Where to Find Hysteresis Loops in Your Circuits

Key Takeaways

• Many physical processes cause hysteresis, and these processes are primarily nonlinear in nature.

• In a  circuit design, there are basic components and common analog circuits that will exhibit hysteresis.

• Some advanced simulation tools can model the effects of hysteresis within larger circuits using custom SPICE subcircuit models.

Hysteresis is normally discussed in terms of magnetic circuits, but hysteresis loops can arise in other systems.

Nonlinear behavior occurs everywhere, and linear behavior is only an approximation to real behavior in any physical system. In electronics, you can often design with many components and ensure a linear response in your circuits, but many components or circuits in your system are always nonlinear and exhibit saturation or rectification. One effect that combines these two aspects of nonlinear behavior is hysteresis, where saturation occurs as an input signal level and is changed along different directions.

When you need to analyze how hysteresis in a circuit or component affects system behavior, you need to simulate a hysteresis loop in an individual circuit. You can also use a hysteresis model in a larger electronic system—as part of any conventional analyses, you would run in a SPICE simulator. When you need to create a SPICE simulation for circuits with hysteresis. 

Where a Hysteresis Loop Can Occur in Electronics

A hysteresis loop occurs in nonlinear components, and in circuits containing nonlinear components. It can arise through many mechanisms, which can be difficult to predict, however, hysteresis is easy to visualize in the time domain, or by looking at a cycled transfer curve (output vs. input signal). When a hysteresis loop occurs, saturation in output results at a high input signal level. Once the input signal level is reversed, the transfer curve briefly exhibits rectification until the input passes some threshold.

After repeated input signal cycles, the output signal will fail to return to its origin when the two quantities are graphed as transfer curves. This is shown in the diagram below. 

The initial output vs. input transfer curve is shown in black and reaches saturation as the input signal increases to the first reversal point. During the first reversal (red curve), the output signal follows a different path to the second reversal point. During the second reversal (blue curve), the signal follows yet another path back to the first reversal point.

Hysteresis loop in a transfer curve.

Hysteresis does not occur in every component, but there are some very common components where it does occur. As an example, a hysteresis loop can be observed in the following components:

• Ferritic components (inductors, ferrite beads/clamps, transformers)

• Thyristors

• Diodes made from unique materials

• Ceramic piezoelectric transducers

• Physisorptive and chemisorptive thin-film sensors

Similarly, a hysteresis loop can occur in the following circuits and systems, even though they may not contain components that exhibit hysteresis:

• Comparators and other circuits with positive feedback

• Frequency multipliers based on varactors

• PID controllers and other control systems

• Cyclic voltammetry measurements with a potentiostat

Hysteresis might be a desirable effect for a circuit’s function, such as in a comparator. In a comparator circuit, hysteresis provides noise immunity, and positive feedback can be used with hysteresis to produce a clean square wave from a noisy signal. In other circuits, such as resonant LLC converters, hysteresis will limit power transfer between the bridge stage on the primary side and the output stage on the secondary side. To determine whether hysteresis is beneficial or problematic, you’ll need to use simulations with the right component models.

Building Simulations with Hysteresis Loops

If you’re working with specialty components, new materials, or unique circuit topologies that exhibit hysteresis, you’ll need to develop SPICE models for your system to simulate the effects of hysteresis. This area of electronics design has remained a current research topic ever since SPICE simulators were first developed. Take a look at this recent IEEE article to learn more about developing specialty SPICE models for transformers with hysteresis.

If you need to simulate the effects of hysteresis in a typical analog circuit, then you need a SPICE model for the particular component that is exhibiting hysteresis. Defining hysteresis in a SPICE model requires defining the output signal level in terms of rising and falling crossing points. This value will then generate different output characteristics as the input signal is cycled. In other words, the output signal level is defined as a piecewise function of the input.

Example results from transient analysis can be used to construct a hysteresis loop. The window below shows a triangle waveform being input to a 1 V CMOS Schmitt trigger buffer and 9 overlaid output curves for 9 input wave cycles.

Hysteresis shown in transient analysis results.

The output waveform varies significantly, even though there is no noise on the input. By plotting output curves vs. input waveform over a single period, the hysteresis loop shown below can be constructed.

Hysteresis loops constructed from transient analysis results for an example inverting Schmitt trigger circuit.

Manufacturers often release standard component models with SPICE models for their components, but these SPICE models do not always account for hysteresis. Be sure to read the SPICE model for your component or check with the model developer before using the SPICE model in a simulation. In the case that a model with hysteresis is unavailable, an existing model will need to be modified to account for hysteresis.

Once you’ve developed SPICE models for each component with a hysteresis loop, you can use the front-end design features from Cadence to build your circuits and create components with your simulation models. The modeling and simulation features in PSpice Simulator can help you create new component models with hysteresis, as well as simulate electrical behavior in circuits with hysteresis loops. Once you’re ready to create a PCB from your system, simply capture your schematics in a blank layout and use Cadence’s board design utilities to finish your design.

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