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Design and Simulation of Schmitt Triggers

Key Takeaways

  • An important IC for creating clean digital pulses is a Schmitt trigger. This device can use hysteresis to output a stream of digital pulses from a noisy reference waveform.

  • A Schmitt trigger acts like an amplifier/comparator with hysteresis where hysteresis is controlled by adjusting the collector resistance in two transistors.

  • If you’re designing a module or IC as a high-frequency Schmitt trigger, you need the right simulation tools to evaluate your design for generating a pulse stream.

Clocking with Schmitt triggers

You can take a noisy analog input and create a clean clock signal with Schmitt triggers.

Schmitt triggers are fundamental circuits in ICs and simpler PCBs, and they can play an important role in cleaning up signals for use in other digital circuits. There are many different Schmitt trigger components and ICs, but they all rely on two important properties for rectification and stabilization of noisy input signals: saturation and hysteresis. Although these circuits are similar to amplifier circuits and even use the same symbol in a schematic, they operate quite differently.

If you’re designing a Schmitt trigger circuit for cleaning up noisy signals and producing digital pulses, you’ll likely find that the classic circuit diagram for Schmitt triggers is surprisingly resilient until you get up to very high frequencies. When you need to evaluate your circuit, you can run some simple SPICE simulations to test circuit behavior and verify that components will function as desired. Here’s what you need to know about designing Schmitt triggers for various applications and how they can help you condition signals.

What Are Schmitt Triggers?

Schmitt triggers are simple circuits that accept an oscillating signal (e.g., a sawtooth or triangle wave) and output a square wave. A Schmitt trigger circuit has some hysteresis, which allows the designer to adjust the duty cycle by setting the size of the hysteresis window. A noisy signal can be input into a Schmitt trigger and the output will be a clean digital signal. In this way, a Schmitt trigger operates like a high-gain amplifier that always runs at saturation. In fact, you can use an op-amp to construct a Schmitt trigger circuit by saturating the differential input, although this is not desired in high-speed circuitry.

A Schmitt trigger is related to another important type of two-state digital circuit: a comparator. A comparator and Schmitt trigger are similar, but they are not the same circuit.

Schmitt Trigger vs. Comparator Circuits

Schmitt triggers are often compared to comparator circuits, as their behavior is quite similar. All Schmitt triggers are comparators, but not all comparators are Schmitt triggers. Both types of circuits use hysteresis to set a threshold for switching between two saturated states. For a comparator, the output is saturated at the supply rail voltages, and the output will cycle between the positive and negative saturation voltages (e.g., rail-to-rail). The reference voltage to induce switching can be set by placing pull-up and pull-down resistors around the inverting input (or the non-inverting input for an inverting comparator). There is always a small hysteresis window in comparator circuits so that they can withstand ~10 mV of fluctuations in the input. The circuit below shows a comparator built from an op-amp where the positive feedback loop causes saturation at the supply rail voltages.

Comparator circuit schematic without hysteresis from an op-amp

Comparator circuit built from an op-amp. The positive feedback loop with high gain ensures the output is saturated at the supply rails as soon as the input voltage falls above or below 0 V.

For Schmitt triggers, hysteresis is intentionally added to set the switching threshold to some desired value. For a transistor-based comparator, hysteresis can be applied to the output voltage with another positive feedback loop using a voltage divider. The values of the resistors in the voltage divider determine the size of the hysteresis window and the duty cycle of the output waveform. A general circuit for an inverting Schmitt trigger is shown below, which includes the hysteresis window on the output signal.

Comparator circuit schematic without hysteresis from an op-amp

Comparator circuit built from an op-amp. The positive feedback loop with high gain ensures the output is saturated at the supply rails as soon as the input voltage falls above or below 0 V.

Note that you can use the same techniques to create a Schmitt trigger with an op-amp, although op-amp manufacturers advise against this. The reason for this advice is that an op-amp is generally not designed to run at high gain deep into saturation. Instead, these components are designed to run in the linear range, and they cannot withstand the thermal demands of switching between saturation states for extended periods.

Input Ripple and Noise Rejection

Because the input is a differential input, a Schmitt trigger has a high common-mode rejection ratio (CMRR). Despite the high CMRR provided by the differential input, natural variations in the input signal could still cause unintended switching between the two output states. This should illustrate the reason that hysteresis can be added to a comparator. By widening the hysteresis window, the rising edge and falling edge transitions become more different, and the circuit can withstand a larger voltage fluctuation without unintended switching.

Comparator circuit with and without hysteresis from an op-amp

Comparator output without hysteresis (left) and with hysteresis (right).

Simulating Schmitt Triggers

A Schmitt trigger circuit can be simulated using transient analysis and DC analysis of the transistor stages involved. When built from transistors, these circuits need to operate at saturation, so a load line will need to be simulated with a DC sweep. Transient analysis allows you to measure the duty cycle of the output square wave, which can then be compared with your earlier analysis from a feedback loop.

If you’re designing high-frequency Schmitt triggers, such as circuits that will operate at high GHz frequencies, you’ll need to use the right model for your SPICE subcircuits. GaAs or GaN-SiC material models are normally used for these high-frequency analog circuits. These types of circuits are still an active area of research, but these circuits can provide a high GHz clock or PWM signal without using a PLL.

Whether you’re designing custom Schmitt triggers from transistors or you’re using an op-amp, you need the best PCB design and analysis software to help you create your circuit diagrams and simulate your signal behavior. The front-end design features from Cadence integrate with the powerful PSpice Simulator to create the ideal system for circuit design and evaluation. Once you’re ready to create a PCB layout, Cadence has a suite of SI/PI Analysis Point Tools for post-layout verification and signal integrity simulations. You’ll have access to a full suite of design and optimization features.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.