Variable Gain Amplifier Biasing for Output Phase Invariance
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Solution Overview
Problem
Conventional variable gain amplifiers (VGAs) suffer from variations in output phase and input impedance as gain settings change, leading to undesirable loading of other RF circuitry and limited gain range, which can result in phase errors and side lobe issues in beamforming applications.
Innovation Solution
The implementation of a VGA with degeneration resistors that maintain a substantially constant bias current across gain settings, providing phase invariance and stable input impedance, achieved by controlling the relative biasing of cascode transistors and using a gain control circuit to manage signal current flow through these transistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional VGAs change gain settings by varying bias current, then gain control is achieved, but output phase and input impedance vary causing loading effects and phase errors
Solution Approach 1:
The patent changes the biasing parameter from variable bias current to constant bias current across all gain settings. By maintaining constant bias current and using degeneration resistors with cascode transistors, the circuit achieves gain control without varying the operating point, thereby stabilizing output phase and input impedance while providing wide gain range through controlled current division.
2Adaptability or versatility
If conventional VGAs use variable bias current for gain control, then gain adjustment is possible, but input impedance matching deteriorates across gain settings
Solution Approach 1:
The patent maintains constant bias current parameter across all gain settings, which stabilizes the input impedance of the gain transistor. Degeneration resistors are introduced to provide gain control through current division rather than bias current variation, ensuring consistent input impedance matching across the entire gain range.
3Adaptability or versatility
If VGA provides wide gain range, then dynamic performance is enhanced, but phase variation increases causing beamforming errors
Solution Approach 1:
The patent uses constant bias current parameter across wide gain range (at least 20 dB), achieving phase invariance through degeneration resistors and cascode transistor configuration. The gain control is implemented by varying the proportion of signal current through different cascode transistors while maintaining constant total bias current, ensuring phase accuracy remains below 5 degrees across the entire gain range.
4Device complexity
If conventional VGA circuitry is simplified, then device complexity is reduced, but phase control precision deteriorates
Solution Approach 1:
The patent segments the signal current into multiple paths using parallel cascode transistor branches, each with degeneration resistors. This segmentation allows independent control of current proportion in each branch while maintaining constant total bias current, achieving precise phase control through current division rather than complex biasing networks.
Data Source
AI summary
Variable gain amplifiers (VGA) with output phase invariance are provided herein. In certain embodiments, a VGA is operable in a selected gain setting chosen from multiple gain settings that provide different amounts of amplification to a radio frequency (RF) input signal. The VGA includes a gain transistor that has a substantially constant bias current across the gain settings, such that the VGA's output phase, input impedance matching, and/or input return loss are substantially constant. The gain setting of the VGA is selected by controlling relative biasing of a pair of cascode transistors each connected to the gain transistor by a corresponding degeneration resistor. The degeneration resistors provide compensation that reduces or eliminates a difference in output phase of the VGA across gain settings, for instance, by introducing a zero in a transfer function of the VGA that cancels a pole arising from the cascode transistors.