Schmitt Trigger Operation

An inverting Schmitt trigger circuit and its voltage transfer characteristic (VTC) are shown below:

[Inverting Schmitt trigger circuit diagram]

Vin	Vout
Vin < VTL	VOH
VTL < Vin < VTH	Previous state
Vin > VTH	VOL

The operation is as follows:

1. When Vin is less than the lower threshold voltage VTL, the output Vout is at the high level VOH.

2. As Vin increases above VTL, Vout remains at VOH. This is because of the hysteresis introduced by the positive feedback.

3. When Vin exceeds the upper threshold voltage VTH, Vout switches low to VOL.

4. As Vin decreases, Vout remains at VOL until Vin drops below VTL, at which point it switches back high to VOH.

The hysteresis voltage Vhys is the difference between the two threshold voltages:

Vhys = VTH – VTL

It determines the minimum input voltage change required to switch the output state. A larger hysteresis provides better noise immunity but reduces sensitivity.

Non-Inverting Schmitt Trigger Operation

A non-inverting Schmitt trigger has the opposite behavior – the output goes high when the input exceeds the upper threshold. The circuit and VTC are shown below:

[Non-inverting Schmitt trigger circuit diagram]

Vin	Vout
Vin < VTL	VOL
VTL < Vin < VTH	Previous state
Vin > VTH	VOH

The operation is similar to the inverting case, just with the high and low output states swapped.

Schmitt Trigger Implementations

Transistor-Based Schmitt Trigger

A simple Schmitt trigger can be built using two transistors as shown:

[Transistor Schmitt trigger schematic]

Resistors R1, R2 and R3 set the threshold voltages according to:

VTL = (VCC * R3) / (R2 + R3)
VTH = VTL + (VCC * R1) / (R1 + R2)

The main drawback of this implementation is that the threshold voltages depend on the transistor characteristics which can vary significantly. More complex designs using current mirrors provide better accuracy.

Op-Amp Based Schmitt Trigger

An op-amp Schmitt trigger offers adjustable thresholds and a clean output signal. A basic inverting op-amp Schmitt trigger is shown below:

[Op-amp Schmitt trigger schematic]

The threshold voltages are set by the resistor ratios:

VTL = (V- * R3) / (R1 + R3)
VTH = VTL + (VOH * R2) / (R1 + R2)

where V- is the op-amp negative supply voltage and VOH is its maximum output voltage. A non-inverting configuration swaps the resistors on the non-inverting input.

Op-amp Schmitt triggers are easy to design and widely used. Their main limitation is speed – op-amps have a limited slew rate which restricts the maximum operating frequency.

Dedicated Schmitt Trigger ICs

Many logic IC families include Schmitt trigger versions of their basic gates, e.g. 74HC14 is a hex inverting Schmitt trigger in the 74HC CMOS family. These dedicated ICs offer several advantages:

• The thresholds are tightly controlled for reliable switching
• They can operate at higher frequencies than op-amp designs
• The hysteresis levels are optimized for typical digital signal levels
• Some types feature selectable hysteresis using an external resistor

The main trade-off is less flexibility compared to a discrete design. The thresholds and hysteresis are fixed or have limited adjustment range.

Applications of Schmitt Triggers

Input Conditioning

A common use of Schmitt triggers is to clean up noisy input signals for reliable switching, for example:

• Mechanical switch debouncing – a Schmitt trigger ignores the contact bounce when a switch is opened or closed
• Converting slowly changing waveforms like sine waves to clean square waves
• Restoring digital signals degraded by noise or distortion during transmission

Oscillator Design

Schmitt triggers are often used in relaxation oscillators. A capacitor charges and discharges between the two threshold voltages, making the output oscillate. The 555 timer IC is a well-known example that uses a Schmitt trigger oscillator.

The oscillation frequency depends on the capacitor value and the charging/discharging currents which are set by resistors. This allows simple oscillators with a wide frequency range.

Analog-to-Digital Conversion

Schmitt triggers can act as 1-bit analog-to-digital converters. The input signal is applied to the Schmitt trigger and the output duty cycle represents the analog level. Slow changing analog signals like temperature can be easily digitized this way.

Level Shifting

Schmitt triggers can translate between different logic voltage levels. For example, a 5V Schmitt trigger can be used to convert 3.3V logic signals to 5V levels. The hysteresis ensures a clean level transition and prevents multiple output transitions due to slow input edges.

Schmitt Trigger Design Considerations

When choosing or designing a Schmitt trigger circuit, the key parameters to consider are:

• Input voltage range – the expected minimum and maximum input voltages must be within the Schmitt trigger input range

• Threshold voltages – VTL and VTH should be selected to provide reliable switching for the input signal range and noise level

• Hysteresis voltage – a larger hysteresis voltage provides better noise immunity but reduces the sensitivity to input changes

• Output voltage levels – VOH and VOL must be compatible with the load circuitry

• Propagation delay – the delay between the input crossing the threshold and the output changing state, important for high-speed applications

• Power consumption – CMOS Schmitt triggers generally have lower power consumption than bipolar types, but may have higher propagation delays

• Package / form factor – Schmitt triggers are available in standard logic IC packages as well as in smaller surface mount types for space constrained designs

Frequently Asked Questions (FAQ)

Q: What is the main advantage of using a Schmitt trigger over a regular comparator?

A: The key advantage of a Schmitt trigger is its hysteresis, which prevents the output from changing state repeatedly when the input is near the threshold voltage. This makes Schmitt triggers much more resistant to noise compared to regular comparators.

Q: Can a Schmitt trigger be used as an amplifier?

A: No, a Schmitt trigger is not designed to amplify signals. Its output only has two states, high and low. For amplification, an operational amplifier or another type of anaLog Amplifier should be used.

Q: How do you set the threshold voltages in a Schmitt trigger circuit?

A: In a transistor-based or op-amp based Schmitt trigger, the threshold voltages are set by the ratios of the resistors connected to the input. In dedicated Schmitt trigger ICs, the thresholds are fixed internally or adjustable via an external resistor.

Q: What determines the oscillation frequency in a Schmitt trigger oscillator?

A: The oscillation frequency depends on the time taken for the capacitor to charge and discharge between the two threshold voltages. This time depends on the capacitor value and the charging/discharging currents which are set by resistors. Smaller capacitances and larger currents result in higher frequencies.

Q: Can a Schmitt trigger be used with negative input voltages?

A: It depends on the specific Schmitt trigger circuit. Transistor-based and op-amp based Schmitt triggers can be designed to work with a wide range of input voltages including negative voltages. However, most dedicated Schmitt trigger ICs are designed for positive input voltages only. If negative inputs are required, a level shifting circuit must be added before the Schmitt trigger input.

Conclusion

Schmitt triggers are versatile circuits that play an important role in many electronic systems. Their key feature of hysteresis allows them to cleanly convert noisy analog signals into digital levels. The different implementations – transistor-based, op-amp based, and dedicated ICs – offer designers flexibility to optimize their designs for parameters like threshold voltages, output levels, speed and power consumption.

Understanding the operating principle and design trade-offs allows effective utilization of Schmitt triggers in applications like input conditioning, oscillator design, analog-to-digital conversion and level translation. As with most engineering decisions, the best implementation choice depends on the specific circuit requirements and constraints.