BJT Early Effect Compensation via Collector-Base Voltage Adjustment
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Solution Overview
Problem
Bipolar junction transistors (BJTs) exhibit nonlinearity due to Early effects, which affect temperature sensors and bandgap circuits by introducing variations in base-emitter voltage differences proportional to absolute temperature (PTAT), complicating the generation of temperature-independent voltage references.
Innovation Solution
A method and apparatus that compensates for Early effects by judiciously biasing individual transistors, combining complementary to absolute temperature (CTAT) and PTAT cells to generate a reference circuit whose output is insensitive to temperature, using specific resistor ratios and amplifier configurations to adjust collector-base voltages and mirror bias currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If two identical BJTs operate at different collector current densities to generate PTAT voltage, then temperature sensing capability is improved, but nonlinearity due to Early effects increases
Solution Approach 1:
The patent changes the operating parameters of the BJTs by applying different collector-base voltages to transistors operating at different collector current densities. This parameter adjustment compensates for the nonlinearity introduced by Early effects, allowing the circuit to maintain both temperature sensing capability and reduced nonlinearity simultaneously.
2Stability of the object's composition
If base-emitter voltage differences are used to generate PTAT output, then temperature independence of voltage reference is improved, but Early effect variations worsen the output accuracy
Solution Approach 1:
The patent employs feedback mechanisms where the output voltage is fed back to adjust the collector-base voltages of the BJTs. This feedback loop compensates for Early effect variations, ensuring that the voltage reference maintains both temperature independence and high output accuracy.
Solution Approach 2:
The patent dynamically adjusts the collector-base voltage parameters based on operating conditions to compensate for Early effect variations. By changing these parameters in response to detected variations, the circuit maintains accurate PTAT output while preserving temperature independence of the voltage reference.
Data Source
AI summary
Early effects are intrinsically present in bipolar junction transistors (BJTs). Described are examples of complimentary to absolute temperature (CTAT) and proportional to absolute temperature (PTAT) cells that reduce errors associated with the Early effects that would otherwise be present.