Transformer-Feedback Low Noise Amplifier for Wideband Linearity
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Solution Overview
Problem
Designing a low noise amplifier for RF receivers is challenging due to conflicting requirements such as high linearity, large conversion gain, low noise figure, and wide input match bandwidth, especially when handling strong interference like 0 dBm RF power, which common source and common gate structures fail to meet, leading to nonlinearities and high noise figures.
Innovation Solution
A low noise amplifier configuration using a common source amplifying transistor with a self-coupled step-up transformer and feedback path, along with a balun circuit and tunable capacitors, to maintain low voltage gain for linearity while boosting feedback voltage gain for input matching, and incorporating a source follower for improved noise figure and linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If common source or common gate low noise amplifier structures are used, then the amplifier can provide voltage gain, but the high input voltage swing from strong interference signals creates nonlinearities at input, interconnection, and output nodes
Solution Approach 1:
The patent divides the amplifier into two separate stages: a first low noise amplifier stage that provides voltage gain with controlled gain magnitude to avoid output saturation, and a second buffer stage that provides current gain with high current drive capability. This segmentation allows each stage to operate within its linear region while achieving overall high power gain, thereby maintaining linearity under strong interference conditions.
2Adaptability or versatility
If feedback path is added to improve input matching, then input match bandwidth is improved, but voltage gain at output node must be increased which contradicts linearity requirement
Solution Approach 1:
The patent separates the voltage gain function and current gain function into different stages. The first stage provides voltage gain with feedback for improved input matching, while the second stage provides current gain without requiring additional voltage gain. This segmentation allows the feedback network to operate effectively for bandwidth expansion without pushing the output voltage swing into nonlinear regions.
Solution Approach 2:
The patent introduces a buffer stage as an intermediary between the voltage gain stage and the output load. This buffer stage acts as a mediator that can drive the output load with high current capability while allowing the voltage gain stage to operate with controlled voltage swing. The buffer isolates the voltage gain stage from output loading effects, enabling effective feedback operation for input matching without compromising linearity.
3Reliability
If voltage gain of low noise amplifier is increased to allow larger feedback resistance, then noise contribution from feedback resistance is reduced, but output voltage swing must be kept low to meet linearity requirement
Solution Approach 1:
The patent segments the gain function into voltage gain in the first stage and current gain in the second stage. The first stage can use a larger feedback resistance to reduce noise contribution while maintaining controlled voltage swing. The second stage then amplifies the current without requiring additional voltage gain, achieving high power gain overall. This segmentation allows optimization of each stage for its specific function without the conflicting requirements.
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
The solution achieves high linearity, wide input match bandwidth, and decent noise figure without increasing power consumption, effectively suppressing out-band interference and providing sufficient conversion gain for RF receivers, even under strong interference conditions.
Implementation Method 1
a self-coupled step-up transformer having at least one primary winding connected to a supply voltage and the drain terminal of the at least one amplifying transistor and at least one self-coupled secondary inductor winding arranged in said feedback path
Data Source
AI summary
A low noise amplifier comprises at least one amplifying transistor (Ts1; Ts2) configured in a common source configuration to receive an input signal (RFin) at a gate terminal and provide an amplified signal at a drain terminal and at least one feedback path arranged to couple a part of the amplified signal back to the gate terminal and comprising a feedback impedance. The low noise amplifier further comprises a self-coupled step-up transformer having at least one primary winding (Lp) connected to a supply voltage (Vdd) and the drain terminal of the at least one amplifying transistor and at least one self-coupled secondary inductor winding (Lf1; Lf2) arranged in the feedback path. The low noise amplifier provides a better suppression for out-band interference and at the same time it has a wider input match bandwidth, decent conversion gain and decent noise figure without increasing power consumption.