Impedance-Matched Source Follower for High-Speed Link Signal Integrity
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Solution Overview
Problem
High speed signal transmission circuits, such as those used in PCI Express, HDMI, and DisplayPort, face challenges with signal attenuation and impedance discontinuities due to large RC time constants and parasitic resistance, leading to degraded signal integrity and unwanted reflections.
Innovation Solution
The implementation of an impedance-matched source follower circuit with a termination resistor and bias current circuit, which maintains constant output impedance over process, voltage, and temperature variations, improving signal integrity and reducing attenuation by matching impedance and providing bias current for transistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a transmission gate is used for signal switching, then signal transmission capability is provided, but large RC time constant causes high speed signals to be degraded
Solution Approach 1:
The patent changes the circuit topology from a transmission gate to a source follower configuration, fundamentally altering the electrical characteristics. This parameter change eliminates the RC time constant issue by using a different transistor operating mode where the source follows the gate voltage, providing low output impedance and high bandwidth suitable for high-speed signals
Solution Approach 2:
The patent replaces the mechanical switching action of the transmission gate with a voltage-follower action of the source follower transistor. This substitution eliminates the parasitic resistance and capacitance effects inherent in transmission gates by using the transistor's inherent voltage buffering capability, achieving better high-frequency response
2Ease of operation
If transmission gate is used, then signal switching function is achieved, but parasitic Ron creates impedance discontinuity and reflections
Solution Approach 1:
The patent changes the output impedance parameter from the parasitic Ron of the transmission gate to the controlled output impedance of the source follower. By biasing the transistor in the active region and using the source follower configuration, the output impedance becomes low and controllable, matching the transmission line impedance and eliminating reflections
Solution Approach 2:
The patent extracts and eliminates the parasitic resistance element inherent in transmission gate operation. By removing the transmission gate topology and replacing it with a source follower, the harmful parasitic Ron is taken out of the circuit, leaving only the controlled impedance of the transistor which can be optimized for impedance matching
Data Source
AI summary
Switching and repeating applications using an impedance matched source follower improve performance of high speed links such as PCI Express, HDMI, DisplayPort and DVI by reducing attenuation and other degradation of high speed signals, including those with transmit pre-emphasis, by avoiding impedance discontinuities over process, voltage and temperature variations and by driving a broader range of loads, e.g., heavily capacitive loads. A circuit for switching or repeating signals on a single-ended or differential high speed link may comprise a source follower with input and output impedances matched to input and output transmission lines on the high speed link. The source follower is biased by a constant transconductance circuit, an external calibration circuit or other circuit to provide an essentially constant output impedance over process, voltage and temperature variations.