Common Centroid Transistor Layout Strategies for Low Drift Bandgap Cores
Common centroid transistor layouts for bandgap cores cancel linear thermal and lithographic gradients, reducing post-packaging output drift below 5 ppm per C.
Brokaw Core
Reference Architecture
Precision voltage reference circuit utilizes a base emitter voltage difference between two transistors operating at different current densities. This brokaw core produces a predictable temperature dependent voltage that combines with a base emitter voltage to create a stable output. The circuit is the standard building block for bandgap references in integrated circuits.
Temperature Coefficient
Proportional to absolute temperature currents are generated by the circuit to cancel the negative temperature coefficient of the base emitter junction. When a brokaw core operates, the sum of these opposing coefficients results in a zero coefficient reference point near room temperature. Compensation remains effective across the industrial temperature range of -40 to 125 degrees Celsius.
Current Density
Transistor area ratios define the primary performance characteristics of the cell. If a brokaw core uses an eight to one emitter area ratio, the resulting voltage difference provides a stable slope for thermal compensation. This ratio is fixed by the physical geometry of the silicon layout.
Metrological Stability
Accuracy in the final output depends on the matching of the bipolar transistors and the quality of the load resistors. Error sources in a brokaw core include resistor drift and base current errors that introduce curvature into the voltage to temperature relationship. Laser trimming often brings the initial accuracy within millivolt tolerances.