Schmitt Trigger Circuit Diagram, Working, Types and Applications

The Schmitt trigger is a bistable circuit widely used in digital and analog applications. It is primarily employed for signal conditioning, converting noisy, analog signals into clean, digital ones, and plays a critical role in eliminating the ambiguity associated with fluctuating or noisy input signals. A distinctive feature of the Schmitt trigger is the concept of hysteresis, providing two distinct threshold voltages – upper and lower that prevent false triggering.

What is a Schmitt Trigger Circuit?

A Schmitt trigger circuit is a type of comparator circuit with hysteresis. This hysteresis means it has two distinct threshold voltages—upper threshold voltage (V_UT) and lower threshold voltage (V_LT). The output of the Schmitt trigger changes state only when the input voltage crosses these thresholds, making it highly resistant to noise.

Types of Schmitt Trigger Circuits:

• Inverting Schmitt Trigger: The output voltage is inverted relative to the input voltage.
• Non-inverting Schmitt Trigger: The output voltage has the same polarity as the input voltage.

Schmitt Trigger Circuit Design:

A Schmitt trigger circuit can be designed using operational amplifiers (op-amps) or transistors. Here, we’ll primarily focus on the op-amp design, which is more common in practical applications.

1. Inverting Schmitt Trigger Circuit:

In the inverting Schmitt trigger circuit, the input signal is applied to the inverting terminal of the op-amp, and feedback is provided through a resistor network. The circuit diagram is shown below:

Circuit Diagram:

• An op-amp with input V_in connected to the inverting terminal.
• Feedback resistor R₂ and reference resistor R₁.
• Non-inverting terminal grounded.
• The output will switch between +V_sat (positive saturation voltage) and -V_sat (negative saturation voltage) depending on the input crossing the threshold levels.

2. Non-inverting Schmitt Trigger Circuit:

In a non-inverting Schmitt trigger, the input signal is applied to the non-inverting terminal. The output follows the same polarity as the input but with hysteresis applied.

Circuit Diagram:

• An op-amp with input V_in connected to the non-inverting terminal.
• Feedback resistor R₂ and series resistor R₁.
• The inverting terminal is grounded.

Threshold Voltages for Schmitt Trigger Circuit:

For both types of Schmitt triggers, the upper and lower threshold voltages can be derived based on the feedback resistor network.

Inverting Schmitt Trigger Thresholds:

Upper Threshold Voltage (V_UT):

V_UT = (R₁ / (R₁ + R_f)) V_sat

Lower Threshold Voltage (V_LT):

V_LT = – (R₁ / (R₁ + R_f)) V_sat

Non-inverting Schmitt Trigger Thresholds:

Upper Threshold Voltage (V_UT):

V_UT = (R₁ + R_f) / R₁ * V_sat

Lower Threshold Voltage (V_LT):

V_LT = – (R₁ + R_f) / R₁ * V_sat

Understanding Hysteresis in Schmitt Trigger Circuit:

The concept of hysteresis in a Schmitt trigger circuit refers to the intentional introduction of two different threshold voltages for switching. This characteristic ensures a stable and noise-resistant operation, making Schmitt triggers ideal for converting noisy signals into clean digital signals.

Key Features of Hysteresis:

Two Threshold Levels:

• Upper Threshold (V_UT): The input voltage at which the output switches from low to high.
• Lower Threshold (V_LT): The input voltage at which the output switches from high to low.

How Hysteresis Works in a Schmitt Trigger:

• When the input voltage rises and crosses V_UT, the output transitions to a high state.
• As the input voltage falls, the output does not immediately transition back to low. Instead, it waits until the input drops below V_LT.
• This ensures the output remains stable, even in the presence of noise or small variations around the threshold levels.

In Schmitt trigger circuit Hysteresis refers to the phenomenon where the circuit has two distinct switching thresholds: one for when the input is increasing (upper threshold) and one for when it is decreasing (lower threshold). This difference between the upper and lower thresholds helps prevent the circuit from toggling back and forth due to minor fluctuations or noise on the input signal.

Hysteresis Width/Band:

ΔVh = VUT - VLT

Hysteresis Band:

• The difference between the upper and lower thresholds (ΔV = V_UT − V_LT) defines the hysteresis band.
• This band prevents small fluctuations in the input signal from causing multiple unwanted output transitions.

Transfer Characteristics of Schmitt Triggers:

Inverting Schmitt Trigger Transfer Characteristics:

In an inverting Schmitt trigger, the output voltage toggles inversely to the input voltage crossing the thresholds:

• When V_in > V_UT, the output transitions to -V_sat.
• When V_in < V_LT, the output transitions to +V_sat.

Non-inverting Schmitt Trigger Transfer Characteristics:

In a non-inverting Schmitt trigger, the output voltage toggles directly with the input:

• When V_in > V_UT, the output transitions to +V_sat.
• When V_in < V_LT, the output transitions to -V_sat.

