Split LNA Drain Sharing for Isolation and Low Noise Figure
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Solution Overview
Problem
Current low noise amplifiers (LNAs) face challenges in achieving high output-to-output isolation and maintaining low front-end losses when operating in Split mode, especially when handling intraband non-contiguous carrier aggregate signals, due to limitations in passive splitters and mode selection switches, leading to interference, distortion, and degradation in receiver sensitivity.
Innovation Solution
The use of multiple low noise amplifiers configured as cascode amplifiers with switchable transistors and capacitors allows for operation in both single and split modes with enhanced isolation and reduced losses, by selectively turning output FETs on or off and coupling drain terminals, while maintaining consistent input impedance through switchable connections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a passive splitter is used to split the signal in Split mode, then the signal can be distributed to multiple DBCs, but the output-to-output isolation between DBCs is insufficient (only 18-20 dB at center frequency)
Solution Approach 1:
The invention divides the single LNA into multiple independent LNA segments (first LNA and second LNA), each handling a specific channel. This segmentation eliminates the need for a passive splitter and provides complete isolation between channels, as each LNA has its own independent signal path to its designated DBC.
Solution Approach 2:
The mode selector switch acts as an intermediary that routes signals from the appropriate LNA output to the selected DBC. In Split mode, the switch enables both LNA outputs to be simultaneously connected to their respective DBCs without interference, achieving perfect isolation through active switching rather than passive splitting.
2Ease of operation
If a passive splitter is used, then signal splitting is achieved, but the insertion loss increases (3 dB reduction plus 1.0-1.5 dB additional loss)
Solution Approach 1:
By segmenting the amplification function across multiple independent LNAs, each LNA provides full-gain amplification to its designated channel without the power division inherent in passive splitters. This eliminates the 3 dB power split loss and additional insertion loss associated with passive splitting components.
3Adaptability or versatility
If the frequency deviates from the optimal frequency for the splitter, then the frequency range coverage is reduced, but the output-to-output isolation degrades
Solution Approach 1:
The invention segments the frequency handling function across multiple LNAs, with each LNA optimized for specific frequency ranges. This allows the system to cover a broad frequency range while maintaining high isolation, as each LNA operates independently within its optimized range without relying on the frequency-dependent performance of a passive splitter.
Solution Approach 2:
The system dynamically selects which LNA to use based on the frequency of the incoming signal. The mode selector switch and control logic adapt the active LNA configuration to match the signal frequency, ensuring optimal performance and isolation across the entire frequency range rather than being constrained to a narrow optimal frequency.
4Object-affected harmful factors
If multiple LNAs are used with switchable transistors and capacitors, then isolation and loss reduction are achieved, but the device complexity increases
Solution Approach 1:
The mode selector switch serves multiple functions: it selects between single-channel and split-channel modes, routes signals from the appropriate LNA to the correct DBC, and enables the system to adapt to different operating conditions. This multi-functionality reduces the need for separate components for each function, offsetting the complexity of using multiple LNAs.
Data Source
AI summary
A receiver front end (300) having low noise amplifiers (LNAs) is disclosed herein. A cascode having a “common source” configured input FET and a “common gate” configured output FET can be turned on or off using the gate of the output FET. A first switch (235) is provided that allows a connection to be either established or broken between the source terminal of the input FET of each LNA. A drain switch (260) is provided between the drain terminals of input FETs to place the input FETs in parallel. This increases the gm of the input stage of the amplifier, thus improving the noise figure of the amplifier.