Band-gap Voltage Reference Circuit with Segmented Branches
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Solution Overview
Problem
Brokaw band-gap circuits suffer from residual process dispersion, variability, and temperature drift due to manufacturing tolerances and mismatch of mirror currents, especially when using PNP transistors with low beta, leading to inaccuracies in reference voltage and parasitic high frequency injection.
Innovation Solution
The proposed solution involves a band-gap voltage reference circuit design that uses pairs of npn and pnp transistors with different emitter current densities, coupled with n-type and p-type FETs in a current mirror configuration, to stabilize the base-emitter voltage and reduce temperature coefficients, while also incorporating a start-up circuit to ensure reliable operation and minimize residual current flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If PNP vertical transistors are used to reduce parasitic effects, then power consumption is reduced, but current mirror mismatch and base current errors increase due to low beta
Solution Approach 1:
The circuit is divided into multiple independent branches (first branch with NPN transistors, second branch with PNP transistors, third branch with complementary transistors). Each branch processes current independently and contributes to the final reference voltage through a weighted sum, allowing optimization of each branch for its specific function while reducing overall errors through diversity
Solution Approach 2:
The invention changes the current density parameter across different branches to compensate for beta errors. By operating transistors at different current densities and using appropriate weighting factors in the voltage summing network, the circuit compensates for the low beta of PNP transistors while maintaining low power consumption
2Temperature
If standard Brokaw circuit is used, then temperature stability is achieved, but manufacturing tolerance dispersion causes voltage reference inaccuracy
Solution Approach 1:
The reference voltage is generated by summing base-emitter voltages from multiple independent branches rather than relying on a single Brokaw cell. This segmentation allows each branch to be optimized for specific functions (temperature compensation, error reduction) and reduces the impact of manufacturing variations in any single branch
Solution Approach 2:
The circuit uses a composite structure combining different transistor types (NPN and PNP) and different branch configurations. This composite approach allows the circuit to benefit from the advantages of each transistor type while compensating for their individual weaknesses through proper weighting and summing of their contributions
3Device complexity
If current mirror configuration is used, then circuit complexity is reduced, but mirror current mismatch increases temperature drift
Solution Approach 1:
Instead of using a single current mirror, the invention uses multiple independent current paths in different branches. Each branch has its own current reference and transistor configuration, which reduces the impact of mirror mismatch errors while maintaining relatively simple circuit structure
Solution Approach 2:
The invention changes the weighting factors applied to different base-emitter voltages to compensate for current mirror mismatches. By adjusting the relative weights of contributions from different branches, the circuit minimizes temperature drift caused by mirror errors without increasing complexity
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design significantly reduces the dispersion of output voltage, from 52 mV to 12 mV (5 sigma), and minimizes the effects of current mirror mismatch and temperature drift, achieving improved stability and accuracy of the reference voltage.
Implementation Method 1
two groups of junction-isolated bipolar transistors run at different emitter current densities. The difference in emitter current densities produces a related difference between the base-emitter voltages of the two groups. This voltage difference is added to the base-emitter voltage of the transistor with higher emitter current density with a suitable ratio defined by a voltage divider. The temperature coefficient of the base-emitter voltage is negative and tends to compensate the positive temperature coefficient of the voltage difference.
Implementation Method 2
The gates of the FETs are control electrodes for current conduction paths extending from the output rail to ground, the sources of the FETs being connected to ground through equal value resistors so that equal currents flow in the branches.
Data Source
AI summary
A band-gap voltage reference circuit having first and second branches respectively including first and second groups of transistors of different emitter current conduction areas and current sources for running the first and second groups of transistors at different emitter current densities to generate respective base-emitter voltages, and output terminals connected to receive a regulated voltage (Vout) which is a function of the base-emitter voltages of the first and second groups of transistors. Each of the first and second groups includes at least one npn-type transistor and at least one pnp transistor connected with their emitter-collector paths in series in the respective one of the branches so as to present cumulated base-emitter voltages across the respective group.