Resistive-Degeneration VGA With Constant Bandwidth Gain Control
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Solution Overview
Problem
Conventional resistively degenerated variable gain amplifiers suffer from bandwidth variation over the range of available gain settings, which affects the performance of automatic gain control loops in high-speed serial communication receivers.
Innovation Solution
A constant-bandwidth variable gain amplifier is designed using a resistive degeneration circuit with compensation capacitors, where each resistive branch includes a parallel combination of a resistor and a compensation capacitor to maintain a constant bandwidth across different gain settings by compensating for parasitic capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional resistively degenerated VGA architecture is used, then gain linearity and immunity to process variations are improved, but bandwidth variation over the range of available gain settings occurs
Solution Approach 1:
The patent applies dynamics by making the degeneration impedance adaptive rather than fixed. Switches dynamically reconfigure the resistive branches in parallel, changing the total degeneration resistance based on the desired gain setting. This dynamic reconfiguration maintains constant bandwidth across different gain values by adjusting the impedance to compensate for gain variations.
Solution Approach 2:
The patent changes the degeneration resistance parameter dynamically. By switching different numbers of resistive branches in parallel, the total degeneration resistance is adjusted to specific values (e.g., Rdeg1, Rdeg2, Rdeg3) corresponding to different gain settings. This parameter change ensures that the product of degeneration resistance and transconductance remains constant, maintaining constant bandwidth.
2Adaptability or versatility
If the number of parallel resistive branches is increased to provide more gain settings, then gain control flexibility is improved, but device complexity increases
Solution Approach 1:
The patent segments the total degeneration resistance into multiple discrete resistive branches that can be independently switched. Each branch represents a portion of the total resistance, and by switching different combinations of these segments in parallel, multiple gain settings are achieved. This segmentation provides flexible gain control while keeping each individual branch simple.
Solution Approach 2:
The patent merges multiple resistive branches in parallel to achieve the desired degeneration resistance values. By combining different numbers of identical resistive branches in parallel, the total resistance is adjusted in discrete steps. This merging approach provides multiple gain settings using repeated simple units, reducing overall circuit complexity compared to using entirely different resistance values.
Data Source
AI summary
The performance of an AGC loop typically depends on several factors, including gain linearity of the VGA and variation in the VGA bandwidth over the range of available gain settings. Although a resistively degenerated VGA provides for excellent gain linearity and immunity to process variations, the conventional architecture for a resistively degenerated VGA suffers from bandwidth variation over the range of available gain settings. Embodiments are provided herein of a constant-bandwidth VGA that utilizes resistive degeneration. To maintain a constant bandwidth over the range of available gain settings, degeneration resistors are coupled in parallel with compensation capacitors. In an embodiment, a compensation capacitor is determined to have a capacitance substantially equal to the decrease in total degeneration resistance that occurs as a result of an associated degeneration resistor being placed in parallel with the total degeneration resistance.