Piecewise Linear Gain Amplifier With Stable Transition Voltages
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Solution Overview
Problem
Log amplifiers exhibit significant variations in output-input voltage transfer characteristics with environmental factors like process and temperature, and their gain saturation behavior is also affected by these factors, especially at high input voltages.
Innovation Solution
A piecewise linear (PWL) gain amplifier circuit is designed with a differential preamplifier and multiple transconductors in parallel, each having distinct linear input ranges, with specific bias currents and degeneration resistances to maintain constant transition voltages across variations in semiconductor manufacturing processes, temperature, and supply voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a log amplifier is used to achieve wide dynamic range, then the dynamic range is improved, but the output-input voltage transfer characteristic varies significantly with process and temperature
Solution Approach 1:
The amplifier is divided into multiple parallel paths, each containing a transconductor with a specific linear input range. These segments handle different portions of the input voltage range, with each segment contributing to the overall output. This segmentation allows the system to maintain linearity across a wide dynamic range while avoiding the logarithmic compression that causes process and temperature sensitivity in traditional log amplifiers.
Solution Approach 2:
The invention changes the operating parameters of the transconductors by applying different bias currents to each parallel path. By carefully selecting bias currents that are proportional to the inverse of load resistors, the transition voltages between linear ranges are made substantially constant over process, temperature, and supply voltage variations. This parameter adjustment resolves the stability issue while maintaining wide dynamic range.
2Adaptability or versatility
If multiple transconductors with different linear input ranges are used in parallel, then the linear input range is extended, but the device complexity increases
Solution Approach 1:
Multiple transconductors are implemented using the same differential pair circuit topology, making each unit functionally similar but differentiated by bias current settings. This universal approach allows reuse of the same circuit design pattern while achieving extended linear input range through parallel operation, reducing design complexity compared to using fundamentally different circuit architectures for each range.
Solution Approach 2:
Instead of changing the circuit topology or structure to achieve different linear input ranges, the invention changes only the bias current parameter for each transconductor. This parameter-based differentiation maintains structural simplicity while achieving the desired functional diversity in terms of input range coverage.
3Reliability
If bias currents are adjusted to maintain constant transition voltages, then the stability over process and temperature is improved, but the manufacturing precision requirements increase
Solution Approach 1:
The invention employs feedback mechanisms where the bias currents are derived from common reference currents that are themselves stabilized against process and temperature variations. By using feedback to maintain current ratios and relationships, the system achieves constant transition voltages without requiring extremely tight absolute current matching, thus reducing manufacturing precision requirements.
Solution Approach 2:
The bias currents are designed with specific relationships (proportional to inverse load resistors) that cause process and temperature variations to cancel out in the transition voltage calculation. This parameter relationship approach transforms the problem from requiring precise absolute current values to requiring only precise current ratios, which are easier to manufacture and match.
Data Source
AI summary
A piecewise linear gain amplifier circuit includes a differential preamplifier and a plurality of transconductors. The differential preamplifier is electrically coupled to a differential input having an input voltage. The transconductors are electrically coupled in parallel with each other. Each transconductor includes a respective differential input that is electrically coupled to a differential output of the differential preamplifier. In addition, each transconductor includes a respective differential output that is electrically coupled to a common differential PWL output. Each transconductor has a different linear input range. An optional attenuation circuit can be electrically coupled in parallel to the differential preamplifier. The differential output of the attenuation circuit can be electrically coupled to a differential input of another transconductor, and that transconductor can have a differential output that is electrically coupled to the common differential PWL output.