Cross-Coupled VGA Gain Steps With Low Phase Error
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Solution Overview
Problem
Variable gain amplifiers (VGAs) face challenges in maintaining a constant phase response across a wide range of gain values, especially at millimeter-wave frequencies, leading to phase errors and degraded performance in phased array systems, particularly in 5G wireless communication systems, due to process variations and mismatches.
Innovation Solution
The implementation of VGAs with cross-couple switching arrangements, which include a transistor arrangement and a cross-couple switching arrangement that selectively changes the coupling of transistor terminals to maintain a constant capacitance and achieve wideband, highly linear operation without significant trade-offs in gain, bandwidth, or noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional VGA designs are used to achieve variable gain, then gain adjustment is possible, but phase response becomes non-constant across gain values
Solution Approach 1:
The patent implements dynamic switching between different gain states using cross-coupled switches that are controlled by gain control signals. The switches dynamically reconfigure the transistor arrangement to achieve different gain values while maintaining constant phase response through the specific cross-coupled topology that preserves signal phase relationships across all gain states.
Solution Approach 2:
The patent changes the electrical parameters of the amplifier by varying the effective transistor width-to-length ratio through the switching arrangement. By selectively connecting transistors in parallel or series configurations based on the desired gain state, the amplifier achieves different gain values while the cross-coupled topology ensures that the phase response remains constant across all parameter states.
2Power
If gain is increased in conventional VGAs, then amplification capability improves, but phase errors increase
Solution Approach 1:
The cross-coupled switching arrangement dynamically adjusts the amplifier configuration based on the desired gain level. When high gain is required, the switches connect transistors in a configuration that maximizes amplification while the cross-coupled topology simultaneously maintains constant phase response, preventing phase errors from increasing with gain.
Solution Approach 2:
The patent employs gain control signals that act as feedback mechanisms to control the switching states. The switching arrangement responds to these control signals to achieve the desired gain while the cross-coupled topology inherently provides phase compensation, ensuring that phase errors do not increase with amplification capability.
3Manufacturing precision
If phase compensation techniques are applied, then phase response improves, but device complexity increases
Solution Approach 1:
The cross-coupled transistor arrangement serves multiple functions simultaneously: it provides variable gain control through the switching mechanism while inherently maintaining constant phase response. This multi-functional design eliminates the need for separate phase compensation circuits, reducing overall device complexity while achieving both gain variability and phase accuracy.
Solution Approach 2:
The patent merges the gain control function and phase response maintenance into a single integrated cross-coupled switching arrangement. By combining these functions in one topology rather than using separate gain control and phase compensation circuits, the design reduces device complexity while achieving both objectives simultaneously.
Data Source
AI summary
An example VGA includes a transistor arrangement having a plurality of transistors configured to realize one or more gain step circuits of the VGA, and a cross-couple switching arrangement having a plurality of switches configured to selectively change the coupling of the terminals of at least some of the transistors depending on whether a given gain step circuit is supposed to be in an ON state or in an OFF state. Using the cross-couple switching arrangement advantageously allows keeping all of the transistors ON at all times during operation and changing the coupling of some transistor terminals to either realize an in-phase addition of currents flowing through various transistors to apply the maximum gain or realize a subtraction of currents to apply the minimum gain. Such a VGA may be inherently wideband, enabling a highly linear, wideband operation without having to resort to significant trade-offs with other performance parameters.