Widlar Current Source

Key Takeaways

• The Widlar Current Source is a circuit configuration that maintains a consistent and well-defined current regardless of changes in the supply voltage or load conditions.

• The Wilson Current Source is another type of circuit configuration that generates a stable and accurate current output in analog and mixed-signal designs. Despite serving a similar purpose to the Widlar configuration, it is designed differently.

• You should consider component placement, traces, grounding, and thermals when designing the PCB using the Widlar Current Source configuration.

Bob Widlar’s patent for the Widlar Current Source configuration

The Widlar Current Source is a circuit configuration that generates a stable current reference. Its primary function is to maintain a consistent and well-defined current regardless of changes in the supply voltage or load conditions. A stable current reference is crucial for accurate and predictable circuit operation, especially in analog and mixed-signal circuits.

The Widlar Current Source uses a transistor (usually a bipolar junction transistor, or BJT) in a common-emitter configuration. If the transistor is biased correctly and the resistor values are correct, the collector current is relatively independent of voltage and temperature changes. The configuration is commonly used as a building block in analog and mixed-signal integrated circuits. It provides a way to generate a stable current reference for biasing other circuit elements, including differential amplifiers and operational amplifiers.

The Widlar Current Source and the Wilson Current Source are two types of current reference circuits that generate stable and accurate current outputs in analog and mixed-signal designs. Despite serving a similar purpose, they're designed differently.

Designed by Bob Widlar, the Widlar Current Source uses a BJT and resistor to generate stable current. This method uses the relatively stable voltage across the transistor's base-emitter junction to establish a reference voltage for current generation. The voltage supply, the base-emitter voltage, and the resistor value determine a circuit's output current. Temperature and manufacturing process variations can affect the accuracy of the Widlar Current Source.

Designed by George Wilson, the Wilson Current Source employs two matched transistors and a feedback loop to achieve better stability and accuracy. It uses a negative feedback mechanism to compensate for temperature variations and transistor parameter differences. The Wilson Current Source's accuracy is less dependent on the absolute value of the base-emitter voltage and the transistor's characteristics than the Widlar Current Source. The Wilson Current Source generally offers better thermal and process compensation, so it is better for high-precision applications.

There are two main differences between the Widlar Current Source and the Wilson Current Source: accuracy and compensation mechanism. The Wilson Current Source uses a feedback loop and matching multiple transistors to address some of Widlar's limitations. If precision and stability are essential, consider using the Wilson Current Source. You'll need to consider the specific requirements of your application, the level of accuracy you need, and the resources you've got for implementation and testing.

Widlar Current Source and PCB Design

You should consider component placement, traces, grounding, and thermals when designing the PCB using the Widlar Current Source configuration. Here's what you need to know:

• Keep traces short and direct between the transistor's collector, emitter, and base. Minimize loop areas to reduce unwanted inductance.
• Abate noise and interference through proper grounding. Connect the ground of the circuit properly and ensure a solid ground plane.
• Ensure good thermal dissipation by using heat sinks or thermal vias to take heat away from the transistor.
• Consider isolating the current source from other circuitry through guard traces or physical separation on the PCB.
• Test the current source under different conditions to ensure it behaves as expected. You should iterate on the PCB design if you encounter unexpected issues.
• Refer to the datasheets of the components you're using and consider simulation tools for a more accurate understanding of circuit behavior before moving on to PCB fabrication.

Your design considerations may vary depending on your application and desired precision.

Cadence Tools Support Designs Using Widlar Current Source

Whether designing with a Widlar Current Source or another current source, you’ll want to use advanced PCB Design and Analysis Software. Cadence offers the  OrCAD PCB Designer to support all your design needs.

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