Waveform Analysis of Schmitt Trigger:

The waveform for a Schmitt trigger circuit will show the input signal gradually increasing and decreasing, while the output signal switches states only when the input crosses the threshold values V_UT and V_LT. Due to hysteresis, there is a lag in the switching, which prevents the output from oscillating due to minor fluctuations near the threshold levels.

Advantages of Schmitt Trigger:

1. Noise Immunity:
   The Schmitt trigger effectively rejects noise on the input signal due to its hysteresis, ensuring a clean output transition even when the input signal has small fluctuations.
2. Improved Signal Integrity:
   Converts noisy or slow rising/falling input signals into sharp digital pulses with defined thresholds.
3. Eliminates Chattering:
   Prevents multiple transitions at the output for input signals that are near the threshold levels, reducing oscillation and glitches.
4. Adjustable Thresholds:
   In designs with external components (e.g., resistors), the upper and lower threshold levels can be customized to suit specific applications.
5. High-Speed Operation:
   Many Schmitt trigger circuits can operate at high speeds, making them suitable for applications like clock generation and pulse shaping.
6. Versatile Applications:
   Used in oscillators, waveform generators, signal conditioning, and level detection circuits.
7. Reduced Power Consumption:
   In many implementations (e.g., CMOS-based), it consumes minimal power during steady states.
8. Simple Implementation:
   Can be implemented easily using a few discrete components or with dedicated ICs like the 74xx14 or 74xx132 series.

Disadvantages of Schmitt Trigger:

1. Fixed Hysteresis in Some Designs:
   In integrated IC versions, the hysteresis level is often fixed, limiting flexibility in some applications.
2. Increased Circuit Complexity:
   For adjustable hysteresis or custom thresholds, external components are needed, adding to circuit complexity.
3. Input Signal Limitations:
   Cannot process very high-frequency signals effectively in some designs due to the response time of the circuit.
4. Power Supply Sensitivity:
   The threshold levels might vary with fluctuations in the power supply voltage unless designed to be independent of it.
5. Static Power Dissipation:
   In some implementations, especially with discrete components, static power dissipation can be higher compared to simpler circuits like comparators.
6. Limited Linear Range:
   As a nonlinear circuit, it is unsuitable for applications requiring linear amplification or processing.
7. Component Dependency:
   Performance is highly dependent on component tolerances in discrete designs, which can affect consistency and reliability.

Applications of Schmitt trigger Circuit:

A Schmitt Trigger circuit, also known as a regenerative comparator, is widely used in electronics for its ability to handle noisy signals and produce clean, sharp transitions. Here are some of its primary applications:

1. Noise Filtering

• Schmitt Triggers are used to remove noise from input signals, ensuring a clean digital output.
• Ideal for converting noisy or slowly varying analog signals into digital signals.

2. Waveform Shaping

• Converts a sine wave or other analog signals into a square wave.
• Common in signal conditioning circuits to ensure consistent digital transitions.

3. Debouncing Switches

• Used in circuits to debounce mechanical switches or push buttons by providing clean transitions, eliminating erratic signal changes.

4. Square Wave Oscillators

• Produces square waves from sinusoidal signals.
• Forms the core of relaxation oscillator circuits.
• Used in timers, clock generation, and waveform generators.

5. Frequency Doublers

• By shaping input waveforms, Schmitt Triggers can effectively double the frequency of an input signal in some configurations.

6. Pulse Shaping

• Converts a pulse with irregular rise and fall times into a sharp and clean square pulse.
• Used in communication systems and digital signal processing.

7. Data Communication

• Cleans up distorted or attenuated signals in data transmission, ensuring reliable digital communication.

8. Threshold Detectors

• Acts as a threshold detector by switching outputs when the input crosses certain predefined voltage levels.
• Used in ADC (Analog-to-Digital Converter) circuits for precise signal conversion.

9. LED Flasher Circuits

• Part of simple LED blinking circuits where the Schmitt Trigger provides stable timing.

10. Memory and Logic Circuits

• Ensures reliable operation in memory elements like flip-flops and latches by providing noise-free signals.

11. Voltage Controlled Oscillators (VCOs)

• Used to stabilize the transitions and improve the performance of VCOs in phase-locked loops (PLLs).

12. Analog Signal Comparators

• Compares two analog signals with hysteresis to provide a stable output in systems prone to noise.

In conclusion, the Schmitt trigger is invaluable in both analog and digital applications due to its noise immunity, sharp transition characteristics, and the unique hysteresis feature that stabilizes output transitions.

Watch these videos for better understanding.

Inverting Schmitt Trigger

Non-Inverting Schmitt Trigger

Different Types of Oscillators with Working and Applications