Reactive Feedback LNA Using Separate Transformers for Gain Matching
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Solution Overview
Problem
Low-noise amplifiers face limitations in gain-boosting and impedance-noise matching due to unwanted mutual inductance between transformer windings, particularly in trifilar designs, which also increase fabrication costs with two thick RF metal layers.
Innovation Solution
The use of separate transformers for negative and positive reactive feedback loops, with the first transformer providing negative feedback ac-coupled to the second transformer without mutual coupling, allowing for dc isolation and reducing fabrication costs to a single thick RF metal layer process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a trifilar transformer is used to provide both positive and negative feedback loops, then gain-boosting is improved, but unwanted mutual inductance between primary and tertiary windings reduces overall gain and impedance-noise matching
Solution Approach 1:
The patent divides the single trifilar transformer into two separate transformers: a first transformer for negative feedback and a second transformer for positive feedback. This segmentation eliminates the unwanted mutual inductance between primary and tertiary windings that existed in the integrated trifilar design, while maintaining the gain-boosting benefits of both feedback loops.
Solution Approach 2:
The patent extracts the tertiary winding function from the primary transformer and implements it as a separate second transformer. This extraction removes the harmful mutual inductance effect while preserving the useful positive feedback function, allowing independent optimization of each transformer's characteristics.
2Reliability
If trifilar and multi-winding transformer designs are implemented as integrated circuits, then high-Q transformer can be achieved, but two thick RF metal layers are required increasing fabrication cost
Solution Approach 1:
By segmenting the feedback loops into separate transformers, the patent enables implementation on a single thick RF metal layer. Each transformer can be independently optimized for high-Q performance without requiring the complex stacked/overlay configuration of multi-winding transformers, thus reducing fabrication cost while maintaining reliability.
3Ease of manufacture
If separate transformers are used for negative and positive feedback loops, then fabrication cost is reduced to single thick RF metal layer, but device complexity increases
Solution Approach 1:
The patent uses a coupling capacitor as an intermediary element to connect the secondary winding of the first transformer to the secondary winding of the second transformer. This intermediary approach simplifies the overall device complexity by providing a straightforward AC coupling mechanism that avoids the need for complex inter-winding connections while maintaining signal integrity between the two feedback loops.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This arrangement enhances gain-boosting capabilities while maintaining impedance-noise matching and significantly reducing the fabrication cost of integrated circuits by allowing high mutual coupling in a single thick metal layer configuration.
Implementation Method 1
a first transformer on the output side of the amplifying element arranged to mutually couple a fraction of the output current from the amplifying element onto the input side of the amplifying element
Implementation Method 2
a second transformer on the input side of the amplifying element arranged to increase the input voltage on the input side via mutual coupling of its primary and secondary windings
Data Source
AI summary
An amplifier, comprising: an amplifying element having an input side and an output side; a first transformer on the output side of the amplifying element arranged to mutually couple a fraction of the output current from the amplifying element onto the input side of the amplifying element; a second transformer on the input side of the amplifying element arranged to increase the input voltage on the input side via mutual coupling of its primary and secondary windings; wherein a primary winding of the first transformer is connected to an output of the amplifying element; wherein a secondary winding of the first transformer is ac connected to a secondary winding of the second transformer; and wherein the primary winding of the first transformer is dc blocked from the secondary winding of the second transformer. The negative and the positive reactive feedback loops are not formed from the same trifilar transformer.