Current Amplifier for High-Speed DAC with Low Distortion
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Solution Overview
Problem
Designing a current-steering digital-to-analog converter (DAC) that operates with large output swings and low distortions at high frequencies is challenging due to the difficulty in achieving high speed and high linear transmission in modern communication systems.
Innovation Solution
A current amplifier is designed with at least two transistors and two impedance circuits, where the first transistor has a gate coupled to a previous-stage circuit and a drain coupled to a current source, and the second transistor has a gate coupled to the current source, with impedance circuits between the gate and source of each transistor, allowing for reduced output swing without affecting transmission efficiency. This design is implemented in both single-ended and pseudo-differential structures to accommodate various digital-to-analog converter configurations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a current mode DAC is designed for high speed operation, then transmission speed is improved, but output swing capability deteriorates
Solution Approach 1:
The transmitter is segmented into two functional blocks: a current mode DAC for high-speed operation and a current amplifier for output swing enhancement. The DAC operates at high speed while the amplifier restores the output swing, resolving the contradiction between speed and swing capability through functional decomposition
Solution Approach 2:
A current amplifier is introduced as an intermediary component between the current mode DAC and the output stage. This mediator takes the high-speed but low-swing output from the DAC and amplifies it to achieve both high speed and large output swing simultaneously
2Strength
If a current mode DAC is designed for large output swings, then output swing capability is improved, but distortion increases
Solution Approach 1:
The system is divided into a current mode DAC that operates with small signals (low distortion) and a current amplifier that restores the output swing. This segmentation allows each component to operate in its optimal region, preventing distortion while maintaining large output capability
Solution Approach 2:
The current amplifier performs preliminary amplification of the DAC output before it reaches the final output stage. By amplifying the signal early in the chain, the system achieves large output swings without requiring the DAC itself to operate with large signals, thus avoiding distortion
3Measurement precision
If a current mode DAC is designed for high resolution, then transmission quality is improved, but bandwidth is reduced
Solution Approach 1:
The transmitter is segmented into a high-resolution current mode DAC and a high-bandwidth current amplifier. The DAC provides high resolution while the amplifier provides high bandwidth, resolving the contradiction through functional separation where each component optimizes for its primary function
Solution Approach 2:
The current amplifier acts as an intermediary that preserves the high-resolution information from the DAC while extending the bandwidth. It amplifies the fine-detail signal without introducing bandwidth limitations, allowing both high resolution and high bandwidth to coexist
Data Source
AI summary
A current amplifier and a transmitter using the same. The current amplifier includes: a first transistor having a gate coupled to a former-stage circuit, a drain coupled to a current source, and a source biased at a constant voltage level; a second transistor having a gate coupled to the current source and having a source and a drain; a first impedance circuit coupled between the gate of the first transistor and the source of the second transistor; and a second impedance circuit coupled between the source of the second transistor and a ground terminal. The current amplifier receives an input current from the former-stage circuit and generates an output current at the drain of the second transistor. Note that no current source is connected to the source of the first transistor.