dB-Linear Voltage-to-Current Converter CMOS Sub-threshold
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Solution Overview
Problem
Designing a voltage-to-current (V/I) converter with an exponential transfer function in CMOS technology is challenging due to MOSFET transistors having a square-law transfer characteristic, unlike bipolar transistors which have an inherent exponential characteristic.
Innovation Solution
The implementation of an exponential V/I converter using a voltage scaler, a current multiplier, and an exponential current converter, where the current multiplier multiplies the input current with a current proportional to absolute temperature, and the exponential current converter applies a temperature-independent exponential transfer function to generate an output current, utilizing a differential MOSFET pair operating in sub-threshold mode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If MOSFET transistors are used in CMOS technology for V/I converter design, then the converter can be implemented in CMOS process, but the transfer characteristic becomes square-law instead of exponential
Solution Approach 1:
The patent changes the operating parameters of MOSFET transistors by operating them in sub-threshold mode rather than strong inversion mode. This parameter change transforms the transfer characteristic from square-law to exponential, enabling CMOS implementation while maintaining the desired exponential V/I conversion function.
Solution Approach 2:
Instead of trying to make MOSFETs behave like bipolar transistors (which have inherent exponential characteristics), the patent inverts the approach by exploiting the sub-threshold region of MOSFET operation where they naturally exhibit exponential characteristics, thus achieving exponential transfer function in CMOS technology.
2Device complexity
If a simple V/I converter design is used, then the device complexity is reduced, but the dB-linear operation range is limited
Solution Approach 1:
The patent divides the V/I converter into multiple functional blocks: a voltage scaler for input voltage scaling, a current multiplier for generating the exponential relationship, and an exponential current converter for final conversion. This segmentation allows each block to be optimized for its specific function, achieving a wide 40 dB linear operation range while maintaining reasonable overall 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 approach allows for a wide dB-linear operation range of about 40 dB, making the converter suitable for CMOS technology and maintaining a temperature-independent exponential transfer characteristic.
Implementation Method 1
The exponential current converter has a differential MOSFET pair operating in a sub-threshold mode and generating an output current that is proportional to a temperature-independent, exponential function of the input voltage
Implementation Method 2
The current multiplier multiplies the input current and a current proportional to absolute temperature and supplies the resulting current to the exponential current converter
Implementation Method 3
Temperature dependence of the current proportional to absolute temperature counteracts temperature dependence of the thermal voltage to cause the output current to be proportional to a temperature-independent, exponential function of the input voltage
Data Source
AI summary
A dB-linear voltage-to-current (V/I) converter is amenable to implementation in CMOS technology. In a representative embodiment, the dB-linear V/I converter has a voltage scaler, a current multiplier, and an exponential current converter serially connected to one another. The voltage scaler supplies an input current to the current multiplier based on an input voltage. The current multiplier multiplies the input current and a current proportional to absolute temperature and supplies the resulting current to the exponential current converter. The exponential current converter has a differential MOSFET pair operating in a sub-threshold mode and generating an output current that is proportional to a temperature-independent, exponential function of the input voltage.