Transconductance Circuit Topology for Wide-Range Linear Output
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Solution Overview
Problem
Existing transconductance circuits face challenges in maintaining linear transconductance and tracking output current changes over a wide range of input voltages, with differential pair circuits exhibiting non-linear behavior and saturation, and doublet circuits having limited output current.
Innovation Solution
The proposed transconductance circuit incorporates complementary sets of differential transistor pairs with common impedance coupling and voltage followers to replicate input voltages, along with area-offset doublet transistor pairs, to improve linearity and output current tracking without saturation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a differential pair circuit is used to implement transconductance, then the circuit structure is simple, but the transconductance becomes non-linear and output current saturates outside a small voltage range
Solution Approach 1:
The circuit is divided into multiple differential pairs (first and second differential pairs) with different tail current sources. Each differential pair operates effectively in different input voltage ranges, allowing the overall circuit to maintain linearity across a wider voltage range while keeping individual segments relatively simple
Solution Approach 2:
The patent changes the operating parameters by using different tail current source values for different differential pairs. The first differential pair uses a first tail current source while the second uses a second tail current source, allowing each pair to be optimized for different input voltage ranges and maintaining overall linearity
2Adaptability or versatility
If the input voltage range is extended beyond a small range, then more signal dynamic range is available, but transconductance non-linearity and output current saturation increase
Solution Approach 1:
The extended input voltage range is divided into different segments, with the first differential pair handling one range and the second differential pair handling another range. This segmentation allows each pair to operate within its optimal linear region while the combined circuit achieves extended range with maintained linearity
Solution Approach 2:
The circuit dynamically switches between different differential pairs based on the input voltage range. As the input voltage changes, different pairs become active, providing adaptive operation that maintains linearity across the extended voltage range
3Stability of the object's composition
If a current source is used to bias the differential pair, then the circuit operates stably, but the output current is limited by the current source capacity
Solution Approach 1:
The total output current capability is segmented across multiple current sources. The first differential pair is biased by a first tail current source and the second by a second tail current source, allowing the overall circuit to provide higher total output current while each individual current source operates stably within its designed range
Solution Approach 2:
Multiple current sources are combined to provide the total bias current for multiple differential pairs. The first and second tail current sources work together to enable the circuit to deliver higher output current while maintaining the stability benefits of current source biasing
4Reliability
If area-offset doublet transistor pairs are added to improve linearity, then transconductance linearity improves, but the circuit complexity increases
Solution Approach 1:
The circuit uses multiple differential pairs with different configurations rather than requiring complex area-offset doublet pairs within a single differential pair. This segmentation achieves linearity improvement through architectural design rather than detailed transistor sizing adjustments
Solution Approach 2:
The patent achieves linearity improvement by changing circuit-level parameters (multiple differential pairs with different current sources) rather than requiring complex transistor-level parameter adjustments (area offsets). This provides a more scalable and manufacturable solution
Data Source
AI summary
A transconductance circuit that improves linearity and output current over a wider range of input voltages than prior designs. The transconductance circuit may include first and second sets of paired differential transistors. In each set, emitters of the paired transistors may be commonly coupled to corresponding nodes of a common impedance, and collectors may be coupled to output terminals of the transconductance circuit. The circuit may further include first and second sets of doublet differential transistor pairs, each doublet pair having transistors of different sizes. Each doublet pair may have current sources coupled between commonly coupled emitters and a source potential. Respective collectors for each doublet pair may be coupled to the output terminals of the transconductance circuit. A pair of voltage followers may be provided to replicate corresponding input voltages across corresponding bases of the differential transistor pairs and the doublet transistor pairs.